A join clause in
SQL – corresponding to a
join operation in relational algebra – combines
columns
A column or pillar in architecture and structural engineering is a structural element that transmits, through compression, the weight of the structure above to other structural elements below. In other words, a column is a compression member. ...
from one or more
tables
Table may refer to:
* Table (furniture), a piece of furniture with a flat surface and one or more legs
* Table (landform), a flat area of land
* Table (information), a data arrangement with rows and columns
* Table (database), how the table data ...
into a new table. Informally, a join stitches two tables and puts on the same row records with matching fields :
INNER
,
LEFT OUTER
,
RIGHT OUTER
,
FULL OUTER
and
CROSS
.
Example tables
To explain join types, the rest of this article uses the following tables:
Department.DepartmentID
is the
primary key
In the relational model of databases, a primary key is a ''specific choice'' of a ''minimal'' set of attributes (Column (database), columns) that uniquely specify a tuple (Row (database), row) in a Relation (database), relation (Table (database), t ...
of the
Department
table, whereas
Employee.DepartmentID
is a
foreign key A foreign key is a set of attributes in a table that refers to the primary key of another table. The foreign key links these two tables. Another way to put it: In the context of relational databases, a foreign key is a set of attributes subject to ...
.
Note that in
Employee
, "Williams" has not yet been assigned to a department. Also, no employees have been assigned to the "Marketing" department.
This is the SQL statement to create the above tables:
CREATE TABLE department(
DepartmentID INT PRIMARY KEY NOT NULL,
DepartmentName VARCHAR(20)
);
CREATE TABLE employee (
LastName VARCHAR(20),
DepartmentID INT REFERENCES department(DepartmentID)
);
INSERT INTO department
VALUES (31, 'Sales'),
(33, 'Engineering'),
(34, 'Clerical'),
(35, 'Marketing');
INSERT INTO employee
VALUES ('Rafferty', 31),
('Jones', 33),
('Heisenberg', 33),
('Robinson', 34),
('Smith', 34),
('Williams', NULL);
Cross join
CROSS JOIN
returns the
Cartesian product
In mathematics, specifically set theory, the Cartesian product of two sets ''A'' and ''B'', denoted ''A''×''B'', is the set of all ordered pairs where ''a'' is in ''A'' and ''b'' is in ''B''. In terms of set-builder notation, that is
: A\ti ...
of rows from tables in the join. In other words, it will produce rows which combine each row from the first table with each row from the second table.
Example of an explicit cross join:
SELECT *
FROM employee CROSS JOIN department;
Example of an implicit cross join:
SELECT *
FROM employee, department;
The cross join can be replaced with an inner join with an always-true condition:
SELECT *
FROM employee INNER JOIN department ON 1=1;
CROSS JOIN
does not itself apply any predicate to filter rows from the joined table. The results of a
CROSS JOIN
can be filtered using a
WHERE
clause, which may then produce the equivalent of an inner join.
In the
SQL:2011 standard, cross joins are part of the optional F401, "Extended joined table", package.
Normal uses are for checking the server's performance.
Inner join
An inner join requires each row in the two joined tables to have matching column values, and is a commonly used join operation in
applications
Application may refer to:
Mathematics and computing
* Application software, computer software designed to help the user to perform specific tasks
** Application layer, an abstraction layer that specifies protocols and interface methods used in a c ...
but should not be assumed to be the best choice in all situations. Inner join creates a new result table by combining column values of two tables (A and B) based upon the join-predicate. The query compares each row of A with each row of B to find all pairs of rows that satisfy the join-predicate. When the join-predicate is satisfied by matching non-
NULL
Null may refer to:
Science, technology, and mathematics Computing
*Null (SQL) (or NULL), a special marker and keyword in SQL indicating that something has no value
*Null character, the zero-valued ASCII character, also designated by , often used ...
values, column values for each matched pair of rows of A and B are combined into a result row.
The result of the join can be defined as the outcome of first taking the
Cartesian product
In mathematics, specifically set theory, the Cartesian product of two sets ''A'' and ''B'', denoted ''A''×''B'', is the set of all ordered pairs where ''a'' is in ''A'' and ''b'' is in ''B''. In terms of set-builder notation, that is
: A\ti ...
(or
Cross join
A join clause in SQL – corresponding to a join operation in relational algebra – combines columns from one or more tables into a new table. Informally, a join stitches two tables and puts on the same row records with matching fields : INNER, ...
) of all rows in the tables (combining every row in table A with every row in table B) and then returning all rows that satisfy the join predicate. Actual SQL implementations normally use other approaches, such as
hash join
The hash join is an example of a join algorithm and is used in the implementation of a relational database management system. All variants of hash join algorithms involve building hash tables from the tuples of one or both of the joined relations ...
s or
sort-merge join
The sort-merge join (also known as merge join) is a join algorithm and is used in the implementation of a relational database management system.
The basic problem of a join algorithm is to find, for each distinct value of the join attribute, the ...
s, since computing the Cartesian product is slower and would often require a prohibitively large amount of memory to store.
SQL specifies two different syntactical ways to express joins: the "explicit join notation" and the "implicit join notation". The "implicit join notation" is no longer considered a best practice, although database systems still support it.
The "explicit join notation" uses the
JOIN
keyword, optionally preceded by the
INNER
keyword, to specify the table to join, and the
ON
keyword to specify the predicates for the join, as in the following example:
SELECT employee.LastName, employee.DepartmentID, department.DepartmentName
FROM employee
INNER JOIN department ON
employee.DepartmentID = department.DepartmentID;
The "implicit join notation" simply lists the tables for joining, in the
FROM
clause of the
SELECT
statement, using commas to separate them. Thus it specifies a
cross join
A join clause in SQL – corresponding to a join operation in relational algebra – combines columns from one or more tables into a new table. Informally, a join stitches two tables and puts on the same row records with matching fields : INNER, ...
, and the
WHERE
clause may apply additional filter-predicates (which function comparably to the join-predicates in the explicit notation).
The following example is equivalent to the previous one, but this time using implicit join notation:
SELECT employee.LastName, employee.DepartmentID, department.DepartmentName
FROM employee, department
WHERE employee.DepartmentID = department.DepartmentID;
The queries given in the examples above will join the Employee and Department tables using the DepartmentID column of both tables. Where the DepartmentID of these tables match (i.e. the join-predicate is satisfied), the query will combine the ''LastName'', ''DepartmentID'' and ''DepartmentName'' columns from the two tables into a result row. Where the DepartmentID does not match, no result row is generated.
Thus the result of the
execution
Capital punishment, also known as the death penalty, is the State (polity), state-sanctioned practice of deliberately killing a person as a punishment for an actual or supposed crime, usually following an authorized, rule-governed process to ...
of the query above will be:
The employee "Williams" and the department "Marketing" do not appear in the query execution results. Neither of these has any matching rows in the other respective table: "Williams" has no associated department, and no employee has the department ID 35 ("Marketing"). Depending on the desired results, this behavior may be a subtle bug, which can be avoided by replacing the inner join with an
outer join
A join clause in SQL – corresponding to a join operation in relational algebra – combines columns from one or more tables into a new table. Informally, a join stitches two tables and puts on the same row records with matching fields : INNER, ...
.
Inner join and NULL values
Programmers should take special care when joining tables on columns that can contain
NULL
Null may refer to:
Science, technology, and mathematics Computing
*Null (SQL) (or NULL), a special marker and keyword in SQL indicating that something has no value
*Null character, the zero-valued ASCII character, also designated by , often used ...
values, since NULL will never match any other value (not even NULL itself), unless the join condition explicitly uses a combination predicate that first checks that the joins columns are
NOT NULL
before applying the remaining predicate condition(s). The Inner Join can only be safely used in a database that enforces
referential integrity
Referential integrity is a property of data stating that all its references are valid. In the context of relational databases, it requires that if a value of one attribute (column) of a relation (table) references a value of another attribute (ei ...
or where the join columns are guaranteed not to be NULL. Many
transaction processing
Transaction processing is information processing in computer science that is divided into individual, indivisible operations called ''transactions''. Each transaction must succeed or fail as a complete unit; it can never be only partially comple ...
relational databases rely on
Atomicity, Consistency, Isolation, Durability (ACID) data update standards to ensure data integrity, making inner joins an appropriate choice. However, transaction databases usually also have desirable join columns that are allowed to be NULL. Many reporting relational database and
data warehouse
In computing, a data warehouse (DW or DWH), also known as an enterprise data warehouse (EDW), is a system used for Business reporting, reporting and data analysis and is considered a core component of business intelligence. DWs are central Repos ...
s use high volume
Extract, Transform, Load (ETL) batch updates which make referential integrity difficult or impossible to enforce, resulting in potentially NULL join columns that an SQL query author cannot modify and which cause inner joins to omit data with no indication of an error. The choice to use an inner join depends on the database design and data characteristics. A left outer join can usually be substituted for an inner join when the join columns in one table may contain NULL values.
Any data column that may be NULL (empty) should never be used as a link in an inner join, unless the intended result is to eliminate the rows with the NULL value. If NULL join columns are to be deliberately removed from the
result set
An SQL result set is a set of rows from a database, as well as metadata about the query such as the column names, and the types and sizes of each column. Depending on the database system, the number of rows in the result set may or may not be kno ...
, an inner join can be faster than an outer join because the table join and filtering is done in a single step. Conversely, an inner join can result in disastrously slow performance or even a server crash when used in a large volume query in combination with database functions in an SQL Where clause. A function in an SQL Where clause can result in the database ignoring relatively compact table indexes. The database may read and inner join the selected columns from both tables before reducing the number of rows using the filter that depends on a calculated value, resulting in a relatively enormous amount of inefficient processing.
When a result set is produced by joining several tables, including master tables used to look up full-text descriptions of numeric identifier codes (a
Lookup table
In computer science, a lookup table (LUT) is an array that replaces runtime computation with a simpler array indexing operation. The process is termed as "direct addressing" and LUTs differ from hash tables in a way that, to retrieve a value v wi ...
), a NULL value in any one of the foreign keys can result in the entire row being eliminated from the result set, with no indication of error. A complex SQL query that includes one or more inner joins and several outer joins has the same risk for NULL values in the inner join link columns.
A commitment to SQL code containing inner joins assumes NULL join columns will not be introduced by future changes, including vendor updates, design changes and bulk processing outside of the application's data validation rules such as data conversions, migrations, bulk imports and merges.
One can further classify inner joins as equi-joins, as natural joins, or as cross-joins.
Equi-join
An equi-join is a specific type of comparator-based join, that uses only
equality
Equality may refer to:
Society
* Political equality, in which all members of a society are of equal standing
** Consociationalism, in which an ethnically, religiously, or linguistically divided state functions by cooperation of each group's elite ...
comparisons in the join-predicate. Using other comparison operators (such as
<
) disqualifies a join as an equi-join. The query shown above has already provided an example of an equi-join:
SELECT *
FROM employee JOIN department
ON employee.DepartmentID = department.DepartmentID;
We can write equi-join as below,
SELECT *
FROM employee, department
WHERE employee.DepartmentID = department.DepartmentID;
If columns in an equi-join have the same name,
SQL-92
SQL-92 was the third revision of the SQL database query language. Unlike SQL-89, it was a major revision of the standard. Aside from a few minor incompatibilities, the SQL-89 standard is forward-compatible with SQL-92.
The standard specificatio ...
provides an optional shorthand notation for expressing equi-joins, by way of the
USING
construct:
SELECT *
FROM employee INNER JOIN department USING (DepartmentID);
The
USING
construct is more than mere
syntactic sugar
In computer science, syntactic sugar is syntax within a programming language that is designed to make things easier to read or to express. It makes the language "sweeter" for human use: things can be expressed more clearly, more concisely, or in an ...
, however, since the result set differs from the result set of the version with the explicit predicate. Specifically, any columns mentioned in the
USING
list will appear only once, with an unqualified name, rather than once for each table in the join. In the case above, there will be a single
DepartmentID
column and no
employee.DepartmentID
or
department.DepartmentID
.
The
USING
clause is not supported by MS SQL Server and Sybase.
Natural join
The natural join is a special case of equi-join. Natural join (⋈) is a
binary operator
In mathematics, a binary operation or dyadic operation is a rule for combining two elements (called operands) to produce another element. More formally, a binary operation is an operation of arity two.
More specifically, an internal binary ope ...
that is written as (''R'' ⋈ ''S'') where ''R'' and ''S'' are
relations. The result of the natural join is the set of all combinations of
tuples
In mathematics, a tuple is a finite ordered list (sequence) of elements. An -tuple is a sequence (or ordered list) of elements, where is a non-negative integer. There is only one 0-tuple, referred to as ''the empty tuple''. An -tuple is defi ...
in ''R'' and ''S'' that are equal on their common attribute names. For an example consider the tables ''Employee'' and ''Dept'' and their natural join:
This can also be used to define
composition of relations
In the mathematics of binary relations, the composition of relations is the forming of a new binary relation from two given binary relations ''R'' and ''S''. In the calculus of relations, the composition of relations is called relative multiplica ...
. For example, the composition of ''Employee'' and ''Dept'' is their join as shown above, projected on all but the common attribute ''DeptName''. In
category theory
Category theory is a general theory of mathematical structures and their relations that was introduced by Samuel Eilenberg and Saunders Mac Lane in the middle of the 20th century in their foundational work on algebraic topology. Nowadays, cate ...
, the join is precisely the
fiber product
In category theory, a branch of mathematics, a pullback (also called a fiber product, fibre product, fibered product or Cartesian square) is the limit of a diagram consisting of two morphisms and with a common codomain. The pullback is often w ...
.
The natural join is arguably one of the most important operators since it is the relational counterpart of logical AND. Note that if the same variable appears in each of two predicates that are connected by AND, then that variable stands for the same thing and both appearances must always be substituted by the same value. In particular, the natural join allows the combination of relations that are associated by a
foreign key A foreign key is a set of attributes in a table that refers to the primary key of another table. The foreign key links these two tables. Another way to put it: In the context of relational databases, a foreign key is a set of attributes subject to ...
. For example, in the above example a foreign key probably holds from ''Employee''.''DeptName'' to ''Dept''.''DeptName'' and then the natural join of ''Employee'' and ''Dept'' combines all employees with their departments. This works because the foreign key holds between attributes with the same name. If this is not the case such as in the foreign key from ''Dept''.''manager'' to ''Employee''.''Name'' then these columns have to be renamed before the natural join is taken. Such a join is sometimes also referred to as an equi-join.
More formally the semantics of the natural join are defined as follows:
:
,
where ''Fun'' is a
predicate
Predicate or predication may refer to:
* Predicate (grammar), in linguistics
* Predication (philosophy)
* several closely related uses in mathematics and formal logic:
**Predicate (mathematical logic)
**Propositional function
**Finitary relation, o ...
that is true for a
relation
Relation or relations may refer to:
General uses
*International relations, the study of interconnection of politics, economics, and law on a global level
*Interpersonal relationship, association or acquaintance between two or more people
*Public ...
''r''
if and only if
In logic and related fields such as mathematics and philosophy, "if and only if" (shortened as "iff") is a biconditional logical connective between statements, where either both statements are true or both are false.
The connective is bicondi ...
''r'' is a function. It is usually required that ''R'' and ''S'' must have at least one common attribute, but if this constraint is omitted, and ''R'' and ''S'' have no common attributes, then the natural join becomes exactly the Cartesian product.
The natural join can be simulated with Codd's primitives as follows. Let ''c''
1, …, ''c''
''m'' be the attribute names common to ''R'' and ''S'', ''r''
1, …, ''r''
''n'' be the attribute names unique to ''R'' and let ''s''
1, …, ''s''
''k'' be the attributes unique to ''S''. Furthermore, assume that the attribute names ''x''
1, …, ''x''
''m'' are neither in ''R'' nor in ''S''. In a first step the common attribute names in ''S'' can now be renamed:
:
Then we take the Cartesian product and select the tuples that are to be joined:
:
A
natural join
In database theory, relational algebra is a theory that uses algebraic structures with a well-founded semantics for modeling data, and defining queries on it. The theory was introduced by Edgar F. Codd.
The main application of relational algebra ...
is a type of equi-join where the join predicate arises implicitly by comparing all columns in both tables that have the same column-names in the joined tables. The resulting joined table contains only one column for each pair of equally named columns. In the case that no columns with the same names are found, the result is a
cross join
A join clause in SQL – corresponding to a join operation in relational algebra – combines columns from one or more tables into a new table. Informally, a join stitches two tables and puts on the same row records with matching fields : INNER, ...
.
Most experts agree that NATURAL JOINs are dangerous and therefore strongly discourage their use. The danger comes from inadvertently adding a new column, named the same as another column in the other table. An existing natural join might then "naturally" use the new column for comparisons, making comparisons/matches using different criteria (from different columns) than before. Thus an existing query could produce different results, even though the data in the tables have not been changed, but only augmented. The use of column names to automatically determine table links is not an option in large databases with hundreds or thousands of tables where it would place an unrealistic constraint on naming conventions. Real world databases are commonly designed with
foreign key A foreign key is a set of attributes in a table that refers to the primary key of another table. The foreign key links these two tables. Another way to put it: In the context of relational databases, a foreign key is a set of attributes subject to ...
data that is not consistently populated (NULL values are allowed), due to business rules and context. It is common practice to modify column names of similar data in different tables and this lack of rigid consistency relegates natural joins to a theoretical concept for discussion.
The above sample query for inner joins can be expressed as a natural join in the following way:
SELECT *
FROM employee NATURAL JOIN department;
As with the explicit
USING
clause, only one DepartmentID column occurs in the joined table, with no qualifier:
PostgreSQL, MySQL and Oracle support natural joins; Microsoft T-SQL and IBM DB2 do not. The columns used in the join are implicit so the join code does not show which columns are expected, and a change in column names may change the results. In the
SQL:2011 standard, natural joins are part of the optional F401, "Extended joined table", package.
In many database environments the column names are controlled by an outside vendor, not the query developer. A natural join assumes stability and consistency in column names which can change during vendor mandated version upgrades.
Outer join
The joined table retains each row—even if no other matching row exists. Outer joins subdivide further into left outer joins, right outer joins, and full outer joins, depending on which table's rows are retained: left, right, or both (in this case ''left'' and ''right'' refer to the two sides of the
JOIN
keyword). Like
inner joins, one can further sub-categorize all types of outer joins as
equi-joins,
natural joins,
ON ''<predicate>''
(
''θ''-join), etc.
No implicit join-notation for outer joins exists in standard SQL.
Left outer join
The result of a left outer join (or simply left join) for tables A and B always contains all rows of the "left" table (A), even if the join-condition does not find any matching row in the "right" table (B). This means that if the
ON
clause matches 0 (zero) rows in B (for a given row in A), the join will still return a row in the result (for that row)—but with NULL in each column from B. A left outer join returns all the values from an inner join plus all values in the left table that do not match to the right table, including rows with NULL (empty) values in the link column.
For example, this allows us to find an employee's department, but still shows employees that have not been assigned to a department (contrary to the inner-join example above, where unassigned employees were excluded from the result).
Example of a left outer join (the
OUTER
keyword is optional), with the additional result row (compared with the inner join) italicized:
SELECT *
FROM employee
LEFT OUTER JOIN department ON employee.DepartmentID = department.DepartmentID;
Alternative syntaxes
Oracle supports the deprecated
[Oracle Left Outer Join](_blank)
/ref> syntax:
SELECT *
FROM employee, department
WHERE employee.DepartmentID = department.DepartmentID(+)
Sybase
Sybase, Inc. was an enterprise software and services company. The company produced software to manage and analyze information in relational databases, with facilities located in California and Massachusetts. Sybase was acquired by SAP in 2010; ...
supports the syntax (Microsoft SQL Server
Microsoft SQL Server is a relational database management system developed by Microsoft. As a database server, it is a software product with the primary function of storing and retrieving data as requested by other software applications—which ma ...
deprecated this syntax since version 2000):
SELECT *
FROM employee, department
WHERE employee.DepartmentID *= department.DepartmentID
IBM Informix
IBM Informix is a product family within IBM's Information Management division that is centered on several relational database management system (RDBMS) offerings. The Informix products were originally developed by Informix Corporation, whose I ...
supports the syntax:
SELECT *
FROM employee, OUTER department
WHERE employee.DepartmentID = department.DepartmentID
Right outer join
A right outer join (or right join) closely resembles a left outer join, except with the treatment of the tables reversed. Every row from the "right" table (B) will appear in the joined table at least once. If no matching row from the "left" table (A) exists, NULL will appear in columns from A for those rows that have no match in B.
A right outer join returns all the values from the right table and matched values from the left table (NULL in the case of no matching join predicate). For example, this allows us to find each employee and his or her department, but still show departments that have no employees.
Below is an example of a right outer join (the OUTER
keyword is optional), with the additional result row italicized:
SELECT *
FROM employee RIGHT OUTER JOIN department
ON employee.DepartmentID = department.DepartmentID;
Right and left outer joins are functionally equivalent. Neither provides any functionality that the other does not, so right and left outer joins may replace each other as long as the table order is switched.
Full outer join
Conceptually, a full outer join combines the effect of applying both left and right outer joins. Where rows in the FULL OUTER JOINed tables do not match, the result set will have NULL values for every column of the table that lacks a matching row. For those rows that do match, a single row will be produced in the result set (containing columns populated from both tables).
For example, this allows us to see each employee who is in a department and each department that has an employee, but also see each employee who is not part of a department and each department which doesn't have an employee.
Example of a full outer join (the OUTER
keyword is optional):
SELECT *
FROM employee FULL OUTER JOIN department
ON employee.DepartmentID = department.DepartmentID;
Some database systems do not support the full outer join functionality directly, but they can emulate it through the use of an inner join and UNION ALL selects of the "single table rows" from left and right tables respectively. The same example can appear as follows:
SELECT employee.LastName, employee.DepartmentID,
department.DepartmentName, department.DepartmentID
FROM employee
INNER JOIN department ON employee.DepartmentID = department.DepartmentID
UNION ALL
SELECT employee.LastName, employee.DepartmentID,
cast(NULL as varchar(20)), cast(NULL as integer)
FROM employee
WHERE NOT EXISTS (
SELECT * FROM department
WHERE employee.DepartmentID = department.DepartmentID)
UNION ALL
SELECT cast(NULL as varchar(20)), cast(NULL as integer),
department.DepartmentName, department.DepartmentID
FROM department
WHERE NOT EXISTS (
SELECT * FROM employee
WHERE employee.DepartmentID = department.DepartmentID)
Another approach could be UNION ALL of left outer join and right outer join MINUS inner join.
Self-join
A self-join is joining a table to itself.
Example
If there were two separate tables for employees and a query which requested employees in the first table having the same country as employees in the second table, a normal join operation could be used to find the answer table. However, all the employee information is contained within a single large table.
Consider a modified Employee
table such as the following:
An example solution query could be as follows:
SELECT F.EmployeeID, F.LastName, S.EmployeeID, S.LastName, F.Country
FROM Employee F INNER JOIN Employee S ON F.Country = S.Country
WHERE F.EmployeeID < S.EmployeeID
ORDER BY F.EmployeeID, S.EmployeeID;
Which results in the following table being generated.
For this example:
* F
and S
are aliases
A pseudonym (; ) or alias () is a fictitious name that a person or group assumes for a particular purpose, which differs from their original or true name (orthonym). This also differs from a new name that entirely or legally replaces an individua ...
for the first and second copies of the employee table.
* The condition F.Country = S.Country
excludes pairings between employees in different countries. The example question only wanted pairs of employees in the same country.
* The condition F.EmployeeID < S.EmployeeID
excludes pairings where the EmployeeID
of the first employee is greater than or equal to the EmployeeID
of the second employee. In other words, the effect of this condition is to exclude duplicate pairings and self-pairings. Without it, the following less useful table would be generated (the table below displays only the "Germany" portion of the result):
Only one of the two middle pairings is needed to satisfy the original question, and the topmost and bottommost are of no interest at all in this example.
Alternatives
The effect of an outer join can also be obtained using a UNION ALL between an INNER JOIN and a SELECT of the rows in the "main" table that do not fulfill the join condition. For example,
SELECT employee.LastName, employee.DepartmentID, department.DepartmentName
FROM employee
LEFT OUTER JOIN department ON employee.DepartmentID = department.DepartmentID;
can also be written as
SELECT employee.LastName, employee.DepartmentID, department.DepartmentName
FROM employee
INNER JOIN department ON employee.DepartmentID = department.DepartmentID
UNION ALL
SELECT employee.LastName, employee.DepartmentID, cast(NULL as varchar(20))
FROM employee
WHERE NOT EXISTS (
SELECT * FROM department
WHERE employee.DepartmentID = department.DepartmentID)
Implementation
Much work in database-systems has aimed at efficient implementation of joins, because relational systems commonly call for joins, yet face difficulties in optimising their efficient execution. The problem arises because inner joins operate both commutative
In mathematics, a binary operation is commutative if changing the order of the operands does not change the result. It is a fundamental property of many binary operations, and many mathematical proofs depend on it. Most familiar as the name o ...
ly and associative
In mathematics, the associative property is a property of some binary operations, which means that rearranging the parentheses in an expression will not change the result. In propositional logic, associativity is a valid rule of replacement f ...
ly. In practice, this means that the user merely supplies the list of tables for joining and the join conditions to use, and the database system has the task of determining the most efficient way to perform the operation. A query optimizer
Query optimization is a feature of many relational database management systems and other databases such as NoSQL and graph databases. The query optimizer attempts to determine the most efficient way to execute a given query by considering the pos ...
determines how to execute a query containing joins. A query optimizer has two basic freedoms:
# Join order: Because it joins functions commutatively and associatively, the order in which the system joins tables does not change the final result set of the query. However, join-order could have an enormous impact on the cost of the join operation, so choosing the best join order becomes very important.
# Join method: Given two tables and a join condition, multiple algorithm
In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific Computational problem, problems or to perform a computation. Algorithms are used as specificat ...
s can produce the result set of the join. Which algorithm runs most efficiently depends on the sizes of the input tables, the number of rows from each table that match the join condition, and the operations required by the rest of the query.
Many join-algorithms treat their inputs differently. One can refer to the inputs to a join as the "outer" and "inner" join operands, or "left" and "right", respectively. In the case of nested loops, for example, the database system will scan the entire inner relation for each row of the outer relation.
One can classify query-plans involving joins as follows:[
]
; left-deep : using a base table (rather than another join) as the inner operand of each join in the plan
; right-deep : using a base table as the outer operand of each join in the plan
; bushy : neither left-deep nor right-deep; both inputs to a join may themselves result from joins
These names derive from the appearance of the query plan
In general, a query is a form of questioning, in a line of inquiry.
Query may also refer to:
Computing and technology
* Query, a precise request for information retrieval made to a database or information system
** Query language, a computer lan ...
if drawn as a tree
In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...
, with the outer join relation on the left and the inner relation on the right (as convention dictates).
Join algorithms
Three fundamental algorithms for performing a join operation exist: nested loop join
A nested loop join is a naive algorithm that joins two sets by using two nested loop (computing), loops. Join operations are important for database management.
Algorithm
Two relations R and S are joined as follows:
algorithm nested_loop_join is
...
, sort-merge join
The sort-merge join (also known as merge join) is a join algorithm and is used in the implementation of a relational database management system.
The basic problem of a join algorithm is to find, for each distinct value of the join attribute, the ...
and hash join
The hash join is an example of a join algorithm and is used in the implementation of a relational database management system. All variants of hash join algorithms involve building hash tables from the tuples of one or both of the joined relations ...
.
Join indexes
Join indexes are database index
A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of additional writes and storage space to maintain the index data structure. Indexes are used to quickly locate data without ...
es that facilitate the processing of join queries in data warehouse
In computing, a data warehouse (DW or DWH), also known as an enterprise data warehouse (EDW), is a system used for Business reporting, reporting and data analysis and is considered a core component of business intelligence. DWs are central Repos ...
s: they are currently (2012) available in implementations by Oracle
An oracle is a person or agency considered to provide wise and insightful counsel or prophetic predictions, most notably including precognition of the future, inspired by deities. As such, it is a form of divination.
Description
The word '' ...
and Teradata
Teradata Corporation is an American software company that provides cloud database and analytics-related software, products, and services. The company was formed in 1979 in Brentwood, California, as a collaboration between researchers at Caltech a ...
.
In the Teradata implementation, specified columns, aggregate functions on columns, or components of date columns from one or more tables are specified using a syntax similar to the definition of a database view
In a database, a view is the result set of a ''stored'' query on the data, which the database users can query just as they would in a persistent database collection object. This pre-established query command is kept in the database dictionary. Unl ...
: up to 64 columns/column expressions can be specified in a single join index. Optionally, a column that defines the primary key
In the relational model of databases, a primary key is a ''specific choice'' of a ''minimal'' set of attributes (Column (database), columns) that uniquely specify a tuple (Row (database), row) in a Relation (database), relation (Table (database), t ...
of the composite data may also be specified: on parallel hardware, the column values are used to partition the index's contents across multiple disks. When the source tables are updated interactively by users, the contents of the join index are automatically updated. Any query whose WHERE clause specifies any combination of columns or column expressions that are an exact subset of those defined in a join index (a so-called "covering query") will cause the join index, rather than the original tables and their indexes, to be consulted during query execution.
The Oracle implementation limits itself to using bitmap indexes. A ''bitmap join index'' is used for low-cardinality columns (i.e., columns containing fewer than 300 distinct values, according to the Oracle documentation): it combines low-cardinality columns from multiple related tables. The example Oracle uses is that of an inventory system, where different suppliers provide different parts. The schema
The word schema comes from the Greek word ('), which means ''shape'', or more generally, ''plan''. The plural is ('). In English, both ''schemas'' and ''schemata'' are used as plural forms.
Schema may refer to:
Science and technology
* SCHEMA ...
has three linked tables: two "master tables", Part and Supplier, and a "detail table", Inventory. The last is a many-to-many table linking Supplier to Part, and contains the most rows. Every part has a Part Type, and every supplier is based in the US, and has a State column. There are not more than 60 states+territories in the US, and not more than 300 Part Types. The bitmap join index is defined using a standard three-table join on the three tables above, and specifying the Part_Type and Supplier_State columns for the index. However, it is defined on the Inventory table, even though the columns Part_Type and Supplier_State are "borrowed" from Supplier and Part respectively.
As for Teradata, an Oracle bitmap join index is only utilized to answer a query when the query's WHERE clause specifies columns limited to those that are included in the join index.
Straight join
Some database systems allow the user to force the system to read the tables in a join in a particular order. This is used when the join optimizer chooses to read the tables in an inefficient order. For example, in MySQL
MySQL () is an open-source relational database management system (RDBMS). Its name is a combination of "My", the name of co-founder Michael Widenius's daughter My, and "SQL", the acronym for Structured Query Language. A relational database o ...
the command STRAIGHT_JOIN
reads the tables in exactly the order listed in the query.
See also
* Join (relational algebra)
In database theory, relational algebra is a theory that uses algebraic structures with a well-founded semantics for modeling data, and defining queries on it. The theory was introduced by Edgar F. Codd.
The main application of relational algebr ...
* Antijoin
In database theory, relational algebra is a theory that uses algebraic structures with a well-founded semantics for modeling data, and defining queries on it. The theory was introduced by Edgar F. Codd.
The main application of relational algebra ...
* Set operations (SQL)
Set operations allow the results of multiple queries to be combined into a single result set. Set operators include UNION, INTERSECT, and EXCEPT.
UNION operator
In SQL the UNION clause combines the results of two SQL queries into a single table ...
References
Citations
Sources
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External links
* Specific to products:
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Sybase ASE 15 Joins
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Joins in Microsoft SQL Server
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{{DEFAULTSORT:Join (SQL)
SQL keywords
Articles with example SQL code