An SQL join clause - corresponding to a join operation in relational algebra - combines columns from one or more tables in a relational database.
SQL is a domain-specific language used in programming and designed for managing data held in a relational database management system, or for stream processing in a relational data stream management system.
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It creates a set that can be saved as a table or used as it is.
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ANSI-standard SQL specifies five types of JOIN: INNER, LEFT OUTER, RIGHT OUTER, FULL OUTER and CROSS.
If the evaluated predicate is true, the combined row is then produced in the expected format, a row set or a temporary table.
Relational databases are usually normalized to eliminate duplication of information such as when entity types have one-to-many relationships.
For example, a department may be associated with a number of employees.
Joining separate tables for department and employee effectively creates another table which combines the information from both tables.
All subsequent explanations on join types in this article make use of the following two tables.
The rows in these tables serve to illustrate the effect of different types of joins and join-predicates.
In the following tables the DepartmentID column of the Department table is the primary key, while Employee.
In the relational model of databases, a primary key is a specific choice of a minimal set of attributes that uniquely specify a tuple in a relation.
DepartmentID is a foreign key.
In the context of relational databases, a foreign key is a set of attributes subject to a certain kind of inclusion dependency constraint, specifically a constraint that the tuples consisting of the foreign key attributes in one relation, R, must also exist in some other relation, S, and furthermore that those attributes must also be a candidate key in S. In simpler words, a foreign key is a set of attributes that references a candidate key.
Note: In the Employee table above, the employee "Williams" has not been assigned to any department yet.
Also, note that no employees are assigned to the "Marketing" department.
This is the SQL statement to create the aforementioned tables.
CROSS JOIN returns the Cartesian product of rows from tables in the join.
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
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: