Join SQL Wikipedia. An SQLjoin clause combines columns from one or more tables in a relational database. It creates a set that can be saved as a table or used as it is. A JOIN is a means for combining columns from one self join or more tables by using values common to each. ANSI standard SQL specifies five types of JOIN INNER, LEFT OUTER, RIGHT OUTER, FULL OUTER and CROSS. As a special case, a table base table, view, or joined table can JOIN to itself in a self join. A programmer declares a JOIN statement to identify rows for joining. If the evaluated predicate is true, the combined row is then produced in the expected format, a row set or a temporary table. Sample tableseditRelational 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 Department. IDcolumn of the Department table which can be designated as Department. Department. ID is the primary key, while Employee. Department. ID is a foreign key.
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