<P> An SQL join 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 - table) 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 . </P> <P> 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 . </P> <P> 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 . </P>

What is a left outer join in sql