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Oracle Semi Join

Semi-join is introduced in Oracle 8.0. It provides an efficient method of performing a WHERE EXISTS sub-query.

A semi-join returns one copy of each row in first table for which at least one match is found.

Semi-joins are written using the EXISTS construct.

Oracle Semi Join Example

Let's take two tables "departments" and "customer"

Departments table

Oracle Semi Join

Customer table

Oracle Semi Join 2

Execute this query

Output

Oracle Semi Join 3

Difference between anti-join and semi-join

While a semi-join returns one copy of each row in the first table for which at least one match is found, an anti-join returns one copy of each row in the first table for which no match is found.

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