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.
Let's take two tables "departments" and "customer"
SELECT departments.department_id, departments.department_name FROM departments WHERE EXISTS ( SELECT 1 FROM customer WHERE customer.department_id = departments.department_id ) ORDER BY departments.department_id;
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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