SQL Join Methods in Oracle

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There are 3 SQL Join Methods in Oracle.

Nested Loop Join

For each row in the outer table, the database retrieves all rows in the inner data set that satisfy the join predicate.

Hash Join

The optimizer uses the smaller of two data sets to build a hash table on the join key in memory. The database then scans the larger data set, probing the hash table to find the rows that meet the join condition.

Sort Merge Join

A sort merge join is a variation on a nested loops join. If the two data sets in the join are not already sorted, then the database sorts them. For each row in the first data set, the database probes the second data set for matching rows and joins them.

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