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Don Burleson Blog 







SQL Exists vs. IN clause

Oracle Database Tips by Donald BurlesonFebruary 29, 2016

Question: I see that I can use either the EXISTS or the IN clause for a subquery.  Which is better, the EXISTS or the IN clause?  Does using EXISTS resul in a different execution plan?  When should I use the IN clause versus the EXISTS clause?

Answer:  Most Oracle IN clause queries involve a series of literal values, and when a table is present a standard join is better. In most cases the Oracle cost-based optimizer will create an identical execution plan for IN vs EXISTS, so there is no difference in query performance.

The Exists keyword evaluates true or false, but the IN keyword will compare all values in the corresponding subuery column.  If you are using the IN operator, the SQL engine will scan all records fetched from the inner query. On the other hand, if we are using EXISTS, the SQL engine will stop the scanning process as soon as it found a match.

The EXISTS subquery is used when we want to display all rows where we have a matching column in both tables.  In most cases, this type of subquery can be re-written with a standard join to improve performance.

   exists (select book_key from sales);

The EXISTS clause is much faster than IN when the subquery results is very large. Conversely, the IN clause is faster than EXISTS when the subquery results is very small.

Also, the IN clause can't compare anything with NULL values, but the EXISTS clause can compare everything with NULLs.

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