EXISTS
is used to return a boolean value, JOIN
returns a whole other table
EXISTS
is only used to test if a subquery returns results, and short circuits as soon as it does. JOIN
is used to extend a result set by combining it with additional fields from another table to which there is a relation.
In your example, the queries are semantically equivalent.
In general, use EXISTS
when:
- You don’t need to return data from the related table
- You have dupes in the related table (
JOIN
can cause duplicate rows if values are repeated) - You want to check existence (use instead of
LEFT OUTER JOIN...NULL
condition)
If you have proper indexes, most of the time the EXISTS
will perform identically to the JOIN
. The exception is on very complicated subqueries, where it is normally quicker to use EXISTS
.
If your JOIN
key is not indexed, it may be quicker to use EXISTS
but you will need to test for your specific circumstance.
JOIN
syntax is easier to read and clearer normally as well.