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MySQL EXISTS Operator
In MySQL, the EXISTS operator is used in conjunction with a subquery to test for the existence of rows returned by the subquery. It returns true if the subquery returns one or more rows, and false otherwise.
SELECT
col_names
FROM
table_name
WHERE
[NOT] EXISTS (subquery)The NOT operator negates the EXISTS operator. In other words, the NOT EXISTS returns true if the subquery returns no row, otherwise it returns false.
Note that you can use
SELECT *, SELECT column, SELECT a_constant, or anything in the subquery. The results are the same because MySQL ignores the select list that appeared in the SELECT clause. The subquery's specific columns selected (col_names) don't matter to EXISTS. It only cares about whether any rows are returned.
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MySQL SELECT EXISTS examples 1
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MySQL SELECT EXISTS examples 2
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EXIST VS JOIN
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