Описание тега where-in

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An SQL-standard condition of the form WHERE SOME_COLUMN IN (1,2,3) or using a subquery to create the list, eg WHERE SOME_COLUMN IN (SELECT X FROM MYTABLE WHERE Y)

For example:

SELECT *
FROM MY_TABLE
WHERE MY_COLUMN IN (1,2,3,5,8,13)

or commonly using a subquery to generate the list:

SELECT *
FROM MY_TABLE
WHERE MY_COLUMN IN (
    SELECT SOME_COLUMN FROM SOME_TABLE WHERE <some condition>
)

The subquery version is a source on many performance problems, because most optimizers do not optimize this properly.

Most IN (subquery) queries can be rewritten to use a join that does perform well.