Описание тега in-subquery
Use this tag for questions related to subqueries (i.e. a query that is nested inside a SELECT, INSERT, etc.), that use IN at their outer statement.
An example of an in-subquery is:
WHERE th.parent = 1015 IN (
SELECT DISTINCT(th1.tid)
...
);
as demonstrated in MySQL DELETE FROM with subquery as condition.
Note that this tag is general in its scope, i.e. that it is not restricted in SQL questions only, but any question referring to this type of IN subqueries is welcome to use this tag.