RE: Select in select subquery

ChrisM@Work

SSC Guru

Points: 186127

August 28, 2013 at 8:17 am

#1645601

It's to support correlated subqueries like this:

select *

from test1 t1

where t1.firstname1 in (select t2.firstname2 from test2 t2 where t2.firstname2 = t1.firstname1)

and is equivalent to this:

select *

from test1 t1

where t1.firstname1 in (select t1.firstname1 from test2 t2)

^{“Write the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw}

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