Shulou(Shulou.com)06/01 Report--

The meaning of sub-query:

Use a query statement as a result set for other SQL statements to use, just like using a normal table

Query statements that are treated as result sets are called subqueries

Almost all places where tables can be used can be replaced by subqueries.

Key child IN

Exists Not Exists subquery returns true or false

SELECT * FROM (SELECT * FROM T2 where FAge

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