subquery vs derived tables

sql cast

Syntax The syntax for the CAST function is: CAST ( value AS type )

cast(click_num_str as int) as click_num

派生表需要别名，这是经常忘记的事情，需要记录和弄懂

Keep in mind that a subquery and derived tables are not the same; a derived table vs subquery comparison will show you that there are many differences between the two.

subquery

select * from tmp where a in (select a from xxx)

derived tables

select count(1) from (select * from a) as t