MySQL where sub-query

A where subquery is the result of an inner query as a comparison condition for the outer layer of SQL.

Select goods_id, goods_name from goods where goods_id = (select Max (goods_id) from goods);

The inner query is select MAX (goods_id) from goods

The outer query is select goods_id, goods_name from goods where goods_id

is to assign the value of the largest goods_id to the outer goods_id.

You can also do this with the comparison operator ' in ':

Select goods_id, cat_id, goods_name from goods where goods_id in (select Max (goods_id) from goods Group by cat_id);

' In ' means that within a set, that is, the largest goods_id in a class grouped by cat_id is assigned to the outer goods_id, so that the maximum goods_id per group can be obtained.

MySQL where sub-query