SQL subquery

MySQL Query statements can be in SQL Statement to implement nested structure , Such a structure is called subquery

• stay from The following subquery , The new table after query will be regarded as an actual table , On the Second inquiry
• stay where After the subquery , about SQL In terms of the execution order of query statements ,group by stay where Execute before statement , So it can't be in where The grouping function is directly used in the statement , But you can where First, calculate a value with the grouping function , Reuse into conditions ：

select name from student where (select avg(score) from student) > 90;