mysql - 如何计算派生表中的行数？

Question

我有 2 张 table

emp (id , name)
cust(id, name)

现在我想要结果中的行数

q=select emp.id  from emp,cust where emp.id not in (select id from cust);

为此 我提出查询:

select count(*) 
from (select emp.id  
      from emp,cust 
      where emp.id not in (select id from cust));

但是我收到错误:

Every derived table must have alias name"

计算 q 结果中的行数的正确查询是什么？

Answer 1

使用别名；

 select count(*) from 
    (
     select emp.id from emp where emp.id not in (select id from cust)
    )A;