获取一个结果集的行数：count(*)和派生表的使用

select * from customers where salesRepEmployeeNumber=( select employeeNumber from employees where jobtitle="sales rep" limit 1) ;

结果集：

 计算行数：

select count(*) from( select * from customers where salesRepEmployeeNumber=( select employeeNumber from employees where jobtitle="sales rep" limit 1) ) mytable ;

结果集：