4.2.1查询条件中的子查询 子查询可用于查询条件。【示例4-10】例如，在表 employees中存储了员工信息。但是其中某些员工并未领取 过工资。那么可以利用子查询作为查询条件来获得所有领取 过工资的员工信息。 SQL>select from employees 2 where employee_id in(select employee_id from salary); EMPLOYEE ID EMPLOYEE NAME EMPLOYEE POSITION EMPLOYEE AGE EMPLOYEE ADDRESS 4-10 employees SQL> select * from employees 2 where employee_id in (select employee_id from salary); EMPLOYEE_ID EMPLOYEE_NAME EMPLOYEE_POSITION EMPLOYEE_AGE EMPLOYEE_ADDRESS