单行比较操作符

代码示例

题目：查询工资大于149号员工工资的员工的信息

SELECT employee_id,last_name,salary
FROM employees
WHERE salary > (
        SELECT salary
        FROM employees
        WHERE employee_id = 149
        );

题目：返回job_id与141号员工相同，salary比143号员工多的员工姓名，job_id和工资

SELECT last_name,job_id,salary
FROM employees
WHERE job_id = (
        SELECT job_id
        FROM employees
        WHERE employee_id = 141
        )
AND salary > (
        SELECT salary
        FROM employees
        WHERE employee_id = 143
        );

题目：返回公司工资最少的员工的last_name,job_id和salary

SELECT last_name,job_id,salary
FROM employees
WHERE salary = (
        SELECT MIN(salary)
        FROM employees
        );

题目：查询与141号员工的manager_id和department_id相同的其他员工的employee_id，manager_id，department_id。

方式1：

SELECT employee_id,manager_id,department_id
FROM employees
WHERE manager_id = (
            SELECT manager_id
            FROM employees
            WHERE employee_id = 141
           )
AND department_id = (
            SELECT department_id
            FROM employees
            WHERE employee_id = 141
           )
AND employee_id <> 141;

方式2：了解

SELECT employee_id,manager_id,department_id
FROM employees
WHERE (manager_id,department_id) = (
                    SELECT manager_id,department_id
                        FROM employees
                    WHERE employee_id = 141
                   )
AND employee_id <> 141;

HAVING 中的子查询

首先执行子查询。
向主查询中的HAVING 子句返回结果。

题目：查询最低工资大于50号部门最低工资的部门id和其最低工资

SELECT department_id,MIN(salary)
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING MIN(salary) > (
            SELECT MIN(salary)
            FROM employees
            WHERE department_id = 110
             );

CASE中的子查询

在CASE表达式中使用单列子查询：

题目：显式员工的employee_id,last_name和location。其中，若员工department_id与location_id为1800 的department_id相同，则location为’Canada’，其余则为’USA’。

SELECT employee_id, last_name, 
                (CASE department_id WHEN (
             SELECT department_id FROM departments WHERE location_id = 1800) 
         THEN 'Canada' ELSE 'USA' END) location 
FROM employees;

子查询中的空值问题

SELECT last_name, job_id
FROM   employees
WHERE  job_id =
                (SELECT job_id
                 FROM   employees
                 WHERE  last_name = 'Haas');

非法使用子查询

错误：Subquery returns more than 1 row

SELECT employee_id, last_name
FROM   employees
WHERE  salary =
                (SELECT   MIN(salary)
                 FROM     employees
                 GROUP BY department_id);