单行子查询

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

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 号或 174 号员工的 manager_id 和 department_id 相同的其他员工的employee_id，manager_id，department_id

实现方式 1：不成对比较

SELECT  employee_id, manager_id, department_id
FROM    employees
WHERE   manager_id IN
          (SELECT  manager_id
                   FROM    employees
                   WHERE   employee_id IN (174,141))
AND     department_id IN 
          (SELECT  department_id
                   FROM    employees
                   WHERE   employee_id IN (174,141))
AND    employee_id NOT IN(174,141);

实现方式 2：成对比较

SELECT    employee_id, manager_id, department_id
FROM    employees
WHERE  (manager_id, department_id) IN
                      (SELECT manager_id, department_id
                       FROM   employees
                       WHERE  employee_id IN (141,174))
AND    employee_id NOT IN (141,174);

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
GROUP BY department_id
HAVING   MIN(salary) >
                       (SELECT MIN(salary)
                        FROM   employees
                        WHERE  department_id = 50);

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');

子查询不返回任何行

非法使用子查询

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

多行子查询使用单行比较符