sql - 单行子查询

sql 复制代码

根据，子查询，返回的结果行数，分为单行子查询、多行子查询，

单行、多行这两类子查询，必须严格匹配对应类型的操作符，混用会直接触发语法报错。

单行子查询，

1、子查询执行后，仅返回 1行1列 的单个标量值（比如：一个数字、一个字符串），

本质是，得到一个可直接比较的具体值。

2、对应操作符：只能使用，单行比较运算符： >、>=、<、<=、=、<>（不等于）。

3、如果，子查询实际返回了多行数据，却使用了=这类单行操作符，数据库会直接报错。

例如：MySQL会提示：Subquery returns more than 1 row。

sql 复制代码

# 查询工资大于149号员工工资的员工信息

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

sql 复制代码

# 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、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 
							)

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# 返回公司工资最少的员工的last_name、job_id、salary
SELECT last_name, job_id, salary
from employees
WHERE salary = (
								SELECT MIN(salary)
								FROM employees
								)

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# 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
SELECT employee_id, manager_id, department_id
from employees
WHERE manager_id in (
											SELECT manager_id
											FROM employees
											WHERE employee_id = 141
											)
and department_id in (
											SELECT department_id
											FROM employees
											WHERE employee_id = 141
											)
and employee_id <> 141

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# 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
											)

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/*
	显示员工的employee_id, last_name, location
	其中，若，员工department_id 与 location_id为1800的department_id相同
	则 location 为'Canada'，其余为'USA'
*/

# 这个查询出来是20
SELECT department_id FROM departments WHERE location_id = 1800

# 首先用 常量，比较简单
SELECT employee_id, last_name, (
	case department_id WHEN '20' THEN 'Canada'
					   ELSE 'USA' END
) "location"
FROM employees

# 再替换成sql语句，这样做比较简单
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

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# 子查询的空值情况
/*
	子查询的结果是空值，所以主查询查出来也是空值
*/
SELECT last_name, job_id
FROM employees
WHERE job_id = (
								SELECT job_id
								from employees
								WHERE last_name = "Haas"
							)