MySQL進階七：子查詢

目錄

SELECT嵌套SELECT

含義

分類

按字查詢出現的位置

按結果集的不同

一、where、having後面

特點

1、標量子查詢

2、列子查詢

3、行子查詢

二、select後面

三、from後面

四、exists後面

語法

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SELECT嵌套SELECT

含義

出現在其他語句中的select語句，稱爲子查詢或者內查詢，外部的查詢語句，稱爲主查詢，或者外查詢。

分類

按字查詢出現的位置

• select後面：僅僅支持標量子查詢
• from後面：支持表子查詢
• where、having後面：支持標量子查詢；列子查詢；行子查詢
• exists後面（相關子查詢）：支持表子查詢

按結果集的不同

• 標量子查詢（結果集只有一行一列）
• 列子查詢（結果集只有一列，但是多行）
• 行子子查詢（結果集可以是多行多列）
• 表子查詢（結果集隨意，一般爲多行多列）

一、where、having後面

特點

• 子查詢放在小括號內
• 子查詢一般放在條件右側
• 標量查詢，一般搭配着單行操作符使用，例如：<、>、>=、<=、=、<> 
• 列子查詢，一般搭配多行操作符使用，in、all/some、all
• 子查詢的執行優先於主查詢的執行，因爲子查詢的結果是主查詢的條件

1、標量子查詢

#案例1：誰的工資比 Abel 高？
#step1. 查詢Abel的工資
SELECT salary
FROM employees
WHERE last_name = 'Abel';
#step2.查詢員工的信息，滿足salary大於1的結果
SELECT *
FROM employees
WHERE salary >(
	SELECT salary
	FROM employees
	WHERE last_name = 'Abel'
);

#案例2：返回job_id與141號員工相同，salary比143號員工多的員工 姓名，job_id和工資
#step1.查詢141號員工的job_id
SELECT job_id
FROM employees
WHERE employee_id = 141;

#step2.查詢143號員工的salary
SELECT salary
FROM employees
WHERE employee_id = 143;

#step3.
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
);

#案例3：返回公司工資最少的員工的last_name, job_id,salary
SELECT last_name, job_id, salary
FROM employees
WHERE salary=(
	SELECT MIN(salary)
	FROM employees
);

#案例4：查詢最低工資大於50號部門最低工資的部門id和其最低工資
#step1.50號部門的最低工資
SELECT MIN(salary)
FROM employees
WHERE department_id=50;

#step2.
SELECT department_id, MIN(salary)
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
	SELECT MIN(salary)
	FROM employees
	WHERE department_id=50
);

注意：非法使用標量子查詢的問題

SELECT department_id, MIN(salary)
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
	SELECT salary # 不是一行一列
	FROM employees
	WHERE department_id=50
);

2、列子查詢

多行操作符

• in/not in：相當於列表中的任意一個（in相當於=any；not in相當於<>all)
• any/some：和子查詢返回的某一個值比較，只要滿足其中一個就可以，經常可以被MIN或者MAX替代
• all：和子查詢返回的所有值比較

#案例1：返回location_id是1400或者1700的部門中的所有員工姓名
#step1.查詢location_id是1400或1700的部門編號
SELECT DISTINCT department_id
FROM departments
WHERE location_id = 1400
OR location_id = 1700;

#step2.查詢員工姓名，要求部門號是1列表中的某一個
SELECT last_name
FROM employees
WHERE department_id IN (
	SELECT DISTINCT department_id
	FROM departments
	WHERE location_id = 1400
	OR location_id = 1700
);

#案例2：返回其他工種中比job_id爲‘IT_PROG’工種任一工資低的員工的：工號、姓名、job_id 以及salary
#step1.
SELECT salary
FROM employees
WHERE job_id='IT_PROG';

#step2.
SELECT department_id, last_name, job_id, salary
FROM employees
WHERE salary < ANY(
	SELECT DISTINCT salary
	FROM employees
	WHERE job_id='IT_PROG'
)
AND job_id<>'IT_PROG';

案例2可以進行如下書寫代替

SELECT department_id, last_name, job_id, salary
FROM employees
WHERE salary < (
	SELECT MAX(salary)
	FROM employees
	WHERE job_id='IT_PROG'
)
AND job_id<>'IT_PROG';

#案例3：返回其他工種中比job_id爲'IT_PROG'工種所有工資都低的員工的工號、姓名、job_id、以及salary
SELECT department_id, last_name, job_id, salary
FROM employees
WHERE salary < ALL(
	SELECT DISTINCT salary
	FROM employees
	WHERE job_id='IT_PROG'
)
AND job_id<>'IT_PROG';

案例3可以進行如下書寫代替

SELECT department_id, last_name, job_id, salary
FROM employees
WHERE salary < (
	SELECT MIN(salary)
	FROM employees
	WHERE job_id='IT_PROG'
)
AND job_id<>'IT_PROG';

3、行子查詢

#案例：查詢員工編號最小並且工資最高的員工信息
#非行子查詢版本
#step1.
SELECT MIN(employee_id)
FROM employees;

#step2.
SELECT MAX(salary)
FROM employees;

#step3.
SELECT *
FROM employees
WHERE employee_id=(
	SELECT MIN(employee_id)
	FROM employees
)
AND salary=(
	SELECT MAX(salary)
	FROM employees
);

#行子查詢版本
SELECT *
FROM employees
WHERE (employee_id, salary)=(
	SELECT MIN(employee_id), MAX(salary)
	FROM employees
);

二、select後面

#案例1：查詢每個部門的員工個數
SELECT d.*, (
	SELECT COUNT(*)
	FROM employees e
	WHERE e.department_id = d.`department_id`
)
FROM departments d;

#案例2：查詢員工號爲102的部門名
SELECT (
	SELECT department_name
	FROM departments d
	INNER JOIN employees e
	ON d.department_id=e.department_id
	WHERE e.employee_id = 102
) 部門名;

三、from後面

將子查詢結果充當一張表，必須起別名

#案例：查詢每個部門的平均工資的工資等級
#step1.
SELECT AVG(salary), department_id
FROM employees
GROUP BY department_id;

#step2.連接1的結果集與job_grades表
SELECT ag_dep.*, g.`grade_level`
FROM (
	SELECT AVG(salary) ag, department_id
	FROM employees
	GROUP BY department_id
) ag_dep
INNER JOIN job_grades g
ON ag_dep.ag BETWEEN g.`lowest_sal` AND g.`highest_sal`;

四、exists後面

語法

exists（完整的查詢語句）

返回結果：1或者0

#案例1：查詢有員工名的部門名
SELECT department_name
FROM departments d
WHERE EXISTS(
	SELECT *
	FROM employees e
	WHERE d.`department_id`=e.`department_id`
);

#使用in代替exists
SELECT department_name
FROM departments d
WHERE d.`department_id` IN(
	SELECT department_id
	FROM employees
);

#案例2：查詢沒有女朋友的男生信息
#in
SELECT bo.*
FROM boys bo
WHERE bo.id NOT IN(
	SELECT boyfriend_id
	FROM beauty
);

#exists
SELECT bo.*
FROM boys bo
WHERE NOT EXISTS(
	SELECT boyfriend_id
	FROM beauty b
	WHERE bo.`id`=b.`boyfriend_id`
);