mysql的子查詢詳解

分類：
按子查詢出現的位置：
    select后面：
        僅僅支持標量子查詢
    
    from后面：
        支持表子查詢
    where或having后面：★
        標量子查詢（單行） √
        列子查詢  （多行） √
        
        行子查詢
        
    exists后面（相關子查詢）
        表子查詢
按結果集的行列數不同：
    標量子查詢（結果集只有一行一列）
    列子查詢（結果集只有一列多行）
    行子查詢（結果集有一行多列）
    表子查詢（結果集一般為多行多列）

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#一、where或having后面
/*
1、標量子查詢（單行子查詢）
2、列子查詢（多行子查詢）

3、行子查詢（多列多行）

特點：
①子查詢放在小括號內
②子查詢一般放在條件的右側
③標量子查詢，一般搭配着單行操作符使用
> < >= <= = <>

列子查詢，一般搭配着多行操作符使用
in、any/some、all  <>all =not in  any=in  any(等於其中的一個)  all(所有滿足)

④子查詢的執行優先於主查詢執行，主查詢的條件用到了子查詢的結果

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 #1.標量子查詢★

#案例1：誰的工資比 Abel 高?

#①查詢Abel的工資

SELECT salary
FROM employees
WHERE last_name = 'Abel'

#②查詢員工的信息，滿足 salary>①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

#①查詢141號員工的job_id

SELECT job_id
FROM employees
WHERE employee_id = 141

#②查詢143號員工的salary

SELECT salary
FROM employees
WHERE employee_id = 143

#③查詢員工的姓名，job_id 和工資，要求job_id=①並且salary>②

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和salary

#①查詢公司的 最低工資

SELECT MIN(salary)
FROM employees

#②查詢last_name,job_id和salary，要求salary=①

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

#①查詢50號部門的最低工資

SELECT  MIN(salary)
FROM employees
WHERE department_id = 50

#②查詢每個部門的最低工資

SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id

#③ 在②基礎上篩選，滿足min(salary)>①

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

#2.列子查詢（多行子查詢）★

#案例1：返回location_id是1400或1700的部門中的所有員工姓名

#①查詢location_id是1400或1700的部門編號

SELECT DISTINCT department_id
FROM departments
WHERE location_id IN(1400,1700)

#②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  <>ALL(
    SELECT DISTINCT department_id
    FROM departments
    WHERE location_id IN(1400,1700)
);

#案例2：返回其它工種中比job_id為‘IT_PROG’工種任一工資低的員工的員工號、姓名、job_id 以及salary

#①查詢job_id為‘IT_PROG’部門任一工資

SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'

#②查詢員工號、姓名、job_id 以及salary，salary<(①)的任意一個

SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<ANY(
    SELECT DISTINCT salary
    FROM employees
    WHERE job_id = 'IT_PROG'
 
) AND job_id<>'IT_PROG';

#或

SELECT last_name,employee_id,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：返回其它部門中比job_id為‘IT_PROG’部門所有工資都低的員工   的員工號、姓名、job_id 以及salary

SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<ALL(
    SELECT DISTINCT salary
    FROM employees
    WHERE job_id = 'IT_PROG'
 
) AND job_id<>'IT_PROG';

#或

SELECT last_name,employee_id,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（結果集一行多列或多行多列）

#案例：查詢員工編號最小並且工資最高的員工信息

SELECT * 
FROM employees
WHERE (employee_id,salary)=(
    SELECT MIN(employee_id),MAX(salary)
    FROM employees
);

#①查詢最小的員工編號

SELECT MIN(employee_id)
FROM employees

#②查詢最高工資

SELECT MAX(salary)
FROM employees

#③查詢員工信息

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

#二、select后面
/*
僅僅支持標量子查詢
*/
#案例：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`
 ) 個數
 FROM departments d;

#案例2：查詢員工號=102的部門名

SELECT (
    SELECT department_name,e.department_id
    FROM departments d
    INNER JOIN employees e
    ON d.department_id=e.department_id
    WHERE e.employee_id=102
    
) 部門名;

#三、from后面
/*
將子查詢結果充當一張表，要求必須起別名
*/

#案例：查詢每個部門的平均工資的工資等級
#①查詢每個部門的平均工資

SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id

SELECT * FROM job_grades;

#②連接①的結果集和job_grades表，篩選條件平均工資 between lowest_sal and highest_sal

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 lowest_sal AND highest_sal;

#四、exists后面（相關子查詢）

/*
語法：  
exists(完整的查詢語句)  外查詢先執行
結果：
1或0

*/

SELECT EXISTS(SELECT employee_id FROM employees WHERE salary=300000);

#案例1：查詢有員工的部門名#in

SELECT department_name
FROM departments d
WHERE d.`department_id` IN(
    SELECT department_id
    FROM employees
 
)

#exists

SELECT department_name
FROM departments d
WHERE EXISTS(
    SELECT *
    FROM employees e
    WHERE d.`department_id`=e.`department_id`
);

#案例2：查詢沒有女朋友的男神信息

#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`
);

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