MySQL_Sql_打怪升級_進階篇_進階7:子查詢
阿新 • • 發佈:2020-09-07
進階7:子查詢
含義:
出現在其他語句中的select語句(或者是增刪改),稱為子查詢或內查詢
外部的查詢語句,稱為主查詢或外查詢
分類:
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 (結果集一行多列或多行多列)
表子查詢 (結果集一般為多行多列)
按子查詢出現的位置:
select後面: 僅僅支援標量子查詢(單行) from後面: 支援表子查詢 where或having後面:★ 標量子查詢(單行) √ 列子查詢 (多行) √ 行子查詢 (多列多行) exists後面(相關子查詢) 表子查詢
一、where或having後面
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(結果集一行多列或多行多列)
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,一般搭配著單行操作符使用 > < >= <= = <> 4、列子查詢,一般搭配著多行操作符使用 in/not in、any/some、all in / not in 等於列表中的任意一個值 any/some 和子查詢返回的某一個值做比較 (any可以用max |some可以用min) all 和子查詢返回的所有值做比較 5、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
1.標量子查詢★
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查詢員工的資訊,滿足 salary>①結果
SELECT last_name,salary
FROM employees
WHERE salary>(
SELECT salary
FROM employees
WHERE last_name = 'Abel'
);
單行子查詢
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
);
分組函式
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
);
子查詢中的 HAVING 子句
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
);
非法使用標量子查詢
案例1.查詢的結果集不是一行一列的情況;例如 > 單行操作符只可以搭配使用標量子查詢
SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
SELECT salary
FROM employees
WHERE department_id = 50
);
#說明:
#SELECT salary --->屬於列子查詢(一列多行)
案例2.返回空值的情況; 例如 department_id 根本就沒有2500號ID
SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
SELECT salary
FROM employees
WHERE department_id = 2500
);
2.列子查詢(多行子查詢)★
特點:
可以將單行的操作放到多行的子查詢中使用
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列表中的所有
SELECT last_name
FROM employees
WHERE department_id IN (
SELECT DISTINCT department_id
FROM departments
WHERE location_id IN(1400,1700)
);
或者
SELECT last_name
FROM employees
WHERE department_id =ANY (
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';
或者
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';
或者
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)
特點:
查詢的條件必須是單行操作符可以連線的
案例:查詢員工編號最小並且工資最高的員工資訊
SELECT *
FROM employees
WHERE (employee_id,salary)=(
SELECT MIN(employee_id),MAX(salary)
FROM employees
);
使用之前學過的也可以實現
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查詢員工資訊
SELECT *
FROM employees
WHERE employee_id=(
SELECT MIN(employee_id)
FROM employees
)AND salary=(
SELECT MAX(salary)
FROM employees
);
二、select後面
特點:
僅僅支援標量子查詢
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`
) 個數
FROM departments d;
案例2:查詢員工號=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
特點:
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連線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後面(相關子查詢)
語法:
exists(完整的查詢語句)
結果:
布林值1或0
注意:exists 執行的順序是先查外查詢再查子查詢
引入
SELECT EXISTS(SELECT employee_id FROM employees WHERE salary=300000);
SELECT EXISTS(SELECT employee_id FROM employees);
案例1:查詢有員工的部門名
exists 實現查詢
SELECT department_name
FROM departments d
WHERE EXISTS(
SELECT *
FROM employees e
WHERE d.`department_id`=e.`department_id`
);
in 多行實現查詢
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神資訊
in實現查詢
SELECT bo.*
FROM boys bo
WHERE bo.id NOT IN(
SELECT boyfriend_id
FROM beauty
);
exists 實現查詢
SELECT bo.*
FROM boys bo
WHERE NOT EXISTS(
SELECT boyfriend_id
FROM beauty b
WHERE bo.`id`=b.`boyfriend_id`
);