#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:一般搭配著單行操作符使用
#  單行操作符: >  >= = > !-
#  列子查詢，一般搭配著多行操作符使用
#  IN,ANY/SOME(任意),ALL
# ④子查詢的執行優先與主查詢執行,主查詢的條件用到了子查詢的結果。

#1.標量子查詢
#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,工資。
#①查141員工的job_id
SELECT job_id
FROM employees
WHERE employee_id='141';

#②查143員工的salary
SELECT salary
FROM employees
WHERE employee_id='143';

#③最后合并結果
SELECT CONCAT(last_name,first_name) AS 姓名,
job_id AS 工種編號,
salary AS 工資
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。
SELECT MIN(salary)
FROM employees;

SELECT
last_name AS 姓,
salary AS 工資,
job_id AS 工種編號
FROM employees
WHERE salary=(
 SELECT MIN(salary)
 FROM employees
     );

#案例4.查詢最低工資大于50號部門最低工資的部門id和其最低工資。
#①查50部門的最低工資
SELECT MIN(salary)
FROM employees
WHERE department_id=50;

#分組后，篩選條件①.【不用排除沒有部門的所以不篩選部門編號】
SELECT department_id AS 部門編號,
MIN(salary) AS 月薪
FROM employees
#WHERE department_id
GROUP BY department_id
HAVING 月薪>(
 SELECT MIN(salary)
 FROM employees
    );

#2.列子查詢(多行子查詢)
#返回多行
#使用多行比較操作符

#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列表的某一個
SELECT CONCAT(last_name,first_name) AS 姓名
FROM employees
WHERE department_id IN (
 SELECT DISTINCT department_id
 FROM departments
 WHERE location_id IN(1400,1700)
 );

用ANY替代IN與上面同樣的結果
SELECT CONCAT(last_name,first_name) AS 姓名
FROM employees
WHERE department_id = ANY(
 SELECT DISTINCT department_id
 FROM departments
 WHERE location_id IN(1400,1700)
 );

#案例.返回location_id不是1400或1700的部門中的所有員工姓名。
SELECT CONCAT(last_name,first_name) AS 姓名
FROM employees
WHERE department_id NOT IN(
 SELECT DISTINCT department_id
 FROM departments
 WHERE location_id IN(1400,1700)
);
==============================
SELECT CONCAT(last_name,first_name) AS 姓名
FROM employees
WHERE department_id > ALL(
 SELECT DISTINCT department_id
 FROM departments
 WHERE location_id IN(1400,1700)
);

#案例2.返回其他工種中比job_id為IT_PROG部門任意一工資低的員工工號,
# 姓名,job_id以及salary
#①把IT_PROG部門中的工資查出來
SELECT DISTINCT salary
FROM employees
WHERE job_id='IT_PROG';

#②把不是IT_PROG部門信息查出來
SELECT *
FROM employees
WHERE job_id != 'IT_PROG';

#③合并①與②在員工表中查出來
SELECT employee_id AS 員工編號,
CONCAT(last_name,first_name) AS 姓名,
job_id AS 工種編號,
salary AS 工資
FROM employees
WHERE job_id != 'IT_PROG'
AND salaryANY(
 SELECT salary
 FROM employees
 WHERE job_id='IT_PROG'
  );

用MAX代替ANY與上面同樣的效果
SELECT employee_id AS 員工編號,
CONCAT(last_name,first_name) AS 姓名,
job_id AS 工種編號,
salary AS 工資
FROM employees
WHERE job_id > 'IT_PROG'
AND salary(
 SELECT MAX(salary)
 FROM employees
 WHERE job_id='IT_PROG'
  );

#案例3.返回其他部門中比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。
#①先把IT_PROG部門的工資查出來。
SELECT DISTINCT salary
FROM employees
WHERE job_id='IT_PROG';

SELECT employee_id AS 員工號,
CONCAT(last_name,first_name) AS 姓名,
job_id AS 工種編號,
salary AS 工資
FROM employees
WHERE salaryALL(
 SELECT DISTINCT salary
 FROM employees
 WHERE job_id='IT_PROG'
)
 AND job_id > 'IT_PROG';
=============================
MIN替代ALL
SELECT employee_id AS 員工號,
CONCAT(last_name,first_name) AS 姓名,
job_id AS 工種編號,
salary AS 工資
FROM employees
WHERE salary(
　　SELECT MIN(salary)
　　FROM employees
　　WHERE job_id='IT_PROG'
)
 AND job_id > 'IT_PROG';

#3.行子查詢(結果集一行多列或者多行多列)
#案例1.查詢員工編號最小并且工資最高的員工信息.引入
SELECT MIN(employee_id)
FROM employees;
=================
SELECT MAX(salary)
FROM employees;

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

這種查詢結果使用虛擬字段，單行操作符必須一致可以使用。查出來與上面同樣的效果。
SELECT *
FROM employees
WHERE (employee_id,salary)=(
 SELECT MIN(employee_id),
 MAX(salary)
 FROM employees
       );

#二.SELECT子查詢
#僅僅支持標量子查詢，結果是一行一列
#案例1.查詢每個部門的員工個數
SELECT d.*,(SELECT COUNT(*) FROM employees)
FROM departments d; 

添加條件
SELECT d.*,(SELECT COUNT(*)
FROM employees e
WHERE e.department_id=d.department_id
) AS 個數
FROM departments d; 

#案例2.查詢員工號=102的部門名。
SELECT department_name
FROM departments;
==============
SELECT employee_id
FROM employees
WHERE employee_id = 102;

SELECT employee_id,
(
 SELECT department_name
 FROM departments d
 WHERE e.department_id=d.department_id
)
FROM employees e
WHERE employee_id=102;

#三.FROM 后面
注意: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等級。
SELECT ROUND(AVG(salary),2),department_id
FROM employees
GROUP BY department_id;

SELECT e.平均工資,j.grade_level
FROM job_grades AS j
,(
SELECT ROUND(AVG(salary),2) AS 平均工資,department_id
 FROM employees
 GROUP BY department_id
) AS e
WHERE e.平均工資 BETWEEN j.lowest_sal AND j.highest_sal;

#1999語法，老師答案
SELECT e.*,j.grade_level
FROM (
　　SELECT ROUND(AVG(salary),2) AS 平均工資,department_id
　　FROM employees
　　GROUP BY department_id
) AS e
INNER JOIN job_grades j
ON e.平均工資 BETWEEN j.lowest_sal AND j.highest_sal;

#四.EXISTS后面(相關子查詢)
語法:EXISTS(完整的查詢語句)
備注:完整的查詢語句可以是一行一列，可以使一行多列
注意:先走外查詢，然后根據某個字段的值再去過濾
EXISTS 判斷(布爾類型)值存不存在,結果只有兩種:1有，0沒有
#引入
SELECT EXISTS(SELECT employee_id FROM employees);

查詢工資3W的員工信息
SELECT EXISTS(SELECT * FROM employees WHERE salary=30000);

#案例引入.查詢員工名和部門名
#查員工名與部門編號
SELECT first_name,department_id
FROM employees
WHERE department_id;

#查部門名
SELECT department_name
FROM departments;

#查員工名與部門名
SELECT e.first_name,d.department_name
FROM employees e
INNER JOIN (  SELECT department_name,department_id
 FROM departments
) AS d
ON e.department_id=d.department_id;

#案例1..查有員工的部門名
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，同樣是上面的結果
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信息
#IN方法
SELECT *
FROM boys bo
WHERE bo.id NOT IN(
 SELECT boyfriend_id
 FROM beauty be
);
===============
#EXISTS方法
SELECT *
FROM boys bo
WHERE NOT EXISTS(
 SELECT boyfriend_id
 FROM beauty be
 WHERE bo.id=be.boyfriend_id
);

進階9:聯合查詢
UNION 聯合 合并:將多條查詢語句的結果合并成一個結果。
語法:
 查詢語句1
 UNION
 查詢語句2
 UNION
 ...

應用場景:
要查詢的結果來自于多個表，且多個表沒有直接的連接關系，
但查詢信息一致時。
網頁搜索內容，內容從不同的表中檢索聯合起來返回給用戶。

特點:
1.要求多條查詢語句的查詢列數是一致的。
2.要求多條查詢語句的查詢的每一列的類型和順序最好一致。
3.使用UNION關鍵字默認去重，如果使用UNION ALL全部展示，包含重復項