MySql的回顾六:子查询(内查询)「终于解决」

#WHERE或HAVING后面 #1.标量子查询(单行子查询) #2.列子查询(多行子查询) #3.行子查询(多列多行) #特点: # ①子查询放在小括号内 # ②子查询一般放在条件的右侧 # ③标量子查询:一般搭配着单行操作符使用 # 单行操作符: > < >=  <= <> !- # 列子查询，一般搭配着多行操作符使用 # IN,ANY/SOME(任意),ALL # ④子查询的执行优先与主查询执行,主查询的条件用到了子查询的结果。

代码100分

代码100分#1.标量子查询 #案例1:谁的工资比Abel高？ #①查询Abel的工资 SELECT salary FROM employees WHERE last_name = "Abel";

#②查询员工的信息，满足Salary>①结果 SELECT * FROM employees WHERE salary>(SELECT salary FROM employees WHERE last_name="Abel");

代码100分#案例2.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。 #①查141员工的job_id SELECT job_id FROM employees WHERE employee_id="141";

#②查143员工的salary SELECT salary FROM employees WHERE employee_id="143";

#③最后合并结果 SELECT CONCAT(last_name,first_name) AS 姓名, job_id AS 工种编号, salary AS 工资 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.返回公司工资最少的员工的last_name,job_id和salary。
SELECT MIN(salary) FROM employees;

SELECT
last_name AS 姓,
salary AS 工资,
job_id AS 工种编号
FROM employees
WHERE salary=(
    SELECT MIN(salary)
    FROM employees
                 );

#案例4.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。 #①查50部门的最低工资 SELECT MIN(salary) FROM employees WHERE department_id=50;

#分组后，筛选条件①.【不用排除没有部门的所以不筛选部门编号】 SELECT department_id AS 部门编号, MIN(salary) AS 月薪 FROM employees #WHERE department_id GROUP BY department_id HAVING 月薪>( SELECT MIN(salary) FROM employees );

#2.列子查询(多行子查询) #返回多行 #使用多行比较操作符

#案例1.返回location_id是1400或1700的部门中的所有员工姓名。 #①查询location_id是1400或1700的部门编号 SELECT DISTINCT department_id FROM departments WHERE location_id IN(1400,1700);

#②查询员工姓名，要求部门号是①列表的某一个 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与上面同样的结果 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的部门中的所有员工姓名。
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.返回其他工种中比job_id为IT_PROG部门任意一工资低的员工工号, # 姓名,job_id以及salary #①把IT_PROG部门中的工资查出来 SELECT DISTINCT salary FROM employees WHERE job_id="IT_PROG";

#②把不是IT_PROG部门信息查出来 SELECT * FROM employees WHERE job_id != "IT_PROG";

#③合并①与②在员工表中查出来 SELECT employee_id AS 员工编号, CONCAT(last_name,first_name) AS 姓名, job_id AS 工种编号, salary AS 工资 FROM employees WHERE job_id != "IT_PROG" AND salary<ANY( SELECT salary FROM employees WHERE job_id="IT_PROG" );

用MAX代替ANY与上面同样的效果 SELECT employee_id AS 员工编号, CONCAT(last_name,first_name) AS 姓名, job_id AS 工种编号, salary AS 工资 FROM employees WHERE job_id <> "IT_PROG" AND salary<( SELECT MAX(salary) FROM employees WHERE job_id="IT_PROG" );

#案例3.返回其他部门中比job_id为‘IT_PROG’部门所有工资都低的员工 #的员工号，姓名，job_id以及salary。 #①先把IT_PROG部门的工资查出来。 SELECT DISTINCT salary FROM employees WHERE job_id="IT_PROG";

SELECT employee_id AS 员工号, CONCAT(last_name,first_name) AS 姓名, job_id AS 工种编号, salary AS 工资 FROM employees WHERE salary<ALL( SELECT DISTINCT salary FROM employees WHERE job_id="IT_PROG" ) AND job_id <> "IT_PROG"; ============================= MIN替代ALL SELECT employee_id AS 员工号, CONCAT(last_name,first_name) AS 姓名, job_id AS 工种编号, salary AS 工资 FROM employees WHERE salary<( 　　SELECT MIN(salary) 　　FROM employees 　　WHERE job_id="IT_PROG" ) AND job_id <> "IT_PROG";

#3.行子查询(结果集一行多列或者多行多列) #案例1.查询员工编号最小并且工资最高的员工信息.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
                 );

这种查询结果使用虚拟字段，单行操作符必须一致可以使用。查出来与上面同样的效果。
SELECT *
FROM employees
WHERE (employee_id,salary)=(
    SELECT MIN(employee_id),
    MAX(salary)
    FROM employees
                           );

#二.SELECT子查询 #仅仅支持标量子查询，结果是一行一列 #案例1.查询每个部门的员工个数 SELECT d.*,(SELECT COUNT(*) FROM employees) FROM departments d; 

添加条件 SELECT d.*,(SELECT COUNT(*) FROM employees e WHERE e.department_id=d.department_id ) AS 个数 FROM departments d; 

#案例2.查询员工号=102的部门名。 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:将子查询结果充当一张表，要求必须起别名 #案例:查询每个部门的平均工资等级。 SELECT ROUND(AVG(salary),2),department_id FROM employees GROUP BY department_id;

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

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

#四.EXISTS后面(相关子查询)
语法:EXISTS(完整的查询语句)
备注:完整的查询语句可以是一行一列，可以使一行多列
注意:先走外查询，然后根据某个字段的值再去过滤
EXISTS 判断(布尔类型)值存不存在,结果只有两种:1有，0没有
#引入
SELECT EXISTS(SELECT employee_id FROM employees);

查询工资3W的员工信息 SELECT EXISTS(SELECT * FROM employees WHERE salary=30000);

#案例引入.查询员工名和部门名
#查员工名与部门编号
SELECT first_name,department_id FROM employees WHERE department_id;

#查部门名
SELECT department_name FROM departments;

#查员工名与部门名 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..查有员工的部门名 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，同样是上面的结果
SELECT department_name FROM departments d WHERE d.department_id IN( SELECT department_id FROM employees );

#案例2.查询没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
);

进阶9:联合查询
UNION 联合 合并:将多条查询语句的结果合并成一个结果。
语法:
    查询语句1
    UNION
    查询语句2
    UNION
    ...

应用场景:
要查询的结果来自于多个表，且多个表没有直接的连接关系，
但查询信息一致时。
网页搜索内容，内容从不同的表中检索联合起来返回给用户。

特点:
1.要求多条查询语句的查询列数是一致的。
2.要求多条查询语句的查询的每一列的类型和顺序最好一致。
3.使用UNION关键字默认去重，如果使用UNION ALL全部展示，包含重复项

#引入案例1.:查询部门编号>90或者邮箱包含A的员工信息 SELECT * FROM employees WHERE email LIKE "%a%" OR department_id>90; 联合查询 SELECT * FROM employees WHERE email LIKE "%a%" UNION SELECT * FROM employees WHERE department_id>90;