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#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。

#1.标量子查询#案例1:谁的工资比Abel高？#①查询Abel的工资SELECT salaryFROM employeesWHERE last_name = 'Abel';

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

#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_idSELECT job_idFROM employeesWHERE employee_id='141';

#②查143员工的salarySELECT salaryFROM employeesWHERE 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 employeesWHERE 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;

SELECTlast_name AS 姓,salary AS 工资,job_id AS 工种编号FROM employeesWHERE 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 employeesWHERE 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_idGROUP BY department_idHAVING 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_idFROM departmentsWHERE 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 employeesWHERE 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 employeesWHERE 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 employeesWHERE 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 employeesWHERE 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 salaryFROM employeesWHERE job_id='IT_PROG';

#②把不是IT_PROG部门信息查出来SELECT *FROM employeesWHERE 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 employeesWHERE 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 employeesWHERE 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 salaryFROM employeesWHERE 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 employeesWHERE salary<ALL( SELECT DISTINCT salary FROM employees WHERE job_id='IT_PROG') AND job_id <> 'IT_PROG';=============================MIN替代ALLSELECT 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 employeesWHERE 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 employeesWHERE 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 employeesWHERE (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 eWHERE 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_nameFROM departments;==============SELECT employee_idFROM employeesWHERE employee_id = 102;

SELECT employee_id,( SELECT department_name FROM departments d WHERE e.department_id=d.department_id)FROM employees eWHERE 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_idFROM employeesGROUP BY department_id;

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

#1999语法，老师答案SELECT e.*,j.grade_levelFROM (　　SELECT ROUND(AVG(salary),2) AS 平均工资,department_id　　FROM employees　　GROUP BY department_id) AS eINNER JOIN job_grades jON 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_idFROM employeesWHERE department_id;

#查部门名SELECT department_nameFROM departments;

#查员工名与部门名SELECT e.first_name,d.department_nameFROM employees eINNER JOIN (  SELECT department_name,department_id FROM departments) AS dON e.department_id=d.department_id;

#案例1..查有员工的部门名SELECT department_nameFROM departments dWHERE 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_nameFROM departments dWHERE 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 boWHERE bo.id NOT IN( SELECT boyfriend_id FROM beauty be);===============#EXISTS方法SELECT *FROM boys boWHERE 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

感谢能认真读到这里的伙伴们，MySQL查询部分结束，相信屏幕前的你照着我博客里的模板可以完成一些简单的SQL查询语句，SQL既然学了，以后还是要多练习一下，SQL1992与1999语法在主流的关系型数据库都是通用的。后续我会继续进行对MySql的知识进行扩展，感兴趣的同志互相关注一呗！o(^▽^)o

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