"Get started quickly with MySQL"-⑨-Compound query

1. Basic query review

• Query employees whose salary is higher than 500 or whose position is MANAGER, and their initials must be capital J;

select * from emp where (sal>500 or job='MANGER') and ename like 'J%';

• Sort by department number in ascending order and employee salary in descending order;

select * from emp order by deptno, sal desc;

• Sort in descending order using annual salary;

select ename, sal*12+ifnull(comm, 0) as '年薪' from emp order by 年薪 desc;

• Display the name and job title of the highest paid employee;

select ename, job from emp where sal = (select max(sal) from emp);

• Display information about employees whose salaries are higher than the average salary;

select ename, sal from emp where sal > (select avg(sal) from emp);

• Shows the average and maximum salary for each department;

select deptno, format(avg(sal), 2), max(sal) from emp group by deptno;

• Display the department number with an average salary below 2000 and its average salary;

select deptno, avg(sal) as avg_sal from emp group by deptno having avg_sal<2000;

• Shows the total number of employees in each position and the average salary

select job, count(*), format(avg(sal), 2) from emp group by job;

2.Multiple table query

In actual development, data often comes from different tables, so multi-table queries are required. In this section, we use a simple company management system with three tables EMP, DEPT, and SALGRADE to demonstrate how to perform multi-table queries.

Case

• Display the employee name, employee salary and the name of the department. Because the above data comes from the EMP and DEPT tables, it needs to be queried jointly;

select emp.ename, emp.sal, dept.dname from emp, dept;

• But in fact, we only need the deptno field record in the emp table = the deptno field record in the dept table;

select emp.ename, emp.sal, dept.dname from emp, dept where emp.deptno = dept.deptno;

• Display the department name, employee name and salary of department number 10;

select emp.ename, emp.sal, dept.dname from emp, dept where emp.deptno = dept.deptno and dept.deptno = 10;

• Display the name, salary, and salary level of each employee;

select ename, sal, grade from emp, salgrade where  emp.sal between losal and hisal;

3.Self link

• 自连接是指在同一张表连接查询；

Case

• Display the number and name of employee FORD’s superior leader (mgr is the employee leader’s number – empno);

Use subquery

select empno, ename from emp where emp.empno =  (select mgr from emp where ename = 'FORD');

Use multi-table query (self-query)

• Use the alias of the table, from emp leader, emp worker, to give your own table an alias. Because you need to do it first笛卡尔积, the alias can be recognized first;

select leader.empno, leader.ename from emp leader, emp worker 
    -> where leader.empno = worker.mgr and worker.ename = 'FORD';

4. Subquery

• 子查询是指嵌入在其他sql语句中的select语句，也叫嵌套查询；

4.1 Single row subquery

• 单行子查询即返回一行记录的子查询；

Case

• Display employees in the same department of SMITH;

select * from emp where deptno = (select deptno from emp where ename='smith');

4.2 Multi-row subquery

• 即返回多行记录的子查询；

Case

• in关键字;Query the name, position, salary, and department number of employees who have the same job position as department No. 10, but do not include 10 themselves;

select ename, job, sal, deptno from emp where job in
    -> (select distinct job from emp where deptno = 10) and deptno <> 10;

• all关键字;Display the name, salary and department number of employees whose salary is higher than that of all employees in department 30;

select ename, sal, deptno from emp where sal > all(
    -> select sal from emp where deptno = 30);

• any关键字;Display the name, salary and department number of employees whose salary is higher than that of any employee in department 30 (including employees in their own department);

select ename, sal, deptno from emp where sal > any(
    -> select sal from emp where deptno = 30);

4.3 Multi-column subquery

• 单行子查询是指子查询只返回单列，单行数据；多行子查询是指返回单列多行数据，都是针对单列而言的，而多列子查询则是指查询返回多个列数据的子查询语句；

Case

• Query all employees with the same department and position as SMITH, excluding SMITH himself;

select ename from emp where (deptno, job) = (
    -> select deptno, job from emp where ename = 'SMITH') and ename <> 'SMITH';

4.4 Using subqueries in the from clause

• 子查询语句出现在from子句中。这里要用到数据查询的技巧，把一个子查询当做一个临时表使用；

Case

• Display the name, department, salary, and average salary of each employee whose salary is higher than the average salary of his or her department;

select ename, deptno, sal, format(asal, 2) from emp,
    -> (select avg(sal) asal, deptno dt from emp group by deptno) tmp
    -> where emp.sal > tmp.asal and emp.deptno = tmp.dt;

• Find the name, salary, department, and maximum salary of the person with the highest salary in each department;

select emp.ename, emp.sal, emp.deptno, ms from emp,
    -> (select max(sal) ms, deptno from emp group by deptno) tmp
    -> where emp.deptno = tmp.deptno and emp.sal = tmp.ms;

• Display the information of each department (department name, number, address) and number of personnel;

• Method 1: Use multiple tables;

select dept.dname, dept.deptno, dept.loc, count(*) '部门人数' from emp,
    -> dept where emp.deptno = dept.deptno group by dept.deptno, dept.dname, dept.loc;

• Method 2: Use subquery;

select dept.deptno, dname, mycnt, loc from dept,
    -> (select count(*) mycnt, deptno from emp group by deptno) tmp
    -> where dept.deptno = tmp.deptno;

5. Combined queries

• 在实际应用中，为了合并多个select的执行结果，可以使用集合操作符 union，union all；

5.1 union

• 该操作符用于取得两个结果集的并集。当使用该操作符时，会自动去掉结果集中的重复行;

Case

• Find people whose salary is greater than 2,500 or whose position is MANAGER;

select ename, sal, job from emp where sal > 2500 union 
    -> select ename, sal, job from emp where job = 'MANGER';

5.2 union all

• 该操作符用于取得两个结果集的并集。当使用该操作符时，不会去掉结果集中的重复行；

Case

• Find people whose salary is greater than 25,000 or whose position is MANAGER

select ename, sal, job from emp where sal > 2500 union 
    -> select ename, sal, job from emp where job = 'MANGER';

6. Inner and outer joins of tables

• Table connections are divided into internal connections and external connections;

6.1 Inner joins

• 内连接实际上就是利用where子句对两种表形成的笛卡儿积进行筛选，我们前面学习的查询都是内连接，也是在开发过程中使用的最多的连接查询；

grammar

select 字段 from 表1 inner join 表2 on 连接条件 and 其他条件;

Case

• Display SMITH’s name and department name;

-- 用前面的写法
select ename, dname from emp, dept where 
    -> emp.deptno = dept.deptno and ename = 'SMITH';

-- 用标准的内连接写法
select ename, dname from emp inner join dept on
    -> emp.deptno = dept.deptno and ename = 'SMITH';

6.2 Outer joins

• Outer joins are divided into left outer joins and right outer joins;

6.2.1 Left outer join

• 如果联合查询，左侧的表完全显示我们就说是左外连接；

grammar

select 字段名  from 表名1 left join 表名2 on 连接条件

Case

-- 建两张表
create table stu (id int, name varchar(30)); -- 学生表
insert into stu values(1,'jack'),(2,'tom'),(3,'kity'),(4,'nono');
create table exam (id int, grade int); -- 成绩表
insert into exam values(1, 56),(2,76),(11, 8);

• Query the scores of all students. If the student has no scores, the student's personal information will also be displayed;

-- 当左边表和右边表没有匹配时，也会显示左边表的数据
select * from stu left join exam on stu.id = exam.id;

6.2.2 Right outer join

• 如果联合查询，右侧的表完全显示我们就说是右外连接；

grammar

select 字段 from 表名1 right join 表名2  on 连接条件

Case

• Jointly query the Stu table and the Exam table to display all scores, even if there is no student corresponding to this score, it must be displayed;

select * from stu right join exam on stu.id=exam.id;