MySQL composite query

As we explained earlier mysql Table query is to query a table , In actual development, we often need to query multiple tables .

Basic query review

1. Check that the salary is higher than 500 Or position is MANAGER Of employees , At the same time, their initials should be capitalized J

2. Sort by department number in ascending order and employee's salary in descending order

3. Use annual salary to sort in descending order

4. Displays the name and position of the highest paid employee

5. Displays information about employees whose wages are higher than the average

6. Displays the average wage and maximum wage for each department

7. It shows that the average wage is lower than 2000 Your department number and its average salary

8. Displays the total number of employees for each position , Average wage

Multi-table query

In actual development, data often comes from different tables , Need multi table query .

Case a ： Show employee name 、 The employee's salary and the name of the Department

Because the above data comes from EMP and DEPT surface , Therefore, joint query

In fact, we only need emp In the table deptno = dept In the table deptno Field records

Case 2 ： The display department number is 10 The name of your department , Employee name and salary

Case three ： Show the name of each employee , Wages , And salary level

Self join

Self join refers to joining queries in the same table

example ：

Show employees FORD The number and name of the superior （mgr Is the employee leader's number –empno）

• Subquery used

• Use multi table query （ Self query ）

Subquery

Subqueries are embedded in other sql Statement select sentence , Also called nested query

Single line sub query

A single row subquery is a subquery that returns one row of records

Example ：

Show SMITH Employees in the same department

Multi line sub query

Multi row subquery is a subquery that returns multi row records

1. in keyword

Query and 10 The name of the employee with the same position in department No , Position , Wages , Department number , But it doesn't include 10 Their own

2. all keyword

Show salary ratio Department 30 The names of all the employees with high salaries 、 Salary and department number

3. any keyword

Show salary ratio Department 30 Name of any employee with a higher salary 、 Salary and department number （ Including employees in their own department ）

Multi column subquery

A single row subquery means that a subquery returns only a single column , Single line data ; Multi row sub query is to return single column and multi row data , It's all about single columns , Multi column subquery refers to the subquery statement that returns multiple column data

example ： Query and SMITH All employees in the same department and position , Not included SMITH I am

stay from Clause using subqueries

Subquery statements appear in from clause . Here we need to use the technique of data query , Use a subquery as a temporary table .

Case study ：

• Displays the name of each employee whose salary is higher than the average salary of his / her department 、 department 、 Wages 、 Average wage

• Find the name of the highest paid person in each department 、 Wages 、 department 、 Maximum wage

• Display information for each department （ Department name , Number , Address ） And the number of people

1. Use multiple tables

2. Use subquery

Merge query

in application , In order to merge multiple select The results of the implementation of , You can use the set operator union,union all

union

This operator is used to get the union of two result sets . When using this operator , Duplicate lines in the result set will be automatically removed .

Case study ： Pay more than 2500 Or position is MANAGER Find out

union all

This operator is used to get the union of two result sets . When using this operator , Duplicate rows in the result set are not removed .

Case study ： Pay more than 25000 Or position is MANAGER Find out