How to connect tables in mysql

In the database, the select statement can not only query the information in the table, but also can be used to query the information of multiple tables and return the queried information. Let's learn with the editor.

Before starting this article, we first create two data tables (employee and employee_record),employeeThe data table stores the employee's ID number, name, salary and department where the employee is located. And employee_record stores the employee’s previous work experience. The following is the data in the table. The operation will be based on these two tables.

Inner join

Select matching data from multiple tables and output it.

Syntax:

（select 表.字段,....  from 表1名,表2名,...  where [匹配的条件比如 表1.字段=表2.字段];

Among them:

• Table: refers to the data table that needs to be operated , for example, here are employee and employee_record;

• field: refers to the field name that needs to be operated.

If you find it difficult to read the words, let’s take a look at an example:

select The statement can give an alias to the field! Just write it directly after the field that needs to be displayed in the query. You can also give an alias to the table.

Outer connection

External link not only selects matching records in multiple tables, but also queries other unmatched records.

Left join

contains all records in the left table (including no matching records in the right table), the so-called left table It is distinguished by left join. The left table on the left side of left join is the left table, and the right table is on the right side.

Grammar:

select * from 表1 left join 表2 on 表1.字段=表2.字段;

We can see that although there is no corresponding one on the right side of the following lines, it is still displayed. .

Right join

Contains all records in the right table (including no matching records in the left table). The syntax of right join is very similar to that of left join, except for individual words.

Syntax:

select * from 表1 right join 表2 on 表1.字段=表2.字段;

You can see that the display result of the right connection is exactly opposite to that of the left connection.

Note: Left join and right join can be converted to each other.

Free learning recommendation: mysql video tutorial

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