What does mysql Chinese and foreign links mean?

Mysql Chinese and foreign joins refer to using comparison operators to compare data in two tables. The join results not only include rows that meet the connection conditions, but also include rows that do not meet the conditions themselves. Rows that do not meet the conditions The field is displayed as null; outer joins are divided into left outer joins and right outer joins.

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Internal connection (Inner Join), also known as simple join or natural join, is a common join query. Inner joins use comparison operators to compare data in two tables, and list the data rows that match the join conditions and combine them into new records. That is to say, in an inner join query, only records that meet the conditions can appear in the query results.

In an inner join query, the returned result only contains data that meets the query conditions and connection conditions. However, sometimes it is also necessary to include unrelated data, that is, the returned result not only contains data that meets the conditions, but also contains the left All the data in the table (left join, left outer join) or right table (right join or right outer join) or both tables (full outer join), then you need to use outer join query.

Mysql Chinese and foreign connections are divided into left outer connections and right outer connections; full outer connections are not supported.

Outer join The syntax format of outer join is as follows:

select 所查字段 from 表1 left | right [outer] join 表2 on 表1.关系字段=表2.关系字段 where 条件包括左外连接、右外连接和全外连接。

Note the use of left join, right join The table to the left of the keyword is called the left table, and the table to the right of the keyword is called the right table .

1. Left outer join

Take the table on the left as the base table and display all rows and columns of the base table. If the conditions in the outer table do not match, all rows and columns in the outer table will be displayed. The field is displayed as null

select * from TestA a left join TestB b on a.id=b.id;

2. Right outer join

takes the table on the right as the base table and displays All rows and columns of the base table and the outer table. If the conditions do not match, all fields in the outer table will be displayed as null

select * from TestA a right join TestB b on a.id=b.id;

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