Add, delete, modify, check in mysql

Multi-table association query:

1. Natural query: the association conditions between multiple tables are connected using where;

2. Inner join: inner join...on, multiple tables The association conditions between multiple tables are used after on;

3. External left join: left join...on. The association conditions between multiple tables are used after on, and the data returned during query is the left join...on. All the data in the table and the data in the right table and the left table that have associated conditions;

4. External right join: right join...on, the association conditions between multiple tables are used after on, And the data returned during the query is to return all the data in the right table and the data in the left table and the right table that have associated conditions;

Subquery:

1. Nested query: Yes It means that a complete query statement is included in a query statement, and this statement can be run independently without errors. It is usually used after where;

2. Related query: It means that it is included in a query statement. There is a query statement, and this statement cannot be run independently. It needs to accept the value of the parent query as its own value, which is usually used in the field list;

3. Parent query and subquery: The external query is the parent A query contained within a parent query is called a subquery.

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