Mysql--5 Sub-Query and connection summary

Subquery: A select that appears in other SQL statements must appear inside the parentheses, the outer layer of the subquery can be a common SELECT statement, and the INSERT statement UPDATE statement DELETE statement can contain multiple keywords and conditions in the subquery (for example, ORDER by, LIMIT), and related functions.

3 ways to raise a subquery: comparison operator, in or not in, EXISTS, or not EXISTS

Connections: SELECT statements in MySQL, multi-table updates, and multiple table deletions can all use connections.

The connection is divided into inner join inner JOIN outer JOIN outer JOIN.

Inner joins: Displays records in both the left and right tables that meet the join criteria.

Outer joins: Left outer (all records in the left table and records with join conditions in the right table) right (all records in the right table and records in the left table that match the join criteria)

Mysql--5 Sub-Query and connection summary