Table Join Query

If the DAO layer can solve the problem, the application layer will be much easier, so it is of great benefit to learn more SQL.

I've never been quite sure what a table connection is, it's actually quite an image, and another table is directly behind the previous table.

For example, select * Form TAB1 LEFT join tab2 on (tab1.size = tab2.size) where tab2.name= ' AAA ';

This creates a temporary intermediate table with the ON clause, and the table structure is as follows

Tab1.size tab1.name tab2.size Tab2.name

Then filter by the WHERE clause

Left connection means that the left table does not restrict, fully display, the right connection, the left side is the default is left outer connection.

Table Join Query