A brief discussion on using Cartesian product principle to query multiple tables in MySQL

MySQL multi-table query (Cartesian product principle)

1. First determine which tables the data will use.
2. Convert multiple tables into one table through Cartesian product.
3. Then remove the illogical data (remove it according to the relationship between the two tables).
4. Finally, just add conditions as if it were a virtual table.

Note: It is best to use table aliases to distinguish column names.

Cartesian Product

Demo:

Left, right, inner, outer joins

Inner join:

Important: All matching records are returned.

select * from a,b where ax = bx ////Inner join

l There are two types of outer joins: left join and right join.

Key points: All matching records are returned plus one record for each row where the primary table foreign key value is null. All columns of the auxiliary table are null values.

select * from a left join b on ax=bx order by ax //Left outer join or left join select * from a right join b on ax=bx order by ax //Right outer join or right join

Select clause order

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