MySQL federated query (multi table query)

One 、 Internal connection

select *from surface 1 [ inner | cross ] join surface 2 [ on Filter conditions ] [ where Query criteria ] ;

[ inner | cross ]:

join Internal connection key ( There must be );

on Grammatical omission , But if you omit it , What will be queried is the Cartesian product of multiple tables ;

1.select *from surface 1 join surface 2 [ on Filter conditions ] [ where Query criteria ] ;

2.select *from surface 1, surface 2 [ where Query criteria ] ;

3.select *from surface 1 inner join surface 2 [ on Filter conditions ] [ where Query criteria ] ;

4.select *from surface 1 cross join surface 2 [ on Filter conditions ] [ where Query criteria ] ;

（1.2 More use ; Multi table queries use 1 good ）

example 1