笛卡儿积

1. select * from t1,t2;

2. select * from t1 join t2;
   =
   select * from t1 inner join t2;

theta连接

1. select * from t1,t2 where t1.属性 θ t2.属性;

2. select * from t1 inner join t2 on t1.属性 θ t2.属性;

自然连接

普通自然连接

1. select * from t1 natural join t2;

2. select * from t1 inner join t2 using(public_field);
   =
   select * from t1 join t2 using(public_field);

自然左连接

1. select * from t1 natural left join t2;

2. select * from t1 left join t2 using(public_field);

自然右连接

1. select * from t1 natural right join t2;

2. select * from t1 r join t2 using(public_field);

mysql连接表