联表查询

drop table if exists t1; CREATE table t1(id int not null,name VARCHAR(20)); insert into t1 values (1,'t1a'); insert into t1 VALUES (2,'t1b'); insert into t1 VALUES (3,'t1c'); insert into t1 VALUES (4,'t1d'); insert into t1 VALUES (5,'t1f'); drop table if exists t2; create table t2(id int not null,name VARCHAR(20)); insert into t2 values (2,'t2b'); insert into t2 values (3,'t2c'); insert into t2 values (4,'t2d'); insert into t2 values (5,'t2f'); insert into t2 values (6,'t2e');

#笛卡尔积

#select * from t1 join t2;

#左连接 两表关联，左表全部保留，右表关联不上的用null表示

#select* from t1 left join t2 on t1.id = t2.id;

#右连接 右表全部保留，左表关联不上用null表示

#select * from t1 right join t2 on t1.id = t2.id;

#内连接 求两个表的交集

#select * from t1 inner join t2 on t1.id = t2.id;

#左表独有的数据

#select * from t1 left join t2 on t1.id=t2.id WHERE t2.id is NULL;

#右表独有的数据

#select * from t1 right join t2 on t1.id = t2.id where t1.id is null;

#全连接 使用union达到全连接的目的

#select * from t1 left join t2 on t1.id = t2.id union select * from t1 right join t2 on t1.id = t2.id;

#并集去交集，也就是找出两个集合不同的地方

select * from t1 left join t2 on t1.id = t2.id WHERE t2.id is null union select * from t1 right join t2 on t1.id = t2.id WHERE t1.id is null;