mysql连接

 1 /*
 2    两个商店，每个店有不同的柜台，每个柜台上卖有不同的商品
 3 */
 4 create table tableA(
 5     id int(10) not null primary key auto_increment,
 6         monorail varchar(20),
 7         variety varchar(50)
 8 );
 9 insert into tableA 
10 values (1,‘a1‘,‘苹果‘),(2,‘a1‘,‘梨‘),(3,‘a3‘,‘香蕉‘),(4,‘a4‘,‘苹果‘),(5,‘a5‘,‘西瓜‘),
11 (6,‘a6‘,‘苹果‘),(7,‘a7‘,‘葡萄‘),(8,‘a8‘,‘桃子‘);
12 create table tableB(
13     id int(10) not null primary key auto_increment,
14         monorail varchar(20),
15         variety varchar(50)
16 );
17 insert into tableB 
18 values (1,‘b1‘,‘虾‘),(2,‘b2‘,‘猪肉‘),(3,‘b3‘,‘苹果‘),(4,‘b4‘,‘西瓜‘),(5,‘b5‘,‘梨‘),
19 (6,‘b6‘,‘香蕉‘);

/*
  内连接，查处两个商店共有的商品
*/
select * from tablea A INNER  JOIN tableb B
on A.variety = B.variety ;

/*
  左连接，以A表为左表
*/
select * from tablea A LEFT  JOIN tableb B
on A.variety = B.variety ;

/*
  右连接，以A表为左表
*/
select * from tablea A RIGHT  JOIN tableb B
on A.variety = B.variety ;

/*
  左外连接，以A表为左表
*/
select * from tablea A LEFT  JOIN tableb B
on A.variety = B.variety Where B.variety is null;

/*
  右外连接，以A表为左表
*/
select * from tablea A RIGHT  JOIN tableb B
on A.variety = B.variety Where A.variety is null;

/*
  全连接
*/
select * from tablea A LEFT  JOIN tableb B
on A.variety = B.variety ;
union
select * from tablea A RIGHT  JOIN tableb B
on A.variety = B.variety ;

select * from tablea A LEFT  JOIN tableb B
on A.variety = B.variety Where B.variety is  null 
union
select * from tablea A RIGHT  JOIN tableb B
on A.variety = B.variety Where A.variety is  null