Mysql 连接的使用（join）

初始化SQL语句:

/*join 建表语句*/
drop database if exists test;
create database test;
use test;

/* 左表t1*/
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');

/* 右表 t2*/
drop table if exists t1;
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,'t2a');

• 1、笛卡尔积

两表关联，把左表的列和右表的列通过笛卡尔积的形式表达出来。

mysql> select * from t1 join t2;

• 1

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

• 1

3、右连接

右表全部保留，左表关联不上的用null表示。

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

• 1

4、内连接

两表关联，保留两表中交集的记录。

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

• 1

5、左表独有

两表关联，查询左表独有的数据。

mysql> select * from t1 left join t2 on t1.id = t2.id where t2.id is null;

• 1

6、右表独有

两表关联，查询右表独有的数据。

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

• 1

7、全连接

两表关联，查询它们的所有记录。

oracle里面有full join,但是在mysql中没有full join。我们可以使用union来达到目的。

mysql> select * from t1 left join t2 on t1. id = t2. id
-> union
-> select * from t1 right join t2 on t1. id = t2. id;

• 1
• 2
• 3

8、并集去交集

两表关联，取并集然后去交集。

mysql> 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;

• 1
• 2
• 3

总结：

SELECT * FROM t1 JOIN t2   on t1.id =t2.id
 取得是两表并集  
SELECT * FROM t1 LEFT JOIN t2 ON t1.id = t2.id 
 左表信息全显示，右表匹配左表id的信息显示
 SELECT * FROM t1 RIGHT JOIN t2 ON t1.id = t2.id 

右表信息全显示，左表匹配左表id的信息显示

SELECT * FROM t1 INNER JOIN t2 ON t1.id = t2.id

两表的交集，公共部分即两表相同的 id 信息显示

SELECT * FROM t1  
     UNION 

 SELECT * FROM t2  

t1和t2两表信息全显示 但两表需结构相同

SELECT * FROM t1 RIGHT JOIN t2 ON t1.id = t2.id WHERE t1.id IS  NULL;

匹配左表的信息 全不显示

SELECT * FROM t1 LEFT JOIN t2 ON t1.id = t2.id
  UNION 

  SELECT * FROM t1 RIGHT JOIN t2 ON t1.id = t2.id;

两表信息全显示  = SELECT * FROM t1 JOIN t2  

出处：https://blog.csdn.net/lc89123/article/details/79518199