SQL中的内连接，左外连接，右外连接，全连接，交叉连接

实验用表

为进行实验，作者在SQL Server数据库下共建立了t1、t2两张表，两张表除主键id外均有个两字段stu_id和teacher_id。

t1表

id	stu_id	teacher_id
1	S0001	T01
2	S0002	T01
3	S0003	T02
4	S0004	T02
5	S0005	T03
6	S0002	T04

---

t2表

id	stu_id	teacher_id
1	S0002	T05
2	S0003	T03
3	S0006	T02
4	S0007	T04
5	S0005	T03

---

连接

把两个或多个表根据某一个或多个共同的字段连接起来。连接查询即使用连接后的表进行查询操作。

内连接

以两张表为例，内连接会把两张表连接字段中共有的值连接起来，抛弃只在一个表中出现的值

SQL脚本

SELECT t1.stu_id,t2.stu_id FROM t1
INNER JOIN t2
ON t1.stu_id=t2.stu_id;

/*只使用JOIN关键字，默认为内连接*/
SELECT t1.stu_id,t2.stu_id FROM t1
JOIN t2
ON t1.stu_id=t2.stu_id;

在SQL Server中结果

stu_id	teacher_id
S0002	S0002
S0003	S0003
S0005	S0005
S0002	S0002

---

在Mysql中结果

stu_id	teacher_id
S0002	S0002
S0003	S0003
S0005	S0005

---

左外连接

以两张表为例，左外连接会把两张表连接字段中共有的值连接起来，同时会把在左表中出现而右表中未出现的值显示出来，在右表中未出现的值以Null表示。

SQL脚本

SELECT t1.stu_id,t2.stu_id FROM t1
LEFT JOIN t2
ON t1.stu_id=t2.stu_id;

在SQL Server中结果

stu_id	teacher_id
S0001	Null
S0002	S0002
S0003	S0003
S0004	Null
S0005	S0005
S0002	S0002

---

在Mysql中结果

stu_id	teacher_id
S0001	Null
S0002	S0002
S0003	S0003
S0004	Null
S0005	S0005

---

右外连接

以两张表为例，右外连接会把两张表连接字段中共有的值连接起来，同时会把在右表中出现而左表中未出现的值显示出来，在左表中未出现的值以Null表示。

SQL脚本

SELECT t1.stu_id,t2.stu_id FROM t1
RIGHT JOIN t2
ON t1.stu_id=t2.stu_id;

在SQL Server中结果

stu_id	teacher_id
S0002	S0002
S0002	S0002
S0003	S0003
Null	S0006
Null	S0007
S0005	S0005

---

在Mysql中结果

stu_id	teacher_id
S0002	S0002
S0003	S0003
Null	S0006
Null	S0007
S0005	S0005

---

全连接

以两张表为例，全连接不仅把两张表连接字段中共有的值连接起来，同时会把在只在左表或只在右表中出现而另一个表中未出现的值显示出来，未出现的值以Null表示。全连接在Mysql中不受支持。

SQL脚本

SELECT t1.stu_id,t2.stu_id FROM t1
FULL JOIN t2
ON t1.stu_id=t2.stu_id;

在SQL Server中结果

stu_id	teacher_id
S0001	Null
S0002	S0002
S0003	S0003
S0004	Null
S0005	S0005
S0002	S0002
Null	S0006
Null	S0007

---

交叉连接

以两张表为例，交叉连接不需指明连接条件，它把表1中的每一行同表2的全部行连接，如果表1有m行记录，表2有n行记录，则交叉连接会产生m*n行记录

SQL脚本

/*使用CROSS JOIN*/
SELECT t1.stu_id,t2.stu_id FROM t1
CROSS JOIN t2;
/*不使用，查询结果同上*/
SELECT t1.stu_id,t2.stu_id FROM t1,t2;

在SQL Server中结果

stu_id	teacher_id
S0001	S0002
S0002	S0002
S0003	S0002
S0004	S0002
S0005	S0002
S0002	S0002
S0001	S0003
S0002	S0003
S0003	S0003
S0004	S0003
S0005	S0003
S0002	S0003
S0001	S0006
S0002	S0006
S0003	S0006
S0004	S0006
S0005	S0006
S0002	S0006
S0001	S0007
S0002	S0007
S0003	S0007
S0004	S0007
S0005	S0007
S0002	S0007
S0001	S0005
S0002	S0005
S0003	S0005
S0004	S0005
S0005	S0005
S0002	S0005

---

在Mysql中结果

stu_id	teacher_id
S0001	S0002
S0002	S0002
S0003	S0002
S0004	S0002
S0005	S0002
S0001	S0003
S0002	S0003
S0003	S0003
S0004	S0003
S0005	S0003
S0001	S0006
S0002	S0006
S0003	S0006
S0004	S0006
S0005	S0006
S0001	S0007
S0002	S0007
S0003	S0007
S0004	S0007
S0005	S0007
S0001	S0005
S0002	S0005
S0003	S0005
S0004	S0005
S0005	S0005

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注：根据实验结果可知：在任何连接类型下，Mysql都会自动把结果中出现的重复记录合并为一条记录。