Mysql数据库内联查询、左连接查询、右连接查询、自连接查询介绍

一.内联查询

1.inner join:只查询键值一致（交叉）的部分 

2.演示

二.左连接

1.left join:以左表为标准，查询输出左表中没有的字段信息

2.演示

三.右连接

1.right join:以右表为标准，查询输出右表中没有的字段信息

2.演示

四.自连接

将一张表通过别名虚拟为两张表，查询比较其中的不同字段

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一.内联查询

1.inner join:只查询键值一致（交叉）的部分 

select (*|指定字段) from 表1 inner join 表2 on 匹配条件;

可以配合where语句、order by语句、逻辑语句等一起使用

2.演示

--t5的id和t6的id具有外键关系
mysql8.0 [SLB]>select * from t5;
+----+---------------+-----------+
| id | name          | major     |
+----+---------------+-----------+
|  1 | sulibao       | dianshang |
|  2 | lixinjin      | dianshang |
|  3 | zhanghongyuan | jike      |
+----+---------------+-----------+
3 rows in set (0.00 sec)

mysql8.0 [SLB]>select * from t6;
+------+--------+------+
| age  | weight | id   |
+------+--------+------+
|   18 |   75.5 |    1 |
|   19 |   75.5 |    2 |
|   25 |     90 |    3 |
+------+--------+------+
3 rows in set (0.00 sec)

mysql8.0 [SLB]>select * from t5 inner join t6 on t5.id=t6.id;  --联合查询所有字段
+----+---------------+-----------+------+--------+------+
| id | name          | major     | age  | weight | id   |
+----+---------------+-----------+------+--------+------+
|  1 | sulibao       | dianshang |   18 |   75.5 |    1 |
|  2 | lixinjin      | dianshang |   19 |   75.5 |    2 |
|  3 | zhanghongyuan | jike      |   25 |     90 |    3 |
+----+---------------+-----------+------+--------+------+
3 rows in set (0.00 sec)

mysql8.0 [SLB]>select t5.id,t5.name,t5.major,t6.age,t6.weight from t5 inner join t6 on t
5.id=t6.id;
--联合查询指定字段
+----+---------------+-----------+------+--------+
| id | name          | major     | age  | weight |
+----+---------------+-----------+------+--------+
|  1 | sulibao       | dianshang |   18 |   75.5 |
|  2 | lixinjin      | dianshang |   19 |   75.5 |
|  3 | zhanghongyuan | jike      |   25 |     90 |
+----+---------------+-----------+------+--------+
3 rows in set (0.00 sec)

mysql8.0 [SLB]>select a.id,a.name,a.major,b.age,b.weight from t5 a inner join t6 b on a.id=b.id;
--也可以为名称较长的表取别名
+----+---------------+-----------+------+--------+
| id | name          | major     | age  | weight |
+----+---------------+-----------+------+--------+
|  1 | sulibao       | dianshang |   18 |   75.5 |
|  2 | lixinjin      | dianshang |   19 |   75.5 |
|  3 | zhanghongyuan | jike      |   25 |     90 |
+----+---------------+-----------+------+--------+
3 rows in set (0.00 sec)

二.左连接

1.left join:以左表为标准，查询输出左表中没有的字段信息

select (*|指定字段) from 左表 left join 从表 on 匹配条件;

可以配合where语句、order by语句、逻辑语句等一起使用

2.演示

(1)

mysql8.0 [SLB]>select * from t5;
+----+---------------+-----------+
| id | name          | major     |
+----+---------------+-----------+
|  1 | sulibao       | dianshang |
|  2 | lixinjin      | dianshang |
|  3 | zhanghongyuan | jike      |
+----+---------------+-----------+
3 rows in set (0.00 sec)

mysql8.0 [SLB]>select * from t6;
+------+--------+------+
| age  | weight | id   |
+------+--------+------+
|   18 |   75.5 |    1 |
|   19 |   75.5 |    2 |
|   25 |     90 |    3 |
+------+--------+------+
3 rows in set (0.00 sec)

mysql8.0 [SLB]>select * from t5 left join t6 on t5.id=t6.id;
+----+---------------+-----------+------+--------+------+
| id | name          | major     | age  | weight | id   |
+----+---------------+-----------+------+--------+------+
|  1 | sulibao       | dianshang |   18 |   75.5 |    1 |
|  2 | lixinjin      | dianshang |   19 |   75.5 |    2 |
|  3 | zhanghongyuan | jike      |   25 |     90 |    3 |
+----+---------------+-----------+------+--------+------+
3 rows in set (0.01 sec)

mysql8.0 [SLB]>select * from t6 left join t5 on t5.id=t6.id;
+------+--------+------+------+---------------+-----------+
| age  | weight | id   | id   | name          | major     |
+------+--------+------+------+---------------+-----------+
|   18 |   75.5 |    1 |    1 | sulibao       | dianshang |
|   19 |   75.5 |    2 |    2 | lixinjin      | dianshang |
|   25 |     90 |    3 |    3 | zhanghongyuan | jike      |
+------+--------+------+------+---------------+-----------+
3 rows in set (0.00 sec)

(2)

左表有的字段而从表没有对应内容则映射为null

mysql8.0 [SLB]>insert t5 values(4,"niexincheng","dianshang");
Query OK, 1 row affected (0.00 sec)

mysql8.0 [SLB]>select * from t5 left join t6 on t5.id=t6.id;
+----+---------------+-----------+------+--------+------+
| id | name          | major     | age  | weight | id   |
+----+---------------+-----------+------+--------+------+
|  1 | sulibao       | dianshang |   18 |   75.5 |    1 |
|  2 | lixinjin      | dianshang |   19 |   75.5 |    2 |
|  3 | zhanghongyuan | jike      |   25 |     90 |    3 |
|  4 | niexincheng   | dianshang | NULL |   NULL | NULL |
+----+---------------+-----------+------+--------+------+
4 rows in set (0.00 sec)

左表没有的字段而从表有则不查询这些字段内容

mysql8.0 [SLB]>select * from t6 left join t5 on t5.id=t6.id;
+------+--------+------+------+---------------+-----------+
| age  | weight | id   | id   | name          | major     |
+------+--------+------+------+---------------+-----------+
|   18 |   75.5 |    1 |    1 | sulibao       | dianshang |
|   19 |   75.5 |    2 |    2 | lixinjin      | dianshang |
|   25 |     90 |    3 |    3 | zhanghongyuan | jike      |
+------+--------+------+------+---------------+-----------+
3 rows in set (0.00 sec)

三.右连接

1.right join:以右表为标准，查询输出右表中没有的字段信息

2.演示

用法同左连接类似

mysql8.0 [SLB]>select * from t6 right join t5 on t5.id=t6.id;
+------+--------+------+----+---------------+-----------+
| age  | weight | id   | id | name          | major     |
+------+--------+------+----+---------------+-----------+
|   18 |   75.5 |    1 |  1 | sulibao       | dianshang |
|   19 |   75.5 |    2 |  2 | lixinjin      | dianshang |
|   25 |     90 |    3 |  3 | zhanghongyuan | jike      |
| NULL |   NULL | NULL |  4 | niexincheng   | dianshang |
+------+--------+------+----+---------------+-----------+
4 rows in set (0.01 sec)

mysql8.0 [SLB]>select * from t5 right join t6 on t5.id=t6.id;
+------+---------------+-----------+------+--------+------+
| id   | name          | major     | age  | weight | id   |
+------+---------------+-----------+------+--------+------+
|    1 | sulibao       | dianshang |   18 |   75.5 |    1 |
|    2 | lixinjin      | dianshang |   19 |   75.5 |    2 |
|    3 | zhanghongyuan | jike      |   25 |     90 |    3 |
+------+---------------+-----------+------+--------+------+
3 rows in set (0.00 sec)

四.自连接

将一张表通过别名虚拟为两张表，查询比较其中的不同字段

示例：yl表中查询yagelage的所有信息

mysql8.0 [SLB]>select y1.* from yl y1 join yl y2 on y1.lname=y2.lname and y1.yageselect * from yl;
+-------------+------+------+-------+
| name        | yage | lage | lname |
+-------------+------+------+-------+
| sulibao     |   20 |   25 | lh    |
| lixinjin    |   21 |   29 | cq    |
| niexincheng |   28 |   25 | zxw   |
+-------------+------+------+-------+
3 rows in set (0.00 sec)

mysql8.0 [SLB]>select y1.* from yl y1 join yl y2 on y1.lname=y2.lname and y1.yage>y2.lage;
+-------------+------+------+-------+
| name        | yage | lage | lname |
+-------------+------+------+-------+
| niexincheng |   28 |   25 | zxw   |
+-------------+------+------+-------+
1 row in set (0.01 sec)