mysql学习笔记六

四种连接查询

1. 内连接 —— join或inner join
2. 外连接
   2.1 左连接 —— left join 或者 left outer join
   2.2 右连接 —— right join 或者 right outer join
   2.3 完全外连接 —— full join 或者 full outer join
3. 使用

3.1 创建2个数据表并添加数据

mysql> create table person(
    -> id int,
    -> name varchar(20),
    -> cardId int
    -> );
Query OK, 0 rows affected (1.67 sec)

mysql> create table card(
    -> id int,
    -> name varchar(20)
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> insert into card values(1,'card1');
mysql> insert into card values(2,'card2');
mysql> insert into card values(3,'card3');
mysql> insert into card values(4,'card4');
mysql> insert into card values(5,'card5');

mysql> insert into person values(1,'zhang',1);
mysql> insert into person values(2,'zeng',3);
mysql> insert into person values(3,'li',5);
mysql> insert into person values(4,'cheng',6);

3.2 inner join练习
内联查询其实就是两张表的数据通过某个相等的字段，进行连接查询。
方法一:相当于多表查询,用的是where

mysql> select * from person,card where person.cardId=card.id;
+------+-------+--------+------+-------+
| id   | name  | cardId | id   | name  |
+------+-------+--------+------+-------+
|    1 | zhang |      1 |    1 | card1 |
|    2 | zeng  |      3 |    3 | card3 |
|    3 | li    |      5 |    5 | card5 |
+------+-------+--------+------+-------+
3 rows in set (0.00 sec)

方法二:（也可直接使用join来实现内连接）

mysql> select * from person inner join card on person.cardId=card.id;
+------+-------+--------+------+-------+
| id   | name  | cardId | id   | name  |
+------+-------+--------+------+-------+
|    1 | zhang |      1 |    1 | card1 |
|    2 | zeng  |      3 |    3 | card3 |
|    3 | li    |      5 |    5 | card5 |
+------+-------+--------+------+-------+
3 rows in set (0.00 sec)

3.3 left join(把左边表person的全部数据取出，右边表的数据如果条件字段有相等的就取出，没有就补null)

mysql> select * from person left join card on person.cardId=card.id;
+------+-------+--------+------+-------+
| id   | name  | cardId | id   | name  |
+------+-------+--------+------+-------+
|    1 | zhang |      1 |    1 | card1 |
|    2 | zeng  |      3 |    3 | card3 |
|    3 | li    |      5 |    5 | card5 |
|    4 | cheng |      6 | NULL | NULL  |
+------+-------+--------+------+-------+
4 rows in set (0.00 sec)

3.4 right join(把右边表card的全部数据取出，左边表的数据如果条件字段有相等的就取出，没有就补null)

mysql> select * from person right join card on person.cardId=card.id;
+------+-------+--------+------+-------+
| id   | name  | cardId | id   | name  |
+------+-------+--------+------+-------+
|    1 | zhang |      1 |    1 | card1 |
| NULL | NULL  |   NULL |    2 | card2 |
|    2 | zeng  |      3 |    3 | card3 |
| NULL | NULL  |   NULL |    4 | card4 |
|    3 | li    |      5 |    5 | card5 |
+------+-------+--------+------+-------+
5 rows in set (0.00 sec)

3.5 full join
mysql不支持full join，实现full join可以使用left join union right join

mysql> select * from person right join card on person.cardId=card.id
    -> union
    -> select * from person left join card on person.cardId=card.id;
+------+-------+--------+------+-------+
| id   | name  | cardId | id   | name  |
+------+-------+--------+------+-------+
|    1 | zhang |      1 |    1 | card1 |
| NULL | NULL  |   NULL |    2 | card2 |
|    2 | zeng  |      3 |    3 | card3 |
| NULL | NULL  |   NULL |    4 | card4 |
|    3 | li    |      5 |    5 | card5 |
|    4 | cheng |      6 | NULL | NULL  |
+------+-------+--------+------+-------+
6 rows in set (0.00 sec)

4. 总结
   四种连接对比图：

   
   
   image.png