MySQL内连接、左外连接、右外连接、全外连接

连接的优势是减少了外键的使用。
内连接：最常见的等值连接，指连接结果仅包含符合连接条件的行，参与连接的两个表都应该符合连接条件。
inner join或join on condition

//首先创建两个表person和card，内容如下
 select * from card;
+------+----------+
| id   | cardName |
+------+----------+
|    1 | cardyi   |
|    2 | carder   |
|    3 | cardsan  |
|    4 | cardsi   |
|    5 | cardwu   |
|    6 | cardliu  |
+------+----------+
 select * from person;
+------+------+--------+
| id   | name | cardId |
+------+------+--------+
|    1 | pyi  |      1 |
|    1 | pyi  |      2 |
|    1 | pyi  |      4 |
|    1 | pyi  |      7 |
|    2 | per  |      6 |
+------+------+--------+

内连接示意：

 select * from person inner join card on person.cardId=card.id;//需要两个表都满足连接条件。
+------+------+--------+------+----------+
| id   | name | cardId | id   | cardName |
+------+------+--------+------+----------+
|    1 | pyi  |      1 |    1 | cardyi   |
|    1 | pyi  |      2 |    2 | carder   |
|    1 | pyi  |      4 |    4 | cardsi   |
|    2 | per  |      6 |    6 | cardliu  |
+------+------+--------+------+----------+

外连接：包含左外连接、右外连接和全外连接
左外连接：left join on或者left outer join on

select * from person left join card on person.cardId =card.id;
+------+------+--------+------+----------+
| id   | name | cardId | id   | cardName |
+------+------+--------+------+----------+
|    1 | pyi  |      1 |    1 | cardyi   |
|    1 | pyi  |      2 |    2 | carder   |
|    1 | pyi  |      4 |    4 | cardsi   |
|    2 | per  |      6 |    6 | cardliu  |
|    1 | pyi  |      7 | NULL | NULL     |
+------+------+--------+------+----------+

左边的全显示，右边有相等就显示，没有就显示null
右外连接：right join或者right outer join

select * from person right join card on person.cardId=card.id;
+------+------+--------+------+----------+
| id   | name | cardId | id   | cardName |
+------+------+--------+------+----------+
|    1 | pyi  |      1 |    1 | cardyi   |
|    1 | pyi  |      2 |    2 | carder   |
|    1 | pyi  |      4 |    4 | cardsi   |
|    2 | per  |      6 |    6 | cardliu  |
| NULL | NULL |   NULL |    3 | cardsan  |
| NULL | NULL |   NULL |    5 | cardwu   |
+------+------+--------+------+----------+

右边有的全显示，左边有的显示，左边没有的显示null。左边有，右边没有的不显示。
全外连接：full join
mysql本身不支持full join
如果要实现full join，就把左连接的实现和右连接的实现进行union

select * from person left join card on person.cardId=card.id
union
select * from person right join card on person.cardId=card.id;
+------+------+--------+------+----------+
| id   | name | cardId | id   | cardName |
+------+------+--------+------+----------+
|    1 | pyi  |      1 |    1 | cardyi   |
|    1 | pyi  |      2 |    2 | carder   |
|    1 | pyi  |      4 |    4 | cardsi   |
|    2 | per  |      6 |    6 | cardliu  |
|    1 | pyi  |      7 | NULL | NULL     |
| NULL | NULL |   NULL |    3 | cardsan  |
| NULL | NULL |   NULL |    5 | cardwu   |
+------+------+--------+--。----+----------+

左边的全显示，右边的全显示，左边有，右边没有，右边为null；右边有，左边没有，左边为null