連線的優勢是減少了外來鍵的使用。
內連線：最常見的等值連線，指連線結果僅包含符合連線條件的行，參與連線的兩個表都應該符合連線條件。
inner join或join on condition
//首先建立兩個表person和card，內容如下
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 |
+------+------+--------+
select * from person inner join card on person.cardid=card.id;//需要兩個表都滿足連線條件。
+------+------+--------+------+----------+
| 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 |
+------+------+--------+------+----------+
左外連線：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 |
+------+------+--------+------+----------+
右外連線：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 |
+------+------+--------+------+----------+
全外連線：full join
mysql本身不支援full join
如果要實現full join，就把左連線的實現和右連線的實現進行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 |
+------+------+--------+--。----+----------+
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