MySQL 使用篇 連線查詢

現有t1t2兩張表

mysql> select * from t1;

+------+------+
| m1 | n1 |
+------+------+
| 1 | a |
| 2 | b |
| 3 | c |
+------+------+
3 rows in set (0.00 sec)
mysql> select * from t2;
+------+------+
| m2 | n2 |
+------+------+
| 2 | b |
| 3 | c |
| 4 | d |
+------+------+
3 rows in set (0.00 sec)
mysql>連線

笛卡爾積就是兩兩組合。

比如（1,2,3）（a,b,c）

組合後就是 1a 1b 1c 2a 2b 2c 3a 3b 3c

select * from t1, t2;

+------+------+------+------+
| m1 | n1 | m2 | n2 |
+------+------+------+------+
| 1 | a | 2 | b |
| 2 | b | 2 | b |
| 3 | c | 2 | b |
| 1 | a | 3 | c |
| 2 | b | 3 | c |
| 3 | c | 3 | c |
| 1 | a | 4 | d |
| 2 | b | 4 | d |
| 3 | c | 4 | d |
+------+------+------+------+
# 下面的方式等價
select t1.m1, t1.n1, t2.m2, t2.n2 from t1, t2;
select m1, n1, m2, n2 from t1, t2;
select t1.*, t2.* from t1, t2;

# 左（外）連線的語法

select * from t1 left [outer] join t2 on 連線條件 [where 普通過濾條件];
# 右（外）連線的語法
select * from t1 right [outer] join t2 on 連線條件 [where 普通過濾條件];
# 內連線的語法
select * from t1 [inner | cross] join t2 [on 連線條件] [where 普通過濾條件];
# mysql中，以下寫法等價
select * from t1 join t2;
select * from t1 inner join t2;
select * from t1 cross join t2;

mysql> select * from t1 inner join t2 on t1.m1 = t2.m2;

+------+------+------+------+
| m1 | n1 | m2 | n2 |
+------+------+------+------+
| 2 | b | 2 | b |
| 3 | c | 3 | c |
+------+------+------+------+
2 rows in set (0.00 sec)
mysql> select * from t1 left join t2 on t1.m1 = t2.m2;
+------+------+------+------+
| m1 | n1 | m2 | n2 |
+------+------+------+------+
| 2 | b | 2 | b |
| 3 | c | 3 | c |
| 1 | a | null | null |
+------+------+------+------+
3 rows in set (0.00 sec)
mysql> select * from t1 right join t2 on t1.m1 = t2.m2;
+------+------+------+------+
| m1 | n1 | m2 | n2 |
+------+------+------+------+
| 2 | b | 2 | b |
| 3 | c | 3 | c |
| null | null | 4 | d |
+------+------+------+------+
3 rows in set (0.00 sec)

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