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)

MySQL 使用篇 組合查詢

組合查詢，也叫合併查詢，就是將多個sql語句的查詢結果集合並為乙個大的結果集。比如有如下兩個sql語句 mysql select m1 from t1 where m1 2 m1 1 1 row in set 0.00 sec mysql select m1 from t1 where m1 2 m...

mysql連線查詢例項 MySQL連線查詢例項詳解

建立表suppliers create table suppliers s id int not null auto increment,s name char 50 not null,s city char 50 null,s zip char 10 null,s call char 50 not...

mysql連線查詢例項 MySQL連線查詢例項詳解

建立表suppliers create table suppliers s id int not null auto increment,s name char 50 not null,s city char 50 null,s zip char 10 null,s call char 50 not...