MySQL学习足迹记录13--联结表--INNER JOIN...ON

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MySQL学习足迹记录13--联结表--INNER JOIN...ON

来源: 作者: 时间:2016-02-18 10:28 【

MySQL学习足迹记录13--联结表--INNER JOIN...ON 学数据库联结表这一节内容几乎吐血,看书上的介绍和百度到的内容,总感到一脸茫然.....也许是我理解能力太差了吧。但我是一个不会轻易放弃
MySQL学习足迹记录13--联结表--INNER JOIN...ON
 
  学联结表这一节内容几乎吐血,上的介绍和百度到的内容,总感到一脸茫然.....也许是我理解能力太差了吧。但我是一个不会轻易放弃的人,研究了半小时,终于搞明白了联结的原理,自我总结如下:关键在于“select语句,如果没有WHERE子句限定,将返回两个表的笛卡尔积.”
 
Example:
    
表一: Kemu
+-----+--------+
| kno | kname  |
+-----+--------+
| b   | yuwen  |
| b1  | shuxue |
+-----+--------+



表二: Chenji;
+------+------+---------+
| sno  | kno  | chengji |
+------+------+---------+
|    1 | a    |      10 |
|    2 | a1   |      20 |
+------+------+---------+
2 rows in set (0.00 sec)




*重点:select语句,如果没有WHERE子句限定,将返回两个表的笛卡尔积.
> SELECT * FROM Kemu,Chenji;
+-----+--------+------+------+---------+
| kno | kname  | sno  | kno  | chengji |
+-----+--------+------+------+---------+
| b   | yuwen  |    1 | a    |      10 |
| b1  | shuxue |    1 | a    |      10 |
| b   | yuwen  |    2 | a1   |      20 |
| b1  | shuxue |    2 | a1   |      20 |
+-----+--------+------+------+---------+
4 rows in set (0.00 sec)

 

 
 Tips:SELECT 语句后面的所有表将会合成一个大表,所以,SELECT后所跟的各种子句就相当于在一个表中筛选数据了。
 
 
1.外键:某个表的一列,它包含另一个表的主键值,定义了两个表之间的关系
 
2. 联结不是物理实体,它在实际的数据库表中不存在,它存在与查询执行当中
 
3.创建联结
  Example:
  mysql> SELECT vend_name,prod_name,prod_price
            -> FROM vendors,products
           -> WHERE vendors.vend_id = products.vend_id        #WHERE子句指示MySQL匹配vendors表的  
           -> ORDER BY vend_name,prod_name;                     #vend_id和products表中的vend_id.

+-------------+----------------+------------+
| vend_name   | prod_name      | prod_price |
+-------------+----------------+------------+
| ACME        | Bird seed      |      10.00 |
| ACME        | Carrots        |       2.50 |
| ACME        | Detonator      |      13.00 |
| ACME        | Safe           |      50.00 |
| ACME        | Sling          |       4.49 |
| ACME        | TNT (1 stick)  |       2.50 |
| ACME        | TNT (5 sticks) |      10.00 |
| Anvils R Us | .5 ton anvil   |       5.99 |
| Anvils R Us | 1 ton anvil    |       9.99 |
| Anvils R Us | 2 ton anvil    |      14.99 |
| Jet Set     | JetPack 1000   |      35.00 |
| Jet Set     | JetPack 2000   |      55.00 |
| LT Supplies | Fuses          |       3.42 |
| LT Supplies | Oil can        |       8.99 |
+-------------+----------------+------------+
14 rows in set (0.01 sec)

 

   
TIPS:
   *WHERE联结两个表时,实际是将第一个表中的每一行和第二个表中的每一行配对.并过滤不符合条件的行.
 
4.内部联结INNER JOIN .... ON
  Example:
  mysql> SELECT vend_name,prod_name,prod_price
           -> FROM vendors INNER JOIN products                     #使用明确的联结语法能狗确保不会忘记联结条件
           -> ON vendors.vend_id = products.vend_id                #联结条件用特定的ON子句,
           -> ORDER BY vend_name,prod_name;                        #传递的实际条件与传递给WHERE的相同
+-------------+----------------+------------+
| vend_name   | prod_name      | prod_price |
+-------------+----------------+------------+
| ACME        | Bird seed      |      10.00 |
| ACME        | Carrots        |       2.50 |
| ACME        | Detonator      |      13.00 |
| ACME        | Safe           |      50.00 |
| ACME        | Sling          |       4.49 |
| ACME        | TNT (1 stick)  |       2.50 |
| ACME        | TNT (5 sticks) |      10.00 |
| Anvils R Us | .5 ton anvil   |       5.99 |
| Anvils R Us | 1 ton anvil    |       9.99 |
| Anvils R Us | 2 ton anvil    |      14.99 |
| Jet Set     | JetPack 1000   |      35.00 |
| Jet Set     | JetPack 2000   |      55.00 |
| LT Supplies | Fuses          |       3.42 |
| LT Supplies | Oil can        |       8.99 |
+-------------+----------------+------------+
14 rows in set (0.00 sec)

 

5.联结多个表
   *SQL对一条SELECT语句中可以联结的表的数目没有限制。
   Example:
    mysql> SELECT prod_name,vend_name,prod_price,quantity
            -> FROM orderitems,products,vendors
            -> WHERE products.vend_id=vendors.vend_id
           -> AND orderitems.prod_id=products.prod_id
           -> AND order_num=20005;
+----------------+-------------+------------+----------+
| prod_name      | vend_name   | prod_price | quantity |
+----------------+-------------+------------+----------+
| .5 ton anvil   | Anvils R Us |       5.99 |       10 |
| 1 ton anvil    | Anvils R Us |       9.99 |        3 |
| TNT (5 sticks) | ACME        |      10.00 |        5 |
| Bird seed      | ACME        |      10.00 |        1 |
+----------------+-------------+------------+----------+
4 rows in set (0.00 sec)

 


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