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.
mysql> 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)