子查询
SELECT order_num FROM orderitems WHERE prod_id='TNT2';
SELECT cust_id FROM orders WHERE order_num IN (SELECT order_num FROM orderitems WHERE prod_id='TNT2');
SELECT cust_name,cust_contact FROM customers WHERE cust_id IN (SELECT cust_id FROM orders WHERE order_num IN (SELECT order_num FROM orderitems WHERE prod_id='TNT2'));
SELECT prod_name,prod_price,vend_name FROM products,vendors WHERE products.vend_id=vendors.vend_id ORDER BY prod_name,vend_name;
SELECT * FROM orders;
| order_num | order_date | cust_id |
| 20005 | 2005-09-01 00:00:00 | 10001 |
| 20006 | 2005-09-12 00:00:00 | 10003 |
| 20007 | 2005-09-30 00:00:00 | 10004 |
| 20008 | 2005-10-03 00:00:00 | 10005 |
SELECT cust_name,cust_state,cust_id FROM customers;
| cust_name | cust_state | cust_id |
| Coyote Inc. | MI | 10001 |
| Mouse House | NULL | 10002 |
| Wascals | IN | 10003 |
| Yosemite Place | AZ | 10004 |
| The Fudds | IL | 10005 |
SELECT COUNT(*) AS orders FROM orders WHERE cust_id=10001;
SELECT cust_name,cust_state,(SELECT COUNT(*) FROM orders WHERE orders.cust_id=customers.cust_id) AS orders FROM customers ORDER BY cust_name;
| cust_name | cust_state | orders |
| Coyote Inc. |
MI | 1 |
| Mouse House | NULL | 0 |
| The Fudds | IL | 1 |
| Wascals | IN | 1 |
| Yosemite Place | AZ | 1 |
关系表的设计
把信息分解成多个表，一类数据一个表，各表通过常用的值互相关联。
主键：主键为某个表的一列，用来唯一标识每行
外键：外键为某个表的一列，它包含另一个表的主键值。
联结
SELECT vend_name,vend_id FROM vendors ORDER BY vend_name;
| vend_name | vend_id |
| ACME | 1003 |
| Anvils R Us | 1001 |
| Furball Inc | 1004 |
| Jet Set | 1005 |
| Jouets Et Ours | 1006 |
| LT Supplies | 1002 |
SELECT prod_name,prod_price,vend_id FROM products ORDER BY prod_name;
| prod_name | prod_price | vend_id |
| .5 ton anvil | 5.99 | 1001 |
| 1 ton anvil | 9.99 | 1001 |
| 2 ton anvil | 14.99 | 1001 |
| Bird seed | 10.00 | 1003 |
| Carrots | 2.50 | 1003 |
| Detonator | 13.00 | 1003 |
| Fuses | 3.42 | 1002 |
| JetPack 1000 | 35.00 | 1005 |
| JetPack2000 | 55.00 | 1005 |
| Oil can | 8.99 | 1002 |
| Safe | 50.00 | 1003 |
| Sling | 4.49 | 1003 |
| TNT (1 stick) | 2.50 | 1003 |
| TNT(5 sticks) | 10.00 | 1003 |
SELECT prod_name,prod_price,vend_name FROM vendors,products WHERE vendors.vend_id=products.vend_id ORDER BY prod_name,vend_name;
| prod_name | prod_price | vend_name |
| .5 ton anvil | 5.99 | Anvils R Us |
| 1 ton anvil | 9.99 | Anvils R Us |
| 2 ton anvil | 14.99 | Anvils R Us |
| Bird seed | 10.00 | ACME |
| Carrots | 2.50 | ACME |
| Detonator | 13.00 | ACME |
| Fuses | 3.42 | LT Supplies |
| JetPack 1000 | 35.00 | Jet Set |
| JetPack2000 | 55.00 | Jet Set |
| Oil can | 8.99 | LT Supplies |
| Safe | 50.00 | ACME |
| Sling | 4.49 | ACME |
| TNT (1 stick) | 2.50 | ACME |
| TNT(5 sticks) | 10.00 | ACME |
SELECT vend_name,prod_name,prod_price,vend_id FROM vendors,products WHERE vendors.vend_id=products.vend_id ORDER BY vend_name,prod_name;
ERROR 1052 (23000): Column 'vend_id' in filed list is ambigous
SELECT vend_name,prod_name,prod_price FROM vendors,products ORDER BY vend_name,prod_name;
SELECT vend_name,prod_name,prod_price FROM vendors,products WHERE vendors.vend_id=products.vend_id ORDER BY vend_name,prod_name;
联结条件：WHERE vendors.vend_id=products.vend_id
有联结条件：vendors表中的每行和products表中的每行匹配，满足匹配条件的才返回；
无联结条件：vendors表中的每行和products表中的每行匹配，返回 vendors的行数*products的行数 （笛卡尔积）