- 查询（query）：
mysql> 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='RGAN01'));
+---------------+--------------------+
| cust_name | cust_contact |
+---------------+--------------------+
| Fun4All | Denise L. Stephens |
| The Toy Store | Kim Howard |
+---------------+--------------------+
2 rows in set (0.01 sec)
子查询的select语句只能查询单个列。
关系数据库与非关系数据库
mysql> select vend_name,prod_name,prod_price
-> from Vendors,Products
-> where Vendors.vend_id=Products.vend_id;
+-----------------+---------------------+------------+
| vend_name | prod_name | prod_price |
+-----------------+---------------------+------------+
| Bears R Us | 8 inch teddy bear | 5.99 |
| Bears R Us | 12 inch teddy bear | 8.99 |
| Bears R Us | 18 inch teddy bear | 11.99 |
| Doll House Inc. | Fish bean bag toy | 3.49 |
| Doll House Inc. | Bird bean bag toy | 3.49 |
| Doll House Inc. | Rabbit bean bag toy | 3.49 |
| Doll House Inc. | Raggedy Ann | 4.99 |
| Fun and Games | King doll | 9.49 |
| Fun and Games | Queen doll | 9.49 |
+-----------------+---------------------+------------+
sql语句中出现笛卡尔乘积 SQL查询入门篇
图片.png
使用别名联结
mysql> select cust_name,cust_contact
-> from Customers as C,Orders as O,OrderItems as OI
-> where C.cust_id=O.cust_id
-> and OI.order_num=O.order_num
-> and prod_id='RGAN01';
+---------------+--------------------+
| cust_name | cust_contact |
+---------------+--------------------+
| Fun4All | Denise L. Stephens |
| The Toy Store | Kim Howard |
+---------------+--------------------+
2 rows in set (0.00 sec)
自联结
mysql> select c1.cust_id,c1.cust_name,c1.cust_contact
-> from Customers as c1, Customers as c2
-> where c1.cust_name = c2.cust_name
-> and c2.cust_contact = 'Jim Jones';
+------------+-----------+--------------------+
| cust_id | cust_name | cust_contact |
+------------+-----------+--------------------+
| 1000000003 | Fun4All | Jim Jones |
| 1000000004 | Fun4All | Denise L. Stephens |
+------------+-----------+--------------------+
2 rows in set (0.00 sec)
自然联结
mysql> select C.*,O.order_num,O.order_date,OI.prod_id,OI.quantity,OI.item_price
-> from Customers as C, Orders as O, OrderItems as OI
-> where C.cust_id = O.cust_id
-> and OI.order_num=O.order_num
-> and prod_id = 'RGAN01';
+------------+---------------+---------------------+-----------+------------+----------+--------------+--------------------+-----------------------+-----------+---------------------+---------+----------+------------+
| cust_id | cust_name | cust_address | cust_city | cust_state | cust_zip | cust_country | cust_contact | cust_email | order_num | order_date | prod_id | quantity | item_price |
+------------+---------------+---------------------+-----------+------------+----------+--------------+--------------------+-----------------------+-----------+---------------------+---------+----------+------------+
| 1000000004 | Fun4All | 829 Riverside Drive | Phoenix | AZ | 88888 | USA | Denise L. Stephens | [email protected] | 20007 | 2012-01-30 00:00:00 | RGAN01 | 50 | 4.49 |
| 1000000005 | The Toy Store | 4545 53rd Street | Chicago | IL | 54545 | USA | Kim Howard | NULL | 20008 | 2012-02-03 00:00:00 | RGAN01 | 5 | 4.99 |
+------------+---------------+---------------------+-----------+------------+----------+--------------+--------------------+-----------------------+-----------+---------------------+---------+----------+------------+
外联结
mysql> select Customers.cust_id,Orders.order_num
-> from Customers inner join Orders
-> on Customers.cust_id = Orders.cust_id;
+------------+-----------+
| cust_id | order_num |
+------------+-----------+
| 1000000001 | 20005 |
| 1000000003 | 20006 |
| 1000000004 | 20007 |
| 1000000005 | 20008 |
| 1000000001 | 20009 |
+------------+-----------+
5 rows in set (0.00 sec)
mysql> select Customers.cust_id,Orders.order_num from Customers left outer join Orders on Customers.cust_id = Orders.cust_id;
+------------+-----------+
| cust_id | order_num |
+------------+-----------+
| 1000000001 | 20005 |
| 1000000001 | 20009 |
| 1000000002 | NULL |
| 1000000003 | 20006 |
| 1000000004 | 20007 |
| 1000000005 | 20008 |
+------------+-----------+
6 rows in set (0.00 sec)
使用带集聚函数的联结
mysql> select Customers.cust_id,count(Orders.order_num) as num_ord
-> from Customers inner join Orders
-> on Customers.cust_id=Orders.cust_id
-> group by Customers.cust_id;
+------------+---------+
| cust_id | num_ord |
+------------+---------+
| 1000000001 | 2 |
| 1000000003 | 1 |
| 1000000004 | 1 |
| 1000000005 | 1 |
+------------+---------+
4 rows in set (0.00 sec)
mysql> select cust_name,cust_contact,cust_email
-> from Customers
-> where cust_state in ('IL','IN','MI');
+---------------+--------------+-----------------------+
| cust_name | cust_contact | cust_email |
+---------------+--------------+-----------------------+
| Village Toys | John Smith | [email protected] |
| Fun4All | Jim Jones | [email protected] |
| The Toy Store | Kim Howard | NULL |
+---------------+--------------+-----------------------+
mysql> select cust_name,cust_contact,cust_email
-> from Customers
-> where cust_name='Fun4All';
+-----------+--------------------+-----------------------+
| cust_name | cust_contact | cust_email |
+-----------+--------------------+-----------------------+
| Fun4All | Jim Jones | [email protected] |
| Fun4All | Denise L. Stephens | [email protected] |
+-----------+--------------------+-----------------------+
mysql> select cust_name,cust_contact,cust_email
-> from Customers
-> where cust_state in ('IL','IN','MI')
-> or cust_name='Fun4All';
+---------------+--------------------+-----------------------+
| cust_name | cust_contact | cust_email |
+---------------+--------------------+-----------------------+
| Village Toys | John Smith | [email protected] |
| Fun4All | Jim Jones | [email protected] |
| Fun4All | Denise L. Stephens | [email protected] |
| The Toy Store | Kim Howard | NULL |
+---------------+--------------------+-----------------------+
mysql> select cust_name,cust_contact,cust_email
-> from Customers
-> where cust_state in ('IL','IN','MI')
-> union
-> select cust_name,cust_contact,cust_email
-> from Customers
-> where cust_name='Fun4All';
+---------------+--------------------+-----------------------+
| cust_name | cust_contact | cust_email |
+---------------+--------------------+-----------------------+
| Village Toys | John Smith | [email protected] |
| Fun4All | Jim Jones | [email protected] |
| The Toy Store | Kim Howard | NULL |
| Fun4All | Denise L. Stephens | [email protected] |
+---------------+--------------------+-----------------------+
- UNION 两条或两条以上select语句组成
- union 中的每一个列必须包含相同的列、表达式、聚集函数
- 列数据类型必须兼容：类型不必完全相同，但必须是DBMA可以隐含转换的类型
union all 返回包含重复的行。