SQL查询语句的基本结构

SQL查询基本结构由三个子语句构成： select     from   where

以instructor关系为例：  创建 关系并插入数据

mysql> insert into instructor value('10101','Srinivasan','Comp.Sci.',65000);
Query OK, 1 row affected (0.50 sec)

ID	name	dept_name	salary
10101	Srinivasan	Comp.Sci.	65000.00
12121	Wu	Finance	90000.00
15151	Mozart	Music	40000.00
22222	Einstein	Physics	95000.00
32343	El Said	History	60000.00
33456	Gold	Physics	87000.00
45565	Katz	Comp.Sci.	75000.00
58583	Califieri	History	62000.00
76543	Singh	Finance	80000.00
76766	Crick	Biology	72000.00
83821	Brandt	Comp.Sci.	92000.00
98345	Kim	Elec.Eng.	80000.00

      The instructor relation

(1)单关系查询语句：

找出所有老师的名字：

mysql> select name from instructor;
+------------+
| name       |
+------------+
| Srinivasan |
| Wu         |
| Mozart     |
| Einstein   |
| El Said    |
| Gold       |
| Katz       |
| Califieri  |
| Singh      |
| Crick      |
| Brandt     |
| Kim        |
+------------+
12 rows in set (0.00 sec)

找出所有老师所在系名：

mysql> select dept_name from instructor;
+-----------+
| dept_name |
+-----------+
| Comp.Sci. |
| Finance   |
| Music     |
| Physics   |
| History   |
| Physics   |
| Comp.Sci. |
| History   |
| Finance   |
| Biology   |
| Comp.Sci. |
| Elec.Eng. |
+-----------+
12 rows in set (0.00 sec)

强行去除重复：

mysql> select distinct dept_name from instructor;
+-----------+
| dept_name |
+-----------+
| Comp.Sci. |
| Finance   |
| Music     |
| Physics   |
| History   |
| Biology   |
| Elec.Eng. |
+-----------+
7 rows in set (0.00 sec)

select语句可带算术表达式：

mysql> select ID,name,dept_name,salary/12 from instructor;
+-------+------------+-----------+-------------+
| ID    | name       | dept_name | salary/12   |
+-------+------------+-----------+-------------+
| 10101 | Srinivasan | Comp.Sci. | 5416.666667 |
| 12121 | Wu         | Finance   | 7500.000000 |
| 15151 | Mozart     | Music     | 3333.333333 |
| 22222 | Einstein   | Physics   | 7916.666667 |
| 32343 | El Said    | History   | 5000.000000 |
| 33456 | Gold       | Physics   | 7250.000000 |
| 45565 | Katz       | Comp.Sci. | 6250.000000 |
| 58583 | Califieri  | History   | 5166.666667 |
| 76543 | Singh      | Finance   | 6666.666667 |
| 76766 | Crick      | Biology   | 6000.000000 |
| 83821 | Brandt     | Comp.Sci. | 7666.666667 |
| 98345 | Kim        | Elec.Eng. | 6666.666667 |
+-------+------------+-----------+-------------+
12 rows in set (0.00 sec)

select * 表示选择所有属性：

mysql> select * from instructor;
+-------+------------+-----------+----------+
| ID    | name       | dept_name | salary   |
+-------+------------+-----------+----------+
| 10101 | Srinivasan | Comp.Sci. | 65000.00 |
| 12121 | Wu         | Finance   | 90000.00 |
| 15151 | Mozart     | Music     | 40000.00 |
| 22222 | Einstein   | Physics   | 95000.00 |
| 32343 | El Said    | History   | 60000.00 |
| 33456 | Gold       | Physics   | 87000.00 |
| 45565 | Katz       | Comp.Sci. | 75000.00 |
| 58583 | Califieri  | History   | 62000.00 |
| 76543 | Singh      | Finance   | 80000.00 |
| 76766 | Crick      | Biology   | 72000.00 |
| 83821 | Brandt     | Comp.Sci. | 92000.00 |
| 98345 | Kim        | Elec.Eng. | 80000.00 |
+-------+------------+-----------+----------+
12 rows in set (0.00 sec)

找出所有在computer science系并且工资超过70000美元的教师名字

mysql> select name from instructor where dept_name='Comp.Sci.' and salary>70000;
+--------+
| name |
+--------+
| Katz |
| Brandt |
+--------+
2 rows in set (0.00 sec)


(2)多关系语句查询