基于 MySQL 的数据库实践（扩展运算）

Posted 2020-10-31 wander4096

select 中的通配符

星号 * 可以用在 select 子句中表示所有的属性。

mysql> select instructor.*
    -> from instructor, teaches
    -> where instructor.ID = teaches.ID;
+-------+------------+------------+----------+
| ID    | name       | dept_name  | salary   |
+-------+------------+------------+----------+
| 10101 | Srinivasan | Comp. Sci. | 65000.00 |
| 10101 | Srinivasan | Comp. Sci. | 65000.00 |
| 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 |
| 45565 | Katz       | Comp. Sci. | 75000.00 |
| 45565 | Katz       | Comp. Sci. | 75000.00 |
| 76766 | Crick      | Biology    | 72000.00 |
| 76766 | Crick      | Biology    | 72000.00 |
| 83821 | Brandt     | Comp. Sci. | 92000.00 |
| 83821 | Brandt     | Comp. Sci. | 92000.00 |
| 83821 | Brandt     | Comp. Sci. | 92000.00 |
| 98345 | Kim        | Elec. Eng. | 80000.00 |
+-------+------------+------------+----------+
15 rows in set (0.01 sec)

它返回了 instructor 中所有的属性。

元组的排序

SQL 为用户提供了一些结果显示的顺序的控制，order by 子句可以让查询结果中元组按升序显示，考虑按字典序列出 Physics 系的所有教师。

mysql> select name
    -> from instructor
    -> where dept_name = ‘Physics‘
    -> order by name;
+----------+
| name     |
+----------+
| Einstein |
| Gold     |
+----------+
2 rows in set (0.00 sec)

为了使用降序，我们可以用 desc 表示降序，完整地说，可以用 asc 表示升序，此外，排序可以在多个属性上进行，例如我们希望按 salary 的降序列出整个 instructor 关系，如果有几名教师工资相同，则按姓名升序排列。

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

where 子句谓词

为了简化 where 子句，SQL 提供 between 运算符来说明一个值落在一个闭区间内。
考虑查询，找出工资在 90000 美元和 100000 美元之间的教师的姓名，直观地有

mysql> select name 
    -> from instructor
    -> where salary <= 100000 and salary >= 90000;
+----------+
| name     |
+----------+
| Wu       |
| Einstein |
| Brandt   |
+----------+
3 rows in set (0.01 sec)

可以用 between 运算符改写为

mysql> select name
    -> from instructor
    -> where salary between 90000 and 100000;
+----------+
| name     |
+----------+
| Wu       |
| Einstein |
| Brandt   |
+----------+
3 rows in set (0.01 sec)

类似的有 not between 运算符

mysql> select name
    -> from instructor
    -> where salary not between 90000 and 100000;
+------------+
| name       |
+------------+
| Srinivasan |
| Mozart     |
| El Said    |
| Gold       |
| Katz       |
| Califieri  |
| Singh      |
| Crick      |
| Kim        |
+------------+
9 rows in set (0.00 sec)

where 子句还支持在元组上进行比较，考虑查询，查找 Biology 系讲授了课程的所有教师的姓名和他们所讲授的课程

mysql> select name, course_id
    -> from instructor, teaches
    -> where (instructor.ID, dept_name) = (teaches.ID, ‘Biology‘);
+-------+-----------+
| name  | course_id |
+-------+-----------+
| Crick | BIO-101   |
| Crick | BIO-301   |
+-------+-----------+
2 rows in set (0.00 sec)