基於 MySQL 的數據庫實踐(擴展運算)
阿新 • • 發佈:2018-04-14
之間 art history 上進 簡化 source 希望 AC 顯示
select 中的通配符
星號 *
可以用在 select
子句中表示所有的屬性。
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 中所有的屬性。
元組的排序
SQL 為用戶提供了一些結果顯示的順序的控制,order by
子句可以讓查詢結果中元組按升序顯示,考慮按字典序列出 Physics 系的所有教師。
mysql> select name
-> from instructor
-> where dept_name = ‘Physics‘
-> order by name;
+----------+
| name |
+----------+
| Einstein |
| Gold |
+----------+
2 rows in set (0.00 sec)
為了使用降序,我們可以用 desc
表示降序,完整地說,可以用 asc
表示升序,此外,排序可以在多個屬性上進行,例如我們希望按 salary 的降序列出整個 instructor 關系,如果有幾名教師工資相同,則按姓名升序排列。
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 子句謂詞
為了簡化 where
子句,SQL 提供 between
運算符來說明一個值落在一個閉區間內。
考慮查詢,找出工資在 90000 美元和 100000 美元之間的教師的姓名,直觀地有
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
運算符改寫為
mysql> select name
-> from instructor
-> where salary between 90000 and 100000;
+----------+
| name |
+----------+
| Wu |
| Einstein |
| Brandt |
+----------+
3 rows in set (0.01 sec)
類似的有 not between
運算符
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
子句還支持在元組上進行比較,考慮查詢,查找 Biology 系講授了課程的所有教師的姓名和他們所講授的課程
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)
基於 MySQL 的數據庫實踐(擴展運算)