《MySQL必知必会》—— 4. 检索数据 SELECT 5.排序检索数据

 

目录

四、检索数据

检索单个列

检索多个列

检索所有列

检索不同的行 DISTINCT

限制结果 LIMIT

使用完全限定的表名

五、排序检索数据

排序数据 ORDER BY

按多个列排序

指定排序方向 DESC(降序)、ASC(升序)

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四、检索数据

使用SELECT检索表数据，必须知道给出两条信息——想选择什么（列），从什么地方选择（表）

检索单个列

SELECT `prod_name` FROM `products`;

mysql> SELECT `prod_name` FROM `products`;
+----------------+
| prod_name      |
+----------------+
| .5 ton anvil   |
| 1 ton anvil    |
| 2 ton anvil    |
| Detonator      |
| Bird seed      |
| Carrots        |
| Fuses          |
| JetPack 1000   |
| JetPack 2000   |
| Oil can        |
| Safe           |
| Sling          |
| TNT (1 stick)  |
| TNT (5 sticks) |
+----------------+
14 rows in set (0.00 sec)

ps. 如果不明确排序查询结果，则返回的数据的顺序没有特殊意义。可能是添加时的顺序，也可能不是。
另外，SQL语句不区分大小写，但是一般SQL关键字大写，列和表名小写，便于阅读和调试。
SQL也不在意空格，但是应该使用空格或多行以便于阅读调试。

检索多个列

mysql> SELECT `prod_name`, `prod_name`, `prod_price` FROM `products`;
+----------------+----------------+------------+
| prod_name      | prod_name      | prod_price |
+----------------+----------------+------------+
| .5 ton anvil   | .5 ton anvil   |       5.99 |
| 1 ton anvil    | 1 ton anvil    |       9.99 |
| 2 ton anvil    | 2 ton anvil    |      14.99 |
| Detonator      | Detonator      |      13.00 |
| Bird seed      | Bird seed      |      10.00 |
| Carrots        | Carrots        |       2.50 |
| Fuses          | Fuses          |       3.42 |
| JetPack 1000   | JetPack 1000   |      35.00 |
| JetPack 2000   | JetPack 2000   |      55.00 |
| Oil can        | Oil can        |       8.99 |
| Safe           | Safe           |      50.00 |
| Sling          | Sling          |       4.49 |
| TNT (1 stick)  | TNT (1 stick)  |       2.50 |
| TNT (5 sticks) | TNT (5 sticks) |      10.00 |
+----------------+----------------+------------+
14 rows in set (0.00 sec)

检索所有列

mysql> SELECT * FROM `products`;
+---------+---------+----------------+------------+----------------------------------------------------------------+
| prod_id | vend_id | prod_name      | prod_price | prod_desc                                                      |
+---------+---------+----------------+------------+----------------------------------------------------------------+
| ANV01   |    1001 | .5 ton anvil   |       5.99 | .5 ton anvil, black, complete with handy hook                  |
| ANV02   |    1001 | 1 ton anvil    |       9.99 | 1 ton anvil, black, complete with handy hook and carrying case |
| ANV03   |    1001 | 2 ton anvil    |      14.99 | 2 ton anvil, black, complete with handy hook and carrying case |
| DTNTR   |    1003 | Detonator      |      13.00 | Detonator (plunger powered), fuses not included                |
| FB      |    1003 | Bird seed      |      10.00 | Large bag (suitable for road runners)                          |
| FC      |    1003 | Carrots        |       2.50 | Carrots (rabbit hunting season only)                           |
| FU1     |    1002 | Fuses          |       3.42 | 1 dozen, extra long                                            |
| JP1000  |    1005 | JetPack 1000   |      35.00 | JetPack 1000, intended for single use                          |
| JP2000  |    1005 | JetPack 2000   |      55.00 | JetPack 2000, multi-use                                        |
| OL1     |    1002 | Oil can        |       8.99 | Oil can, red                                                   |
| SAFE    |    1003 | Safe           |      50.00 | Safe with combination lock                                     |
| SLING   |    1003 | Sling          |       4.49 | Sling, one size fits all                                       |
| TNT1    |    1003 | TNT (1 stick)  |       2.50 | TNT, red, single stick                                         |
| TNT2    |    1003 | TNT (5 sticks) |      10.00 | TNT, red, pack of 10 sticks                                    |
+---------+---------+----------------+------------+----------------------------------------------------------------+
14 rows in set (0.00 sec)

ps. 除了检索未知列的情况，或者确实需要每个列，否则最好别用 * 通配符（影响性能）

检索不同的行 DISTINCT

mysql> SELECT `vend_id` FROM `products`;
+---------+
| vend_id |
+---------+
|    1001 |
|    1001 |
|    1001 |
|    1002 |
|    1002 |
|    1003 |
|    1003 |
|    1003 |
|    1003 |
|    1003 |
|    1003 |
|    1003 |
|    1005 |
|    1005 |
+---------+
14 rows in set (0.00 sec)

mysql> SELECT DISTINCT `vend_id` FROM `products`;
+---------+
| vend_id |
+---------+
|    1001 |
|    1002 |
|    1003 |
|    1005 |
+---------+
4 rows in set (0.00 sec)

ps. 不能部分使用DOSTINCT。该关键字应用于所有的列，而不仅仅是前置它的列。select多个列则选出组合不同的列。

mysql> SELECT DISTINCT `vend_id`, `prod_price` FROM `products`;
+---------+------------+
| vend_id | prod_price |
+---------+------------+
|    1001 |       5.99 |
|    1001 |       9.99 |
|    1001 |      14.99 |
|    1003 |      13.00 |
|    1003 |      10.00 |
|    1003 |       2.50 |
|    1002 |       3.42 |
|    1005 |      35.00 |
|    1005 |      55.00 |
|    1002 |       8.99 |
|    1003 |      50.00 |
|    1003 |       4.49 |
+---------+------------+
12 rows in set (0.00 sec)

限制结果 LIMIT

LIMIT 5; 返回结果不多于5行
LIMIT 5, 5; 第一个数为开始的位置，第二个数为要检索的行数。（只有一个值的时候默认从0行开始）
LIMIT 4 OFFSET 3; MySQL5的新写法，从行3开始取4行。= LIMIT 3, 4;

mysql> SELECT `prod_name`
    -> FROM `products`
    -> LIMIT 5;
+--------------+
| prod_name    |
+--------------+
| .5 ton anvil |
| 1 ton anvil  |
| 2 ton anvil  |
| Detonator    |
| Bird seed    |
+--------------+
5 rows in set (0.00 sec)

mysql> SELECT `prod_name`
    -> FROM `products`
    -> LIMIT 5,5;
+--------------+
| prod_name    |
+--------------+
| Carrots      |
| Fuses        |
| JetPack 1000 |
| JetPack 2000 |
| Oil can      |
+--------------+
5 rows in set (0.00 sec)

使用完全限定的表名

1.FROM后的表名加上指定数据库
2.SELECT后的列指定表名

SELECT `products`.`prod_name`
FROM `crashcourse`.`products`;

五、排序检索数据

排序数据 ORDER BY

一般检索出来的数据会按照添加到表中的顺序来显示，但是当进行过更新或删除，数据会受到MySQL重用回收存储空间的影响。所以不应该依赖于这个默认的顺序。

mysql> SELECT `prod_name`
    -> FROM `products`
    -> ORDER BY `prod_name`;
+----------------+
| prod_name      |
+----------------+
| .5 ton anvil   |
| 1 ton anvil    |
| 2 ton anvil    |
| Bird seed      |
| Carrots        |
| Detonator      |
| Fuses          |
| JetPack 1000   |
| JetPack 2000   |
| Oil can        |
| Safe           |
| Sling          |
| TNT (1 stick)  |
| TNT (5 sticks) |
+----------------+
14 rows in set (0.00 sec)

ps. 一般排序基准列都是被选择的、但可以使用非检索列作为排序的基准列。

按多个列排序

mysql> SELECT `prod_id`, `prod_price`, `prod_name`
    -> FROM `products`
    -> ORDER BY `prod_price`,`prod_name`;
+---------+------------+----------------+
| prod_id | prod_price | prod_name      |
+---------+------------+----------------+
| FC      |       2.50 | Carrots        |
| TNT1    |       2.50 | TNT (1 stick)  |
| FU1     |       3.42 | Fuses          |
| SLING   |       4.49 | Sling          |
| ANV01   |       5.99 | .5 ton anvil   |
| OL1     |       8.99 | Oil can        |
| ANV02   |       9.99 | 1 ton anvil    |
| FB      |      10.00 | Bird seed      |
| TNT2    |      10.00 | TNT (5 sticks) |
| DTNTR   |      13.00 | Detonator      |
| ANV03   |      14.99 | 2 ton anvil    |
| JP1000  |      35.00 | JetPack 1000   |
| SAFE    |      50.00 | Safe           |
| JP2000  |      55.00 | JetPack 2000   |
+---------+------------+----------------+
14 rows in set (0.01 sec)

ps. 首先按第一个字段排序，在第一个字段值相同的情况下，按下一个字段排序。如果第一个字段的值都不相同，第二字段的存在没有意义。

指定排序方向 DESC(降序)、ASC(升序)

mysql> SELECT `prod_id`, `prod_price`, `prod_name`
    -> FROM `products`
    -> ORDER BY `prod_price` DESC,`prod_name`;
+---------+------------+----------------+
| prod_id | prod_price | prod_name      |
+---------+------------+----------------+
| JP2000  |      55.00 | JetPack 2000   |
| SAFE    |      50.00 | Safe           |
| JP1000  |      35.00 | JetPack 1000   |
| ANV03   |      14.99 | 2 ton anvil    |
| DTNTR   |      13.00 | Detonator      |
| FB      |      10.00 | Bird seed      |
| TNT2    |      10.00 | TNT (5 sticks) |
| ANV02   |       9.99 | 1 ton anvil    |
| OL1     |       8.99 | Oil can        |
| ANV01   |       5.99 | .5 ton anvil   |
| SLING   |       4.49 | Sling          |
| FU1     |       3.42 | Fuses          |
| FC      |       2.50 | Carrots        |
| TNT1    |       2.50 | TNT (1 stick)  |
+---------+------------+----------------+
14 rows in set (0.00 sec)

ps. 
默认就是升序，所以不用写ASC
DESC关键字只作用于其前面那一个列。
ORDER BY 结合 LIMIT 可以得到某列中最低/最高值，但是ORDER BY要在FROM之后，LIMIT要在ORDER BY之后。

mysql> SELECT `prod_id`, `prod_price`, `prod_name`
    -> FROM `products`
    -> ORDER BY `prod_price` DESC
    -> LIMIT 1;
+---------+------------+--------------+
| prod_id | prod_price | prod_name    |
+---------+------------+--------------+
| JP2000  |      55.00 | JetPack 2000 |
+---------+------------+--------------+
1 row in set (0.00 sec)

mysql> SELECT `prod_id`, `prod_price`, `prod_name`
    -> FROM `products`
    -> LIMIT 1
    -> ORDER BY `prod_price` DESC;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'ORDER BY `prod_price` DESC' at line 4
mysql>