读书笔记--SQL必知必会03--排序检索数据

3.1 排序数据

子句（clause）
SQL语句由子句构成。一个子句通常由一个关键字加上所提供的数据组成。
ORDER BY子句可以取一个或多个列的名字，将SELECT语句检索出的数据进行排序。
ORDER BY子句可以使用非检索的列排序数据。
ORDER BY子句必须作为SELECT语句中最后一条子句。

MariaDB [sqlbzbh]> SELECT prod_name FROM Products ORDER BY prod_name;
+---------------------+
| prod_name           |
+---------------------+
| 12 inch teddy bear  |
| 18 inch teddy bear  |
| 8 inch teddy bear   |
| Bird bean bag toy   |
| Fish bean bag toy   |
| King doll           |
| Queen doll          |
| Rabbit bean bag toy |
| Raggedy Ann         |
+---------------------+
9 rows in set (0.00 sec)MariaDB [sqlbzbh]> 
MariaDB [sqlbzbh]> SELECT prod_name FROM Products ORDER BY vend_id;
+---------------------+
| prod_name           |
+---------------------+
| 8 inch teddy bear   |
| 12 inch teddy bear  |
| 18 inch teddy bear  |
| Fish bean bag toy   |
| Bird bean bag toy   |
| Rabbit bean bag toy |
| Raggedy Ann         |
| King doll           |
| Queen doll          |
+---------------------+
9 rows in set (0.00 sec)MariaDB [sqlbzbh]> 

3.2 按多个列排序

要按多个列排序，简单指定列名，列名之间用逗号分开即可。
在按多个列排序时，排序的顺序完全按规定进行。也就是说，仅在前一列的多个行具有相同的值时，才按照下一列进行排序。

MariaDB [sqlbzbh]> SELECT prod_id,prod_price,prod_name FROM Products ORDER BY prod_price,prod_name;
+---------+------------+---------------------+
| prod_id | prod_price | prod_name           |
+---------+------------+---------------------+
| BNBG02  |       3.49 | Bird bean bag toy   |
| BNBG01  |       3.49 | Fish bean bag toy   |
| BNBG03  |       3.49 | Rabbit bean bag toy |
| RGAN01  |       4.99 | Raggedy Ann         |
| BR01    |       5.99 | 8 inch teddy bear   |
| BR02    |       8.99 | 12 inch teddy bear  |
| RYL01   |       9.49 | King doll           |
| RYL02   |       9.49 | Queen doll          |
| BR03    |      11.99 | 18 inch teddy bear  |
+---------+------------+---------------------+
9 rows in set (0.00 sec)MariaDB [sqlbzbh]> 

3.3 按列位置排序

ORDER BY支持按相对列位置进行排序。
优点是不用重新输入列名，缺点是容易造成错误排序。个人不建议使用。
如果有必要，可以混合使用实际列名和相对列位置。

MariaDB [sqlbzbh]> SELECT prod_id,prod_price,prod_name FROM Products ORDER BY 2,3;
+---------+------------+---------------------+
| prod_id | prod_price | prod_name           |
+---------+------------+---------------------+
| BNBG02  |       3.49 | Bird bean bag toy   |
| BNBG01  |       3.49 | Fish bean bag toy   |
| BNBG03  |       3.49 | Rabbit bean bag toy |
| RGAN01  |       4.99 | Raggedy Ann         |
| BR01    |       5.99 | 8 inch teddy bear   |
| BR02    |       8.99 | 12 inch teddy bear  |
| RYL01   |       9.49 | King doll           |
| RYL02   |       9.49 | Queen doll          |
| BR03    |      11.99 | 18 inch teddy bear  |
+---------+------------+---------------------+
9 rows in set (0.00 sec)MariaDB [sqlbzbh]> 

3.4 制定排序方向

默认的排序顺序是升序排序（从A到Z）。
可以使用ORDER BY子句进行降序排序（从Z到A），但必须指定DESC关键字。
DESC关键字只应用到直接位于其前面的列名。如果想在多个列上进行降序排序，必须对每一列指定DESC关键字。
DESC是DESCENDING的缩写，这两个关键字都可以使用。
ASC是ASCENDING的缩写，因为升序是默认的，所以不必指定关键字。

MariaDB [sqlbzbh]> SELECT prod_id,prod_price,prod_name FROM Products ORDER BY prod_price;
+---------+------------+---------------------+
| prod_id | prod_price | prod_name           |
+---------+------------+---------------------+
| BNBG01  |       3.49 | Fish bean bag toy   |
| BNBG02  |       3.49 | Bird bean bag toy   |
| BNBG03  |       3.49 | Rabbit bean bag toy |
| RGAN01  |       4.99 | Raggedy Ann         |
| BR01    |       5.99 | 8 inch teddy bear   |
| BR02    |       8.99 | 12 inch teddy bear  |
| RYL01   |       9.49 | King doll           |
| RYL02   |       9.49 | Queen doll          |
| BR03    |      11.99 | 18 inch teddy bear  |
+---------+------------+---------------------+
9 rows in set (0.00 sec)MariaDB [sqlbzbh]>  
MariaDB [sqlbzbh]> SELECT prod_id,prod_price,prod_name FROM Products ORDER BY prod_price DESC;
+---------+------------+---------------------+
| prod_id | prod_price | prod_name           |
+---------+------------+---------------------+
| BR03    |      11.99 | 18 inch teddy bear  |
| RYL01   |       9.49 | King doll           |
| RYL02   |       9.49 | Queen doll          |
| BR02    |       8.99 | 12 inch teddy bear  |
| BR01    |       5.99 | 8 inch teddy bear   |
| RGAN01  |       4.99 | Raggedy Ann         |
| BNBG01  |       3.49 | Fish bean bag toy   |
| BNBG02  |       3.49 | Bird bean bag toy   |
| BNBG03  |       3.49 | Rabbit bean bag toy |
+---------+------------+---------------------+
9 rows in set (0.00 sec)MariaDB [sqlbzbh]> 
MariaDB [sqlbzbh]> SELECT prod_id,prod_price,prod_name FROM Products ORDER BY prod_price DESC, prod_name;
+---------+------------+---------------------+
| prod_id | prod_price | prod_name           |
+---------+------------+---------------------+
| BR03    |      11.99 | 18 inch teddy bear  |
| RYL01   |       9.49 | King doll           |
| RYL02   |       9.49 | Queen doll          |
| BR02    |       8.99 | 12 inch teddy bear  |
| BR01    |       5.99 | 8 inch teddy bear   |
| RGAN01  |       4.99 | Raggedy Ann         |
| BNBG02  |       3.49 | Bird bean bag toy   |
| BNBG01  |       3.49 | Fish bean bag toy   |
| BNBG03  |       3.49 | Rabbit bean bag toy |
+---------+------------+---------------------+
9 rows in set (0.00 sec)MariaDB [sqlbzbh]>