读书笔记--SQL必知必会10--分组数据

10.1 数据分组

使用分组可以将数据分为多个逻辑组，对每个组进行聚集计算。

10.2 创建分组

使用SELECT语句的GROUP BY子句建立分组。

• GROUP BY子句必须出现在WHERE之后，ORDER BY子句之前。
• GROUP BY子句可以包含任意数目的列，可以对分组进行嵌套。
• 如果GUOUP BY子句中嵌套了分组，数据将在最后指定的分组上进行汇总。
• GUOUP BY子句中的列必须是实际有效的检索列或表达式，不能使用别名。
• 除聚集计算语句外，SELECT语句中的每一列都必须在GROUP BY子句中给出。
• 一行或多行NULL值，将作为一个分组返回。

某些DBMS允许根据SELECT列表中的相对位置指定列，但不建议使用，容易导致编辑SQL语句时出错。

MariaDB [sqlbzbh]> SELECT * FROM Products;
+---------+---------+---------------------+------------+-----------------------------------------------------------------------+
| prod_id | vend_id | prod_name           | prod_price | prod_desc                                                             |
+---------+---------+---------------------+------------+-----------------------------------------------------------------------+
| BNBG01  | DLL01   | Fish bean bag toy   |       3.49 | Fish bean bag toy, complete with bean bag worms with which to feed it |
| BNBG02  | DLL01   | Bird bean bag toy   |       3.49 | Bird bean bag toy, eggs are not included                              |
| BNBG03  | DLL01   | Rabbit bean bag toy |       3.49 | Rabbit bean bag toy, comes with bean bag carrots                      |
| BR01    | BRS01   | 8 inch teddy bear   |       5.99 | 8 inch teddy bear, comes with cap and jacket                          |
| BR02    | BRS01   | 12 inch teddy bear  |       8.99 | 12 inch teddy bear, comes with cap and jacket                         |
| BR03    | BRS01   | 18 inch teddy bear  |      11.99 | 18 inch teddy bear, comes with cap and jacket                         |
| RGAN01  | DLL01   | Raggedy Ann         |       4.99 | 18 inch Raggedy Ann doll                                              |
| RYL01   | FNG01   | King doll           |       9.49 | 12 inch king doll with royal garments and crown                       |
| RYL02   | FNG01   | Queen doll          |       9.49 | 12 inch queen doll with royal garments and crown                      |
+---------+---------+---------------------+------------+-----------------------------------------------------------------------+
9 rows in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT vend_id, COUNT(*) AS num_prods FROM Products GROUP BY vend_id;
+---------+-----------+
| vend_id | num_prods |
+---------+-----------+
| BRS01   |         3 |
| DLL01   |         4 |
| FNG01   |         2 |
+---------+-----------+
3 rows in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT vend_id, COUNT(*) AS num_prods FROM Products GROUP BY 1;
+---------+-----------+
| vend_id | num_prods |
+---------+-----------+
| BRS01   |         3 |
| DLL01   |         4 |
| FNG01   |         2 |
+---------+-----------+
3 rows in set (0.00 sec)

MariaDB [sqlbzbh]>

10.3 过滤分组

使用HAVING子句可以过滤分组。

相比之下，WHERE子句过滤指定的是行而不是分组。也可以理解为: WHERE在数据分组前进行过滤，HAVING在数据分组后进行过滤。

HAVING子句支持所有WHERE操作符，也就是说有关WHERE的句法都适用于HAVING，只是关键字可能有差别。

MariaDB [sqlbzbh]> SELECT * FROM Orders;
+-----------+---------------------+------------+
| order_num | order_date          | cust_id    |
+-----------+---------------------+------------+
|     20005 | 2012-05-01 00:00:00 | 1000000001 |
|     20006 | 2012-01-12 00:00:00 | 1000000003 |
|     20007 | 2012-01-30 00:00:00 | 1000000004 |
|     20008 | 2012-02-03 00:00:00 | 1000000005 |
|     20009 | 2012-02-08 00:00:00 | 1000000001 |
+-----------+---------------------+------------+
5 rows in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT cust_id, COUNT(*) AS orders FROM Orders GROUP BY cust_id;
+------------+--------+
| cust_id    | orders |
+------------+--------+
| 1000000001 |      2 |
| 1000000003 |      1 |
| 1000000004 |      1 |
| 1000000005 |      1 |
+------------+--------+
4 rows in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT cust_id, COUNT(*) AS orders FROM Orders GROUP BY cust_id HAVING COUNT(*) >=2;
+------------+--------+
| cust_id    | orders |
+------------+--------+
| 1000000001 |      2 |
+------------+--------+
1 row in set (0.00 sec)

MariaDB [sqlbzbh]>

同时使用WHERE子句和HAVING子句

MariaDB [sqlbzbh]> SELECT * FROM Products;
+---------+---------+---------------------+------------+-----------------------------------------------------------------------+
| prod_id | vend_id | prod_name           | prod_price | prod_desc                                                             |
+---------+---------+---------------------+------------+-----------------------------------------------------------------------+
| BNBG01  | DLL01   | Fish bean bag toy   |       3.49 | Fish bean bag toy, complete with bean bag worms with which to feed it |
| BNBG02  | DLL01   | Bird bean bag toy   |       3.49 | Bird bean bag toy, eggs are not included                              |
| BNBG03  | DLL01   | Rabbit bean bag toy |       3.49 | Rabbit bean bag toy, comes with bean bag carrots                      |
| BR01    | BRS01   | 8 inch teddy bear   |       5.99 | 8 inch teddy bear, comes with cap and jacket                          |
| BR02    | BRS01   | 12 inch teddy bear  |       8.99 | 12 inch teddy bear, comes with cap and jacket                         |
| BR03    | BRS01   | 18 inch teddy bear  |      11.99 | 18 inch teddy bear, comes with cap and jacket                         |
| RGAN01  | DLL01   | Raggedy Ann         |       4.99 | 18 inch Raggedy Ann doll                                              |
| RYL01   | FNG01   | King doll           |       9.49 | 12 inch king doll with royal garments and crown                       |
| RYL02   | FNG01   | Queen doll          |       9.49 | 12 inch queen doll with royal garments and crown                      |
+---------+---------+---------------------+------------+-----------------------------------------------------------------------+
9 rows in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT vend_id, COUNT(*) AS num_prods FROM Products GROUP BY vend_id HAVING COUNT(*) >=2;
+---------+-----------+
| vend_id | num_prods |
+---------+-----------+
| BRS01   |         3 |
| DLL01   |         4 |
| FNG01   |         2 |
+---------+-----------+
3 rows in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT vend_id, COUNT(*) AS num_prods FROM Products WHERE prod_price >= 4 GROUP BY vend_id HAVING COUNT(*) >=2;
+---------+-----------+
| vend_id | num_prods |
+---------+-----------+
| BRS01   |         3 |
| FNG01   |         2 |
+---------+-----------+
2 rows in set (0.00 sec)

MariaDB [sqlbzbh]>

10.4 分组和排序

对比GROUP BY 与 ORDER BY

GROUP BY	OREDR BY
输出可能不是分组的顺序	对产生的输出排序
只能使用选择列或表达式列，而且必须使用每个选择列表达式	任意列都可以使用，包括非选择列
必须使用与聚集函数一起的列或表达式	不一定需要

GROUP BY子句的分组数据输出并不一定是顺序的，输出的顺序也不一定满足实际的需要，所以千万不要仅仅依赖GROUP BY排序数据。

一般在使用GROUP BY子句的同时也使用ORDER BY子句，保证数据正确排序，满足实际需要。

MariaDB [sqlbzbh]> SELECT * FROM OrderItems;
+-----------+------------+---------+----------+------------+
| order_num | order_item | prod_id | quantity | item_price |
+-----------+------------+---------+----------+------------+
|     20005 |          1 | BR01    |      100 |       5.49 |
|     20005 |          2 | BR03    |      100 |      10.99 |
|     20006 |          1 | BR01    |       20 |       5.99 |
|     20006 |          2 | BR02    |       10 |       8.99 |
|     20006 |          3 | BR03    |       10 |      11.99 |
|     20007 |          1 | BR03    |       50 |      11.49 |
|     20007 |          2 | BNBG01  |      100 |       2.99 |
|     20007 |          3 | BNBG02  |      100 |       2.99 |
|     20007 |          4 | BNBG03  |      100 |       2.99 |
|     20007 |          5 | RGAN01  |       50 |       4.49 |
|     20008 |          1 | RGAN01  |        5 |       4.99 |
|     20008 |          2 | BR03    |        5 |      11.99 |
|     20008 |          3 | BNBG01  |       10 |       3.49 |
|     20008 |          4 | BNBG02  |       10 |       3.49 |
|     20008 |          5 | BNBG03  |       10 |       3.49 |
|     20009 |          1 | BNBG01  |      250 |       2.49 |
|     20009 |          2 | BNBG02  |      250 |       2.49 |
|     20009 |          3 | BNBG03  |      250 |       2.49 |
+-----------+------------+---------+----------+------------+
18 rows in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT order_num, COUNT(*) AS items FROM OrderItems GROUP BY order_num;
+-----------+-------+
| order_num | items |
+-----------+-------+
|     20005 |     2 |
|     20006 |     3 |
|     20007 |     5 |
|     20008 |     5 |
|     20009 |     3 |
+-----------+-------+
5 rows in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT order_num, COUNT(*) AS items FROM OrderItems GROUP BY order_num HAVING COUNT(*) >= 3;
+-----------+-------+
| order_num | items |
+-----------+-------+
|     20006 |     3 |
|     20007 |     5 |
|     20008 |     5 |
|     20009 |     3 |
+-----------+-------+
4 rows in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT order_num, COUNT(*) AS items FROM OrderItems GROUP BY order_num HAVING COUNT(*) >= 3 ORDER BY items, order_num;
+-----------+-------+
| order_num | items |
+-----------+-------+
|     20006 |     3 |
|     20009 |     3 |
|     20007 |     5 |
|     20008 |     5 |
+-----------+-------+
4 rows in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT order_num, COUNT(*) AS items FROM OrderItems GROUP BY order_num HAVING COUNT(*) >= 3 ORDER BY items;
+-----------+-------+
| order_num | items |
+-----------+-------+
|     20006 |     3 |
|     20009 |     3 |
|     20007 |     5 |
|     20008 |     5 |
+-----------+-------+
4 rows in set (0.01 sec)

MariaDB [sqlbzbh]>

10.5 SELECT子句顺序

在SELECT语句中使用子句时，必须遵循的次序。

子句	说明	是否必须使用
SELECT	要返回的列或表达式	是
FROM	从表中检索数据	仅在从表选择数据时使用
WHERE	行过滤	否
GROUP BY	分组	仅在按组计算聚集时使用
HAVING	组过滤	否
ORDER BY	输出排序	否