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 |