读书笔记--SQL必知必会09--汇总数据

9.1 聚集函数

聚集函数（aggregate function），对某些行运行的函数，计算并返回一个值。

使用聚集函数可以汇总数据而不必将涉及的数据实际检索出来。

可利用标准的算术操作符，实现更高级的汇总。

AVG() --- 返回某列的平均值
COUNT() --- 返回某列的行数
MAX() --- 返回某列的最大值
MIN() --- 返回某列的最小值
SUM() --- 返回某列值之和

9.1.1 AVG()函数

AVG()函数可用来返回特定列或行的平均值。

也可以返回多个列的平均值，此时必须使用多个AVG()函数。

AVG()函数忽略列值为NULL的行。

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]> SELECT AVG(prod_price) AS avg_price FROM Products;
+-----------+
| avg_price |
+-----------+
|  6.823333 |
+-----------+
1 row in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT AVG(prod_price) AS avg_price FROM Products WHERE vend_id = 'DLL01';
+-----------+
| avg_price |
+-----------+
|  3.865000 |
+-----------+
1 row in set (0.00 sec)

MariaDB [sqlbzbh]>

9.1.2 COUNT()函数

可利用COUNT()函数获取表中行的数目或者符合特定条件的行的数目。

• COUNT(*)统计表中行的数目，包括NULL的行。
• COUNT(column)统计特定列中的行的数目，忽略NULL的行。

MariaDB [sqlbzbh]> SELECT * FROM Customers;
+------------+---------------+----------------------+-----------+------------+----------+--------------+--------------------+-----------------------+
| cust_id    | cust_name     | cust_address         | cust_city | cust_state | cust_zip | cust_country | cust_contact       | cust_email            |
+------------+---------------+----------------------+-----------+------------+----------+--------------+--------------------+-----------------------+
| 1000000001 | Village Toys  | 200 Maple Lane       | Detroit   | MI         | 44444    | USA          | John Smith         | sales@villagetoys.com |
| 1000000002 | Kids Place    | 333 South Lake Drive | Columbus  | OH         | 43333    | USA          | Michelle Green     | NULL                  |
| 1000000003 | Fun4All       | 1 Sunny Place        | Muncie    | IN         | 42222    | USA          | Jim Jones          | jjones@fun4all.com    |
| 1000000004 | Fun4All       | 829 Riverside Drive  | Phoenix   | AZ         | 88888    | USA          | Denise L. Stephens | dstephens@fun4all.com |
| 1000000005 | The Toy Store | 4545 53rd Street     | Chicago   | IL         | 54545    | USA          | Kim Howard         | NULL                  |
+------------+---------------+----------------------+-----------+------------+----------+--------------+--------------------+-----------------------+
5 rows in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT COUNT(*) AS num_cust FROM Customers;
+----------+
| num_cust |
+----------+
|        5 |
+----------+
1 row in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT COUNT(cust_email) AS num_cust FROM Customers;
+----------+
| num_cust |
+----------+
|        3 |
+----------+
1 row in set (0.00 sec)

MariaDB [sqlbzbh]>

9.1.3 MAX()函数

MAX()函数一般用来找出最大的数值或日期值。必须指定列名。

在用于文本数据时，则是返回该列排序后的最后一行。

MAX()函数忽略列值为NULL的行。

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]> SELECT MAX(prod_price) AS max_price FROM Products;
+-----------+
| max_price |
+-----------+
|     11.99 |
+-----------+
1 row in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT MAX(prod_name) AS max_prod_name FROM Products;
+---------------+
| max_prod_name |
+---------------+
| Raggedy Ann   |
+---------------+
1 row in set (0.00 sec)

MariaDB [sqlbzbh]>

9.1.4 MIN()函数

MIN()函数返回最小的数值或日期值。必须指定列名。

在用于文本数据时，则是返回该列排序后的最前面的行。

MIN()函数忽略列值为NULL的行。

MariaDB [sqlbzbh]> SELECT MIN(prod_price) AS min_price FROM Products;
+-----------+
| min_price |
+-----------+
|      3.49 |
+-----------+
1 row in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT MIN(prod_name) AS min_prod_name FROM Products;
+--------------------+
| min_prod_name      |
+--------------------+
| 12 inch teddy bear |
+--------------------+
1 row in set (0.00 sec)

MariaDB [sqlbzbh]>

9.1.5 SUM()函数

SUM()函数用来返回指定列值的和。

SUM()函数忽略列值为NULL的行。

MariaDB [sqlbzbh]> SELECT * FROM OrderItems WHERE order_num = 20005;
+-----------+------------+---------+----------+------------+
| order_num | order_item | prod_id | quantity | item_price |
+-----------+------------+---------+----------+------------+
|     20005 |          1 | BR01    |      100 |       5.49 |
|     20005 |          2 | BR03    |      100 |      10.99 |
+-----------+------------+---------+----------+------------+
2 rows in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT SUM(quantity) AS items_ordered FROM OrderItems WHERE order_num = 20005;
+---------------+
| items_ordered |
+---------------+
|           200 |
+---------------+
1 row in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT SUM(item_price*quantity) AS total_price FROM OrderItems WHERE order_num = 20005;
+-------------+
| total_price |
+-------------+
|     1648.00 |
+-------------+
1 row in set (0.00 sec)

MariaDB [sqlbzbh]>

9.2 聚集不同值

对以上5个聚集函数都可以如下使用：

• 对所有行执行计算，指定ALL参数或不指定参数（ALL参数是默认行为）
• 只包含不同的值，指定DISTINCT参数。

MariaDB [sqlbzbh]> SELECT AVG(prod_price) AS avg_price FROM Products WHERE vend_id = 'DLL01';
+-----------+
| avg_price |
+-----------+
|  3.865000 |
+-----------+
1 row in set (0.00 sec)

MariaDB [sqlbzbh]>
MariaDB [sqlbzbh]> SELECT AVG(DISTINCT prod_price) AS avg_price FROM Products WHERE vend_id = 'DLL01';
+-----------+
| avg_price |
+-----------+
|  4.240000 |
+-----------+
1 row in set (0.00 sec)

MariaDB [sqlbzbh]>

9.3 组合聚集函数

SELECT语句可根据需要包含多个聚集函数。

注意：指定别名，不应该使用表中实际的列名。

MariaDB [sqlbzbh]> SELECT COUNT(*) AS num_items, MIN(prod_price) AS price_min, MAX(prod_price) AS price_max, AVG(prod_price) AS avg_price FROM Products;
+-----------+-----------+-----------+-----------+
| num_items | price_min | price_max | avg_price |
+-----------+-----------+-----------+-----------+
|         9 |      3.49 |     11.99 |  6.823333 |
+-----------+-----------+-----------+-----------+
1 row in set (0.00 sec)

MariaDB [sqlbzbh]>