当前位置：首页 -> AI编程基础 -> 数据库编程

MySQL学习足迹记录10--汇总数据--MAX(),MIN(),AVG(),SUM(),COUNT()(一)

2014-11-24 17:06:30 · 作者: · 浏览: 4
标签: MySQL 学习 足迹 记录 10-- 汇总 数据 --MAX MIN AVG SUM COUNT
MySQL学习足迹记录10--汇总数据--MAX(),MIN(),AVG(),SUM(),COUNT()
本文所用到的数据 
mysql> SELECT prod_price FROM products;
+------------+
| prod_price |
+------------+
|       5.99 |
|       9.99 |
|      14.99 |
|      13.00 |
|      10.00 |
|       2.50 |
|       3.42 |
|      35.00 |
|      55.00 |
|       8.99 |
|      50.00 |
|       4.49 |
|       2.50 |
|      10.00 |
+------------+
14 rows in set (0.00 sec)

1.聚集函数
AVG(): 返回某列的平均值
COUNT(): 返回会某列的行数
MAX(): 返回会某列的最大值
MIN(): 返回会某列的最小值
SUM(): 返回会某列值之和
2.AVG()函数 
Examples:
mysql> SELECT AVG(prod_price) AS avg_price
         -> FROM products;
+-----------+
| avg_price |
+-----------+
| 16.133571 |
+-----------+
1 row in set (0.01 sec)

*返回特定列或行的平均值
 Examples: 
   mysql> SELECT AVG(prod_price) AS avg_price        #过滤出vend_id为1003的产品，再求平均值
            -> FROM products
           -> WHERE vend_id = 1003;
+-----------+
| avg_price |
+-----------+
| 13.212857 |
+-----------+
1 row in set (0.00 sec)

Tips:
AVG()只能用来求特定数值列的平均值，为了获得多个列的平均值，必须使用多个AVG()函数
AVG()函数忽略列值为NULL的行
3.COUNT()函数
*COUNT（*）对表中行的数目进行计数，不管列标中包含的是空值（NULL）还是非空值
*COUNT（column)对特定的列中具有值的行进行计数，忽略NULL值 
 Examples:
   mysql> select COUNT(*) AS count_prod from products;
+------------+                            #products表中行的数目进行计数
| count_prod |
+------------+
|         14 |
+------------+
1 row in set (0.00 sec)


先列出cust_email的内容
mysql> SELECT cust_email FROM customers;
+---------------------+
| cust_email          |
+---------------------+
| ylee@coyote.com     |
| NULL                |
| rabbit@wascally.com |
| sam@yosemite.com    |
| NULL                |
+---------------------+
5 rows in set (0.00 sec)

    
对cust_email进行计数
mysql> SELECT COUNT(cust_email) AS num_cust
         -> FROM customers;                   #忽略NULL值
+----------+
| num_cust |
+----------+
|        3 |
+----------+
1 row in set (0.00 sec)

4.MAX（）函数
返回指定列中的最大值,忽略NULL值 
Examples:
 mysql> SELECT MAX(prod_price) AS max_price
          -> FROM products;
+-----------+
| max_price |
+-----------+
|     55.00 |
+-----------+
1 row in set (0.00 sec)

5.MIN（）函数
*返回指定列的最小值 
mysql> SELECT MIN(prod_price) AS min_price
         -> FROM products;
+-----------+
| min_price |
+-----------+
|      2.50 |
+-----------+
1 row in set (0.00 sec)

6.SUM（）函数
*返回指定列值的和 
mysql> SELECT SUM(prod_price) AS sum_price
          -> FROM products;
+-----------+
| sum_price |
+-----------+
|    225.87 |
+-----------+
1 row in set (0.00 sec)

*SUM也可用来合计计算值
Examples:
下面先列出要计算的数据 
mysql> SELECT item_price,quantity 
         -> FROM orderitems
         -> WHERE order_num = 20005;
+------------+----------+
| item_price | quantity |
+------------+----------+
|       5.99 |       10 |
|       9.99 |        3 |
|      10.00 |        5 |
|      10.00 |        1 |
+------------+----------+
4 rows in set (0.01 sec)

mysql> SELECT SUM(item_price*quantity) AS total_price
         -> FROM orderitems                         #返回订单中所有的物品价钱之和
         -> WHERE order_num = 20005;
+-------------+
| total_price |
+-------------+
|      149.87 |
+-------------+
1 row in set (0.00 sec)

7.聚集不同的值，关键字DISTINCT
对于SUM（），MAX（），MIN（），AVG（），COUNT（），默认的参数为ALL，如果要计算只包含不同的值，需指定DISTINCT参数 
 EXAMPLES:
   mysql> SELECT AVG(DISTINCT prod_price) AS avg_price
            -> FROM products
            -> WHERE vend_id = 1003;
+-----------+
| avg_price |
+-----------+
| 15.998000 |
+-----------+
1 row in set (0.02 sec)

8.组