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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)
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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)
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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 |+---------------------+| [email protected] || NULL || [email protected] || [email protected] || 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)
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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)
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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)
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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)
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*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)
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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)
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8.组合聚集函数
eg: mysql> SELECT COUNT(*) AS num_items, -> MIN(prod_price) AS price_min, -> MAX(prod_price) AS price_min, -> AVG(prod_price) AS price_avg -> FROM products;+-----------+-----------+-----------+-----------+| num_items | price_min | price_min | price_avg |+-----------+-----------+-----------+-----------+| 14 | 2.50 | 55.00 | 16.133571 |+-----------+-----------+-----------+-----------+1 row in set (0.00 sec)
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