数据库教程:mysql统计数据时sum if、count if、ifnull的使用

sum if 的使用

条件格式：sum(if(条件,字段名或固定值,0))
注解：sum是求和函数，条件为true时，列值（字段名）或固定值累加求和，条件为false时用0累加

1.单条件判断格式，sum(if(条件,字段名或固定值,0))

2.多条件判断格式，sum(if(条件 AND 条件 AND 条件,字段名或固定值,0))

注解：固定值如果为1时，等价于count if计数，固定值如果为2时，结果=计数值 × 2

count if 的使用

条件格式：

1.统计总数，count(if(条件,true,null))

2.某字段去重统计总数，count(distinct 去重字段名,if(条件,true,null))

ifnull 的使用：

格式：IFNULL(expression, alt_value)
注解：expression 表达式为 NULL 时返回alt_value
常规用法：SUM 累加的数据如果不存在则返回null，所以配合IFNULL可实现null转为0，代码层则无需做null转0的操作了。

示例：

mysql统计数据的写法一：

SELECT COUNT(*) 'ORDER_COUNT' , SUM(IF(true, o.amount, 0)) 'ORDER_SUM_AMOUNT' , SUM(IF(o.`status` = 0, 1, 0)) 'PENDING_PAYMENT' , SUM(IF(o.`status` = 1, 1, 0)) 'PENDING_REVIEW' , SUM(IF(o.`status` = 2, 1, 0)) 'PENDING_SHIPMENT' , SUM(IF(o.`status` = 3, 1, 0)) 'SHIPPED' , SUM(IF(o.`status` = 4, 1, 0)) 'RECEIVED' , SUM(IF(o.`status` = 5, 1, 0)) 'COMPLETED' , SUM(IF(o.`status` = 6, 1, 0)) 'FAILED' , SUM(IF(o.`status` = 7, 1, 0)) 'CANCELED' , SUM(IF(o.`status` = 8, 1, 0)) 'DENIED' , SUM(IF(o.`status` in (0,1) and o.expire is not null and o.expire < NOW(), 1, 0)) 'EXPIRED' , SUM(IF(o.`status` in (0,1) and o.createdDate >= CURRENT_DATE and o.expire is not null and o.expire < NOW(), 1, 0)) 'TODAY_EXPIRED' , SUM(IF(o.`status` = 0 and (o.expire is null or o.expire > NOW()), 1, 0)) 'PENDING_PAYMENT_ORDER_COUNT' , SUM(IF(o.`status` in (1,2) and (o.expire is null or o.expire > NOW()), 1, 0)) 'PENDING_SHIPMENT_ORDER_COUNT' , SUM(IF(o.`status` in (4,5) and (o.isReviewed is null or o.isReviewed = 0), 1, 0)) 'PENDING_REVIEW_COUNT' FROM Orders o WHERE o.member_id = 66666 

mysql统计数据的写法二：

SELECT COUNT(*) 'ORDER_SUM' , IFNULL(SUM(o.amount), 0) 'ORDER_SUM_AMOUNT' , COUNT(IF(o.`status` = 0, true, null)) 'PENDING_PAYMENT' , COUNT(IF(o.`status` = 1, true, null)) 'PENDING_REVIEW' , COUNT(IF(o.`status` = 2, true, null)) 'PENDING_SHIPMENT' , COUNT(IF(o.`status` = 3, true, null)) 'SHIPPED' , COUNT(IF(o.`status` = 4, true, null)) 'RECEIVED' , COUNT(IF(o.`status` = 5, true, null)) 'COMPLETED' , COUNT(IF(o.`status` = 6, true, null)) 'FAILED' , COUNT(IF(o.`status` = 7, true, null)) 'CANCELED' , COUNT(IF(o.`status` = 8, true, null)) 'DENIED' , COUNT(IF(o.`status` in (0,1) and o.expire is not null and o.expire < NOW(), true, null)) 'EXPIRED' , COUNT(IF(o.`status` in (0,1) and o.createdDate >= CURRENT_DATE and o.expire is not null and o.expire < NOW(), true, null)) 'TODAY_EXPIRED' , COUNT(IF(o.`status` = 0 and (o.expire is null or o.expire > NOW()), true, null)) 'PENDING_PAYMENT_ORDER_COUNT' , COUNT(IF(o.`status` in (1,2) and (o.expire is null or o.expire > NOW()), true, null)) 'PENDING_SHIPMENT_ORDER_COUNT' , COUNT(IF(o.`status` in (4,5) and (o.isReviewed is null or o.isReviewed = 0), true, null)) 'PENDING_REVIEW_COUNT' FROM Orders o WHERE o.member_id = 66666 

写法一和写法二的结果一样，但是写法二列结果为null时返回0

需要了解更多数据库技术:mysql统计数据时sum if、count if、ifnull的使用，都可以关注数据库技术分享栏目—计算机技术网(www.ctvol.com)!