'SELECT'语句中的'IF' – 根据列值select输出值

SELECT id, amount FROM report 

如果report.type='P' ,则需要amountamount ,如果report.type='N' -amount 。 我如何将这个添加到上面的查询?

 SELECT id, IF(type = 'P', amount, amount * -1) as amount FROM report 

请参阅http://dev.mysql.com/doc/refman/5.0/en/control-flow-functions.html

此外,您可以处理条件为空时。 在空数量的情况下:

 SELECT id, IF(type = 'P', IFNULL(amount,0), IFNULL(amount,0) * -1) as amount FROM report 

部分IFNULL(amount,0)表示当金额不为空时返回金额,否则返回0

使用case语句:

 select id, case report.type when 'P' then amount when 'N' then -amount end as amount from `report` 
 SELECT CompanyName, CASE WHEN Country IN ('USA', 'Canada') THEN 'North America' WHEN Country = 'Brazil' THEN 'South America' ELSE 'Europe' END AS Continent FROM Suppliers ORDER BY CompanyName; 
 select id, case when report_type = 'P' then amount when report_type = 'N' then -amount else null end from table 

最简单的方法是使用IF() 。 是的,Mysql允许你做条件逻辑。 IF函数需要3个参数条件,实际结果,错误结果。

所以逻辑是

 if report.type = 'p' amount = amount else amount = -1*amount 

SQL

 SELECT id, IF(report.type = 'P', abs(amount), -1*abs(amount)) as amount FROM report 

如果所有的no都是+ ve,你可以跳过abs()

 SELECT id, amount FROM report WHERE type='P' UNION SELECT id, (amount * -1) AS amount FROM report WHERE type = 'N' ORDER BY id; 

你也可以试试这个

  Select id , IF(type=='p', IFNULL(amount,0), IFNULL(amount,0) * -1) as amount from table 

让我们试试这个。

  SELECT id , IF(report.type == 'p', IFNULL(amount,0), IFNULL(amount,0) * -1) as amount FROM report