'SELECT'语句中的'IF' - 根据列值选择输出值

Question

SELECT id, amount FROM report

我需要amount成为amountif report.type='P'和-amountif report.type='N'.如何将其添加到上述查询中？

Answer 1

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,您可以在条件为null时进行处理.在空金额的情况下:

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

该部分IFNULL(amount,0)表示当金额不为空时返回金额,否则返回0.

Answer 2

使用case声明:

select id,
    case report.type
        when 'P' then amount
        when 'N' then -amount
    end as amount
from
    `report`

Answer 3

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;

Answer 4

select 
  id,
  case 
    when report_type = 'P' 
    then amount 
    when report_type = 'N' 
    then -amount 
    else null 
  end
from table

Answer 5

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,真实的结果,错误的结果.

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

如果所有不是+ ve,你可以跳过abs()

Answer 6

SELECT id, amount
FROM report
WHERE type='P'

UNION

SELECT id, (amount * -1) AS amount
FROM report
WHERE type = 'N'

ORDER BY id;

Answer 7

你也可以尝试这个

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