[mysql] 'IF' in 'SELECT' statement - choose output value based on column values

SELECT id, amount FROM report

I need amount to be amount if report.type='P' and -amount if report.type='N'. How do I add this to the above query?

Use a case statement:

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

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

UNION

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

ORDER BY id;

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

You can try this also

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

Most simplest way is to use a IF(). Yes Mysql allows you to do conditional logic. IF function takes 3 params CONDITION, TRUE OUTCOME, FALSE OUTCOME.

So Logic is

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

You may skip abs() if all no's are +ve only