IF statement within a formula in a SQL query - sql

Question

Let's say I have a table with two numeric columns: NUM and DEN.
I need to extract the ratio NUM/DEN only if DEN isn't 0: otherwise the ratio should be 0.
Something like this:
select ID, [...] AS RATIO
from Table
where [...] is some kind of equivalent of the excel formula IF(DEN=0;0;NUM/DEN).
Is there a way to perform this kind of query?
Many thanks!

Answer 1

This should work:
case
when DEN = 0 then 0
else NUM/DEN
end

Answer 2

Yes, what you are looking for is case. It has two versions:
case [variable]
when [value1] then [output1]
when [value2] then [output2]
when [value3] then [output3]
...
else [outputdefault] end
and
case when [Boolean(True/false) expression 1] then [output1]
when [Boolean(True/false) expression 2] then [output2]
when [Boolean(True/false) expression 3] then [output3]
...
else [outputdefault] end

Answer 3

If SQL Server 2012 you can use : IIF
IIF ( boolean_expression, true_value, false_value )

Answer 4

If you are using SQL Server you could use case statement like w0lf mentioned or you could use iif statement like so:
select iif(age > 21, 'Allowed', 'Not Allowed') as status
from test;
Example:
create table test (
fullname varchar (20),
age int
);
insert into test values
('John', 10),
('Matt', 90),
('Jane', 25),
('Ruby', 80),
('Randy', null);
Result
| fullname | status |
|----------|-------------|
| John | Not Allowed |
| Matt | Allowed |
| Jane | Allowed |
| Ruby | Allowed |
| Randy | Not Allowed |
The same thing can be written as
select case when age > 21 then 'Allowed' else 'Not Allowed' end as status
from test;
case statement is used by many database engines.
If you are dealing with null and not null values, you could also use coalesce like so:
select fullname, coalesce(age, 999) as status
from test;
The result will be:
| fullname | status |
|----------|--------|
| John | 10 |
| Matt | 90 |
| Jane | 25 |
| Ruby | 80 |
| Randy | 999 |
At first you may think that coalesce does if age is null then 999 else age. It does that, sort-of, but in particular coalesce outputs the first non-null value in a list. So, coalesce(null, null, 45) will result in 45. coalesce(null, 33, 45) will result in 33.
Feel free to play around with SQL Fiddle: http://sqlfiddle.com/#!6/a41a7/6