Writing CASE Statement in SQL - sql

Question

I have a requirement to display two columns in a report,whose values will be determined using the same input expression as following:
SELECT
CASE WHEN id>10
THEN 'AAAA'
ELSE
'BBBB'
END as 'FirstColumn',
CASE WHEN id>10
THEN
'BBBB'
ELSE
'DDDD' END as 'SecondColumn'
Can I construct this expression without repeating input expression twice as they are same?

Answer 1

You will need to repeat the CASE statment for each field as while the condition might be the same teh results are differnt. The only other alternative is to use a UNion all statement whenre the first select takes the records WHERE id<=10 and the other one does the WHERE ID>10. This can bea viable alternative but it is a littel hareder to maintain, so if the performance is good enough, Iwoudl stick to repeating teh CASE condition.

Answer 2

If some sophisticated is supposed instead of id>10 then, to make it a bit shorter and a bit more readable:
select
IIF(p.b=1, 'AAA', 'BBB') [Col1],
IIF(p.b=1, 'BBB', 'DDD') [Col2]
from
TableName t
outer apply (select cast(case when t.id>10 then 1 else NULL end as bit) as b) p
However, this is only available in SqlServer 2012. In earlier versions, parhaps, you will have to write your own IIF-like scalar function:
create function dbo.IIF (#b bit, #ifValue varchar(50), #elseValue varchar(50))
returns varchar(50)
as begin
return (select case when #b = 1 then #ifValue else #elseValue end)
end
GO
select
dbo.IIF(p.b, 'AAA', 'BBB') [Col1],
dbo.IIF(p.b, 'BBB', 'DDD') [Col2]
from
TableName t
outer apply (select cast(case when t.id>10 then 1 else NULL end as bit) as b) p

Answer 3

If it's worth the trouble, you could created a User-defined function (UDF) that would do this transforamation logic:
CREATE FUNCTION dbo.fColTransform(#id Int)
RETURNS Varchar(20)
AS BEGIN
DECLARE #ret Varchar(20)
SET #ret =
CASE
WHEN #id IS NULL THEN 'Unknown'
WHEN #id > 10 THEN 'BBB'
ELSE 'DDD'
END
RETURN #ret
END
Then, in your SELECT, you could structure it like this:
SELECT dbo.fColTransform(id1) AS FirstColumn,
dbo.fColTransform(id2) AS SecondColumn
FROM MyTable

Answer 4

You can store it in a variable and use Execute to call it:
Declare #foo as nvarchar(max)
set #foo='MyTable'
execute ('Select * from ' + #foo)
The downside is that your entire query would be all red (since it's now a string).

Answer 5

Try with this query, maybe will be useful (in the case if you do not use a function):
WITH temp AS (
SELECT
CASE WHEN x.ID > 10
THEN 'AAAA'
ELSE 'BBBB'
END as Col
FROM (
SELECT 1 AS ID
UNION
SELECT 11 AS ID
UNION
SELECT 13 AS ID
UNION
SELECT 9 AS ID
UNION
SELECT 7 AS ID
) x
)
SELECT temp.Col AS FirstColumn, temp.Col AS SecondColumn
FROM temp
Basically:
With name As (
/*yourquery with one CASE-COLUMN*/
)
SELECT name.COL AS COLUMN1, name.COL AS COLUMN2 FROM name
You can try this here