SQL Server : how to declare variable in condition case when - sql-server

Question

I have a problem when running queries with a 'case when'. so I want to create a variable to hold the value of the results of the query 'select' to other tables. like the example below. please help me to get the results I want. thank you.
SELECT
a.field1, a.field2,
a.field3 =
CASE
WHEN a.field1 = 'alfa'
THEN
WHEN
-- I want to declare variable to check get value from another table with query select and condition 'WHERE' with variable declare
var varTest = (SELECT TOP 1 b.field1 FROM Table2 as b WHERE b.field2=a.field2) Then
if varTest = 'actif' then
SELECT (c.field4 * C.field5) as hasil FROM Table2 as c WHERE c.field1=varTest)
ELSE
a.field3
END
FROM
Table1 a(NOLOCK)
WHERE
a.field1 = 'alfa'
This is sample data :
field1 | field2 | field3 | > Table1
alfa idAlfa 0
beta idBeta 0
carlie idCarlie 0
field1 | field2 | field4 | field5 | > Table2
actif idAlfa 80 5
pasif idBeta 50 5
other idCarlie 10 5
Result :
field1 | field2 | field3
alfa idAlfa 400
beta idBeta 250
carlie idCarlie 50

Answer 1

It looks to me like this is what you intended. Hard to be entirely sure. It's also possible that this isn't the most efficient query as it's generally a good idea to avoid subqueries inside case expressions.
a.field3 =
CASE WHEN a.field1 = 'alfa' THEN
CASE WHEN (
SELECT TOP 1 b.field1 FROM Table2 as b
WHERE b.field2 = a.field
) = 'actif'
THEN (
SELECT c.field4 * C.field5 as hasil FROM Table2 as c
WHERE c.field1 = 'actif'
)
ELSE a.field3
END
ELSE null
END

Answer 2

You don't need a variable of any type to accomplish this.
A simple JOIN and CASE will get the active (actif) record, calculate your field4 * field5 result (hasil) or use 0 for all passive (pasif) or other records like so:
SELECT
a.field1,
a.field2,
CASE b.field1 -- check id...
WHEN 'actif'
THEN b.field4 * b.field5 -- if 'actif' record, calculate result
ELSE
a.field3 -- not the 'actif' record, use default value
END AS hasil
FROM Table1 AS a -- join the two tables by id name
LEFT JOIN Table2 AS b
ON a.field2 = b.field2;
If you're using SQL Server 2012, you can condense this CASE statement by using IIF:
SELECT
a.field1,
a.field2,
IIF(b.field1 = 'actif', b.field3 * b.field4, a.field3) AS hasil
FROM Table1 AS a
LEFT JOIN Table2 AS b
ON a.field2 = b.field2;
The results using your sample data from these queries are:
field1 | field2 | hasil
alfa idAlfa 400
beta idBeta 0
charlie idCharlie 0
For future reference, the IF syntax you showed in your sample SQL is invalid. That is used in flow control statements that affect how SQL is executed; usually seen in stored procedures. IF can't be used in expressions. Instead, use CASE or IIF statements.