Database

CREATE FUNCTION DoFunction
(
    @Op1  BIGINT
,
    @Oper NVARCHAR(2)
,
    @Op2  BIGINT
)
RETURNS BIT
AS
BEGIN
    IF ( @Oper = '>=' AND @Op1 >= @Op2 ) RETURN 1
 
    ELSE IF ( @Oper = '<=' AND @Op1 <= @Op2 ) RETURN 1
 
-- More
 
    RETURN 0
 
END

select * from test where dbo.dofunction ( VOLUME_TRADED , '<=' , @Volume )=1

SELECT * FROM test WHERE CASE WHEN @Oper='<=' AND VOLUME_TRADED <= @Volume THEN 1 WHEN @Oper='>=' AND VOLUME_TRADED >= @Volume THEN 1 ELSE 0 END = 1

SELECT [Call].*
,Operator.Fullname
FROM 
    (SELECT [Caller],FirstCall=MIN(CallTime) FROM [Call] WHERE DATEPART(MM,CallTime)=4 GROUP BY [Caller]) T
INNER JOIN [Call]
ON T.[Caller]=[Call].[Caller]
AND T.FirstCall=[Call].CallTime
INNER JOIN [Operator]
ON [Call].Operator=Operator.ID