Below is the simplest way.

There are alternative ways where you don't have to repeat the SELECT statement. I would go tend to use the simpler way unless the SELECT statement is highly complicated and I don't want to repeat it.

CREATE TABLE #Test (TranCode varchar(20))

INSERT INTO #Test (TranCode) VALUES ('BUY')

INSERT INTO #Test (TranCode) VALUES ('SELL')

INSERT INTO #Test (TranCode) VALUES ('CONSTRAIN')

INSERT INTO #Test (TranCode) VALUES ('HOLD')

INSERT INTO #Test (TranCode) VALUES ('ON ACCOUNT')

DECLARE @TC varchar(50)

--SELECT @TC = 'BUY'

SELECT @TC = 'ALL'

IF @TC = 'BUY' OR @TC = 'SELL'

BEGIN

SELECT

*

FROM #Test

WHERE

TranCode = @TC

END

ELSE

BEGIN

SELECT

*

FROM #Test

END

DROP TABLE #Test