DECLARE @IsRowType1Req bit;
DECLARE @IsRowType2Req bit;

DECLARE @tbl1 table (ID int, RowType varchar(50));

DECLARE @tmpTbl table (ID int, RowType varchar(50));

IF @IsRowType1Req = 1
    INSERT INTO
        @tmpTbl
    SELECT
        *
    FROM
        @tbl1
    WHERE
        RowType = 'RowType1' AND
        @IsRowType1Req = 1;

IF @IsRowType2Req = 1
    INSERT INTO
        @tmpTbl
    SELECT
        *
    FROM
        @tbl1
    WHERE
        RowType = 'RowType2' AND
        @IsRowType2Req = 1;

SELECT * FROM @tmpTbl;

DECLARE @IsRowType1Req bit;
DECLARE @IsRowType2Req bit;

DECLARE @tbl1 table (ID int, RowType varchar(50));

SELECT
    *
FROM
    @tbl1
WHERE
    RowType = 'RowType1' AND
    @IsRowType1Req = 1

UNION

SELECT
    *
FROM
    @tbl1
WHERE
    RowType = 'RowType2' AND
    @IsRowType2Req = 1