Select TOP N and set variable if more could be selected - sql

Question

Is there a more efficient way of doing the following SQL?
I want to select the top 50 results, but I also want to set a variable to tell me if I would have gotten more results back without the TOP
DECLARE #MoreExists BIT
SET #MoreExists = 0
DECLARE #Count INT
SELECT #Count = Count(*)
FROM MyTable WHERE ... --some expensive where clause
IF #Count > 50
SET #MoreExists = 1
SELECT TOP 50 Field1, Field2, ...
FROM MyTable WHERE ... --same expensive where clause

Answer 1

Select 51 results instead, use the top 50 in the client layer, and use the count to know if there are more.

Answer 2

A spin on #Dougs answer
SET NOCOUNT ON
SELECT TOP 51 Field1, Field2, ...
into #t
FROM MyTable WHERE ... --same expensive where clause
if ##rowcount > 50
SET #MoreExists = 1
SET NOCOUNT OFF
SELECT TOP 50 Field1, Field2, ...
from #t
-- maintain ordering with an order by clause

Answer 3

Yes.
The common approach is to use ROW_NUMBER():
WITH MyTableEntries AS
(
SELECT ROW_NUMBER() OVER (ORDER BY Date DESC) AS Row, col1, col2
FROM MyTable
WHERE
-- some expensive WHERE clause
)
SELECT col1, col2
FROM MyTableEntries
WHERE Row BETWEEN(#PageIndex - 1) * #PageSize + 1 and #PageIndex*#PageSize
The efficient approach shown at this SqlServercentral article:
DECLARE #startRow INT ; SET #startrow = 50
;WITH cols
AS
(
SELECT table_name, column_name,
ROW_NUMBER() OVER(ORDER BY table_name, column_name) AS seq,
ROW_NUMBER() OVER(ORDER BY table_name DESC, column_name desc) AS totrows
FROM [INFORMATION_SCHEMA].columns
)
SELECT table_name, column_name, totrows + seq -1 as TotRows
FROM cols
WHERE seq BETWEEN #startRow AND #startRow + 49
ORDERBY seq

Answer 4

How about using COUNT(*) OVER... in a sub-query?
DECLARE #ReqCount int;
SET #ReqCount = 50;
SELECT TOP (#ReqCount) *
FROM
(
SELECT *, Count(*) OVER() AS TotalCnt
FROM MyTable WHERE ...
) t
ORDER BY ...
;
And if you want to use ROW_NUMBER() too, then try:
SELECT *
FROM
(
SELECT *, ROW_NUMBER() OVER (ORDER BY ...) AS RowNum, Count(*) OVER() AS TotalCnt
FROM MyTable WHERE ...
) t
WHERE RowNum BETWEEN #StartRange AND #EndRange
ORDER BY ...
;
And then you can easily check to see if TotalCnt > #ReqCount (or #EndRange), to be able to see if there are more to fetch.
Rob