Why my T-SQL (WHILE) does not work? - sql

Question

In my code, I need to test whether specified column is null and the most close to 0 as possible (it can holds numbers from 0 to 50) so I have tried the code below.
It should start from 0 and for each value test the query. When #Results gets null, it should return. However, it does not work. Still prints 0.
declare #hold int
declare #Result int
set #hold0
set #Result=0
WHILE (#Result!=null)
BEGIN
select #Result=(SELECT Hold from Numbers WHERE Name='Test' AND Hold=#hold)
set #hold=#hold+1
END
print #hold

Answer 1

First, you can't test equality of NULL. NULL means an unknown value, so you don't know whether or not it does (or does not) equal any specific value. Instead of #Result!=NULL use #result IS NOT NULL
Second, don't use this kind of sequential processing in SQL if you can at all help it. SQL is made to handle sets, not process things sequentially. You could do all of this work with one simple SQL command and it will most likely run faster anyway:
SELECT
MIN(hold) + 1
FROM
Numbers N1
WHERE
N1.name = 'Test' AND
NOT EXISTS
(
SELECT
*
FROM
Numbers N2
WHERE
N2.name = 'Test' AND
N2.hold = N1.hold + 1
)
The query above basically tells the SQL Server, "Give me the smallest hold value plus 1 (MIN(hold) + 1) in the table Numbers where the name is test (name = 'Test') and where the row with name of 'Test' and hold of one more that that does not exist (the whole "NOT EXISTS" part)". In the case of the following rows:
Name Hold
-------- ----
Test 1
Test 2
NotTest 3
Test 20
SQL Server finds all of the rows with name of "Test" (1, 2, 20) then finds which ones don't have a row with name = Test and hold = hold + 1. For 1 there is a row with Test, 2 that exists. For Test, 2 there is no Test, 3 so it's still in the potential results. For Test, 20 there is no Test, 21 so that leaves us with:
Name Hold
-------- ----
Test 2
Test 20
Now SQL Server looks for MIN(hold) and gets 2 then it adds 1, so you get 3.
SQL Server may not perform the operations exactly as I described. The SQL statement tells SQL Server what you're looking for, but not how to get it. SQL Server has the freedom to use whatever method it determines is the most efficient for getting the answer.
The key is to always think in terms of sets and how do those sets get put together (through JOINs), filtered (through WHERE conditions or ON conditions within a join, and when necessary, grouped and aggregated (MIN, MAX, AVG, etc.).

Answer 2

have you tried
WHILE (#Result is not null)
BEGIN
select #Result=(SELECT Hold from Numbers WHERE Name='Test' AND Hold=#hold)
set #hold=#hold+1
END

Answer 3

Here's a more advanced version of Tom H.'s query:
SELECT MIN(N1.hold) + 1
FROM Numbers N1
LEFT OUTER JOIN Numbers N2
ON N2.Name = N1.Name AND N2.hold = N1.hold + 1
WHERE N1.name = 'Test' AND N2.name IS NULL
It's not as intuitive if you're not familiar with SQL, but it uses identical logic. For those who are more familiar with SQL, it makes the relationship between N1 and N2 easier to see. It may also be easier for the query optimizer to handle, depending on your DBMS.

Answer 4

Try this:
declare #hold int
declare #Result int
set #hold=0
set #Result=0
declare #max int
SELECT #max=MAX(Hold) FROM Numbers
WHILE (#hold <= #max)
BEGIN
select #Result=(SELECT Hold from Numbers WHERE Name='Test' AND Hold=#hold)
set #hold=#hold+1
END
print #hold
While is tricky in T-SQL - you can use this for (foreach) looping through (temp) tables too - with:
-- Foreach with T-SQL while
DECLARE #tempTable TABLE (rownum int IDENTITY (1, 1) Primary key NOT NULL, Number int)
declare #RowCnt int
declare #MaxRows int
select #RowCnt = 1
select #MaxRows=count(*) from #tempTable
declare #number int
while #RowCnt <= #MaxRows
begin
-- Number from given RowNumber
SELECT #number=Number FROM #tempTable where rownum = #RowCnt
-- next row
Select #RowCnt = #RowCnt + 1
end