SQL - concatenate strings in variable while loop with += - sql

Question

I can't concatenate in this example bellow!
When I loop I get my 2 correct results.
When I concatenate #MaterialCompositionEn += ', ' it works fine
When I try to concatenate #MaterialCompositionEn += same query to get the 2nd row, I have a null!
DECLARE #MaterialCompositionId int = 475;
DECLARE #MaterialCompositionKey nvarchar(50) = '202071512324138';
DECLARE #Records nvarchar(250);
DECLARE #RecordProceed int;
DECLARE #MaterialCompositionEn nvarchar(500);
SET #Records = (SELECT STRING_AGG(Id, ',') FROM MaterialCompositions mc WHERE mc.MaterialCompositionId = #MaterialCompositionId)
WHILE len(#Records) > 0
BEGIN
SET #RecordProceed = CAST(LEFT(#Records,4) AS int)
if #RecordProceed > 0
BEGIN
SET #Records = REPLACE(#Records,substring(#Records, 1, 4),'')
END
if len(#Records) > 4
BEGIN
SET #Records = REPLACE(#Records,substring(#Records, 1, 1),'')
END
if len(#MaterialCompositionEn) > 0
BEGIN
SET #MaterialCompositionEn += ', '
END
PRINT 'MaterialCompositionEn1: ' + #MaterialCompositionEn
SET #MaterialCompositionEn =
(SELECT COALESCE (CAST(MaterialProportion AS nvarchar) + '% ', '') +
(SELECT mp.MaterialPrimaryEn +
COALESCE(
(SELECT ' (' + ms.MaterialSecondaryEn + ')' AS MS1 FROM dbo.MaterialSecondaries AS ms WHERE ms.Id = mc.MaterialSecondaryId)
, '')
FROM dbo.MaterialPrimaries AS mp WHERE mp.Id = mc.MaterialPrimaryId)
FROM MaterialCompositions mc WHERE mc.Id = #RecordProceed
)
PRINT 'MaterialCompositionEn2: ' + #MaterialCompositionEn
END
Result:
MaterialCompositionEn2: 20% Cashmere
MaterialCompositionEn1: 20% Cashmere,
MaterialCompositionEn2: 80% Wool
Now when I change to:
SET #MaterialCompositionEn +=
(SELECT COALESCE......
I am expecting 20% Cashmere, 80% Wool
instead my 3 prints are NULL
I tried to CAST but won't help.
Any idea?
Thanks in advance

Answer 1

I'm guessing there is a much simpler way to do what you want. However, I think the problem is that you need to initialize the string. So at the top of the code block put:
SET #MaterialCompositionEn = '';

Answer 2

SET #MaterialCompositionEn =
SELECT
CONCAT(
mp.MaterialPrimaryEn, ' ', MaterialProportion, '% ', --always show primary
', ' + ms.MaterialSecondaryEn + CONCAT(100 - MaterialProportion, '%') --sometimes show secondary
)
FROM
MaterialCompositions mc
INNER JOIN dbo.MaterialPrimaries mp ON mp.Id = mc.MaterialPrimaryId
LEFT JOIN dbo.MaterialSecondaries ms ON ms.Id = mc.MaterialSecondaryId
WHERE mc.Id = #RecordProceed
)
Something like this might be neater.. Note that I'm not clear where MaterialProportion comes from (I suspect MaterialCompositions), so this perhaps isn't a solution, just a note as to how you might use CONCAT/avoid having boatloads of nested selects. The use of INNER/OUTER join links compositions and definitely a primary, possibly a secondary material. If the secondary material is null then the aim is to hide it with a mix and match of CONCAT and +
CONCAT treats nulls as empty strings, where as + causes the whole expression to become null. This can be useful to mix and match e.g. in something like ', ' + ms.MaterialSecondaryEn + CONCAT(100 - MaterialProportion, '%'):
the CONCAT(100 - MaterialProportion, '%') would be 20% (if the primary material was 80%) but
the ', ' + ms.MaterialSecondaryEn + CONCAT(...) as a whole is NULL if MaterialSecondaryEn IS NULL from a left join fail to match, so where there is only a primary material, a the string describing the secondary should disappear entirely as NULL, which the outer CONCAT handles as an empty string