Why is this column always NULL in a LEFT JOIN query? - sql

Question

I am having some trouble . I have been writing a SQL query that fetches 5 columns from 2 tables. In a simple join, the column named CustomerCode is displayed correctly, but in the below query, it is displayed NULL. In fact, I want to select from the two tables with a left join.
This is my query:
select fff.*,CustomerCode,SUBSTRING(( SELECT DISTINCT
RIGHT('0000'
+ LTRIM(RTRIM(CustomerCode)),
4)
+ '-'
FROM TatEstelam.dbo.CustomerInfo
WHERE ltrim(rtrim(CodeMelli)) <> '' and CodeMelli <> '0000000000' AND CODEMELLI <> '0' AND Nationalcode = fff.CodeMelli
FOR
XML PATH('')
), 0,
LEN(( SELECT DISTINCT
RIGHT('0000'
+ LTRIM(RTRIM(CustomerCode)),
4)
+ '-'
FROM TatEstelam.dbo.CustomerInfo
WHERE ltrim(rtrim(CodeMelli)) <> '' and CodeMelli <> '0000000000' AND CODEMELLI <> '0' AND Nationalcode = fff.CodeMelli
FOR
XML PATH('')
)))
AS customerNumber from ( SELECT NameOutput AS Fullname ,
shobehNew AS BRANCHNO ,
ShenaseMelli AS Nationalcode ,
[CodeMelli],
CreateDate AS RegisterDate ,
N'ساها حقوقی' as type
FROM TatEstelam.dbo.tblCodeMRequest_0 where CreateDate BETWEEN '920101' AND '930501' and CusType = 6) as fff
LEFT JOIN (select DISTINCT Nationalcode,customercode,NationalID from TatEstelam.dbo.CustomerInfo where InqueryType <> '01' ) ci ON fff.Nationalcode = ci.NationalID and ltrim(rtrim(CodeMelli)) <> '' and CodeMelli <> '0000000000' AND CODEMELLI <> '0'
ORDER BY CODEmELLI
but in this query customer code will display :
select * from TatEstelam.dbo.CustomerInfo c right join TatEstelam.dbo.tblCodeMRequest_0 m
on c.Nationalcode = m.ShenaseMelli