Get name from variable using index in T-SQL - sql-server

Question

Using the following two queries
Query 1:
DECLARE #ContentColumnNamesSRC NVARCHAR(4000) = NULL,
SELECT
#ContentColumnNamesSRC = COALESCE(#ContentColumnNamesSRC + ', ', '') + '[' + name + ']'
FROM
tempdb.sys.columns
WHERE
1 = 1
AND object_id = OBJECT_ID('tempdb..#tempTable')
AND column_id < 9 -- First 8 columns are ID data, which is what I am after
Query 2:
DECLARE #ContentColumnNamesDST NVARCHAR(4000) = NULL,
SELECT
#ContentColumnNamesDST = COALESCE(#ContentColumnNamesDST + ', ', '') + '[' + name + ']'
FROM
tempdb.sys.columns
WHERE
1 = 1
AND object_id = OBJECT_ID('Import.dbo.ContentTable')
AND column_id < 9 -- First 8 columns are ID data, which is what I am after
I can get the first 8 columns from each table into a variable.
What I would like to do is find a way to get the values out of the variable, such that I can match the column names.
They should be identical in each table, and I need it to be able to create a dynamic merge statement, such that the columnsnames from each variable
#ContentColumnNamesSRC
and
#ContentColumnNamesDST
line up, so I can use it in a merge statement.
The point of this is to be able to use it in a loop, and all i would have to do is change which tables it looks at and the merge statements would still work.
Ideally, id like to end up with something like the following:
SELECT #StageSQLCore = N'USE Staging;
BEGIN TRANSACTION
MERGE '+#StageTableCore+' AS DST
USING '+#ImportTableCore+' AS SRC
ON (SRC.[Key] = DST.[Key])
WHEN NOT MATCHED THEN
INSERT ('+#StageTableCoreColumns+')
VALUES (
'+#ImportTableCoreColumns+',GETDATE())
WHEN MATCHED
THEN UPDATE
SET
DST.'+#ContentColumnNamesDST[i]' = SRC.'+#ContentColumnNamesSRC[i] +'
,DST.'+#ContentColumnNamesDST[i]' = SRC.'+#ContentColumnNamesSRC[i] +'
,DST.'+#ContentColumnNamesDST[i]' = SRC.'+#ContentColumnNamesSRC[i] +'
,DST.'+#ContentColumnNamesDST[i]' = SRC.'+#ContentColumnNamesSRC[i] +'
,DST.[ETLDate] = GETDATE()
;
COMMIT'
EXEC (#StageSQLCore)

Answer 1

You can generate Merge SQL like this if both the ordinal are matching
DECLARE #MergeSQL NVARCHAR(4000) = NULL
SELECT --*--,
#MergeSQL = COALESCE(#MergeSQL + ', DST.=', '') + QUOTENAME(bc.column_name) + ' = SRC.' + QUOTENAME(bc.COLUMN_NAME) + char(13)
FROM
test.INFORMATION_SCHEMA.COLUMNS tc
inner join testb.INFORMATION_SCHEMA.COLUMNS bc
on tc.TABLE_NAME = bc.TABLE_NAME
and tc.ORDINAL_POSITION = bc.ORDINAL_POSITION
and tc.TABLE_NAME = 'History'
WHERE
tc.ORDINAL_POSITION < 5 -- First 8 columns are ID data, which is what I am after
and bc.ORDINAL_POSITION < 5
select #MergeSQL