generate_merge.sql

-- Generate a vanilla MERGE statement for Microsoft SQL Server that performs an
-- upsert. You may need to further modify the generated statement to fit your
-- needs, but it should at least save you from typing the same fields in
-- quadruplicate.
--
-- History:
--   2021-04-15: Daniel Rothfus - Initial version

DECLARE @MergeColumns TABLE([Name] SYSNAME NOT NULL);
INSERT INTO @MergeColumns
VALUES ('Key Column 1'), ('Key Column 2');
DECLARE @TargetTable SYSNAME = 'TargetTable';
DECLARE @Source NVARCHAR(MAX) = 'SourceTable';

DECLARE @Command NVARCHAR(MAX);

WITH AllColumns AS (
	SELECT c.*
	FROM sys.columns AS c
	JOIN sys.objects AS o
	ON c.object_id = o.object_id
	WHERE o.name = @TargetTable
), AllQuotedColumnNames AS (
	SELECT QUOTENAME([name]) AS [Name]
	FROM AllColumns
), UpdateColumnNamesQuoted AS (
	SELECT QUOTENAME([name]) AS [Name]
	FROM AllColumns
	WHERE [name] NOT IN (
		SELECT [Name]
		FROM @MergeColumns
	)
)
SELECT @Command = CONCAT(
	-- See https://weblogs.sqlteam.com/dang/2009/01/31/upsert-race-condition-with-merge/ for motivation behind HOLDLOCK
	'MERGE INTO ', QUOTENAME(@TargetTable), ' WITH (HOLDLOCK) AS t ', CHAR(10),
	'USING ', QUOTENAME(@Source), ' AS s', CHAR(10),
	'ON ', (
		SELECT STRING_AGG(CONCAT('t.', QUOTENAME([Name]), ' = s.', QUOTENAME([Name])), CONCAT(CHAR(10), '    AND '))
		FROM @MergeColumns
	), CHAR(10),
	'WHEN MATCHED THEN UPDATE SET ', (
		SELECT STRING_AGG(CONCAT('t.', [Name], ' = s.', [Name]), CONCAT(',', CHAR(10), '    '))
		FROM UpdateColumnNamesQuoted
	), CHAR(10),
	'WHEN NOT MATCHED THEN INSERT (', CHAR(10),
	(
		SELECT STRING_AGG(CONCAT('    ', [Name]), CONCAT(', ', CHAR(10)))
		FROM AllQuotedColumnNames
	), CHAR(10),
	') VALUES (', CHAR(10),
	(
		SELECT STRING_AGG(CONCAT('    s.', [Name]), CONCAT(', ', CHAR(10)))
		FROM AllQuotedColumnNames
	), CHAR(10),
	')'
)

PRINT(@Command);