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-- put the entire stored procedure code in a variable
-- have it start with "PROC" so we can easily either create or alter the 
-- procedure based on whether it already exists or not
DECLARE @sp_code NVARCHAR(MAX) = 
'
 PROC [dbo].[usp_some_proc] AS
	SELECT DB_NAME()
'
-- get a list of databases to install the stored procedure to
select [s].[Name]
INTO #tbl_databases
from [sys].[databases] as [s]

-- define some variables to use in the loop
DECLARE @sql NVARCHAR(MAX);
DECLARE @execute_sql NVARCHAR(MAX);
DECLARE @database_name NVARCHAR(500);

-- iterate through each database
WHILE EXISTS (SELECT * FROM #tbl_databases)
BEGIN

    -- get this iteration's database
    SELECT TOP 1
        @database_name = [name]
    FROM #tbl_databases

    -- determine whether stored procedure should be created or altered
    IF OBJECT_ID(QUOTENAME(@database_name) + '.[dbo].[usp_some_proc]') IS NULL
        SET @sql = 'CREATE' + @sp_code;
    ELSE
        SET @sql = 'ALTER' + @sp_code;

    -- define some dynamic sql to execute against the appropriate database
    SET @execute_sql = 'EXEC ' + QUOTENAME(@database_name) + '.[dbo].[sp_executesql] @sql';

    -- execute the code to create/alter the procedure
    EXEC [dbo].[sp_executesql] @execute_sql, N'@sql NVARCHAR(MAX)', @sql;

    -- delete this database so the loop will process the next one
    DELETE FROM #tbl_databases
    WHERE   [name] = @database_name

END

-- clean up :)
DROP TABLE #tbl_databases