How To Define Output Parameters in Stored Procedures in SQL Server Transact-SQL?

Sometime a stored procedure not only want to take input values from the calling statement batch, but it also want to send output values back to the calling statement batch. This can be done by defining output parameters in the CREATE PROCEDURE statement.

To define an output parameter, you should use this format: "@parameter_name data_type OUTPUT", as shown in the following tutorial exercise:

DROP PROCEDURE diff_in_days;
GO

-- Defining an output parameter
CREATE PROCEDURE diff_in_days 
   @start_date DATETIME,
   @end_date DATETIME = '19-May-2007',
   @days VARCHAR(40) OUTPUT
AS BEGIN
   SET @days = CONVERT(VARCHAR(20),@end_date,107)
      + ' - ' 
      + CONVERT(VARCHAR(20),@start_date,107)
      + ' = ' 
      + STR(DATEDIFF(DAY, @start_date, @end_date));
END;
GO
Command(s) completed successfully.

EXEC diff_in_days
   @start_date='01-Jan-2007'
GO
Msg 201, Level 16, State 4, Procedure diff_in_days, Line 0
Procedure or Function 'diff_in_days' expects 
parameter '@days', which was not supplied.

⇒Using Stored Procedures in SQL Server Transact-SQL

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