T-SQL: Useful Tips

SCOPE_IDENTITY()	To retrieve identity value from the last inserted record, use SCOPE_IDENTITY() instead of @@IDENTITY.
EXISTS	To gain better performance, use EXISTS instead of IN. BONUS: SQL Server: JOIN vs IN vs EXISTS - the logical difference
SET NOCOUNT ON	Use it in a stored proc. It reduces network traffic by eliminating sending "done" messages to client.
@@ROWCOUNT	It returns number of rows affected by the last statement.
RETURN	It is usually used for returning status/error code. RETURN can be used to exit from a stored proc. Any statements that follow RETURN are not executed.
@@ERROR	It returns the error number for the last statement. Zero (0) means no errors.
ERROR_NUMBER()	The result of ERROR_NUMBER() is the same as @@ERROR. BEGIN TRY -- Generate a divide-by-zero error. SELECT 1/0; END TRY BEGIN CATCH -- All of these must be used inside CATCH. SELECT ERROR_NUMBER() AS ErrorNumber, ERROR_SEVERITY() AS ErrorSeverity, ERROR_STATE() AS ErrorState, ERROR_PROCEDURE() AS ErrorProcedure, ERROR_LINE() AS ErrorLine, ERROR_MESSAGE() AS ErrorMessage; END CATCH;

T-SQL: Variable Assignment Using SET vs. SELECT

Use SET instead of SELECT to make a variable assignment because it is ANSI standard and recommended in SQL Server doc.

References:

• SET @local_variable (Transact-SQL)
• T-SQL: SET vs SELECT when assigning variables
• Differences between SET and SELECT in SQL Server

-- You could assign variable with SELECT.
DECLARE @total_rows INT = 0;
SELECT @total_rows = COUNT(*) FROM customer;
PRINT @total_rows;

-- However, it is better to use SET.
DECLARE @total_rows INT = 0;
SET @total_rows = (SELECT COUNT(*) FROM customer); -- Parentheses around SELECT statement is required.
PRINT @total_rows;