Check if a temp table exists (TSQL)

if object_id('##temp_table') is null print 'DOES NOT EXIST' else print 'EXISTS'

Generate row ID (T-SQL)

Let's assume we have all our data in a table MyTable, with a column named MyTableID, that we want to fill with unique consecutive values.

DECLARE @Index BIGINT
SET @Index = 0

UPDATE MyTable
SET @Index = MyTableID = @Index + 1

How to declare a function (T-SQL)

CREATE FUNCTION MyFunction
(
@EntryParam BIGINT
)
RETURNS BIGINT --the returned type
AS
BEGIN
DECLARE @ReturnVar AS BIGINT -- the var to keep the return value
..... -- set @ReturnVar's value
RETURN @ReturnVar
END