What's the purpose of SQL keyword “AS”? [closed] - sql-server

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I am wondering about what AS Keyword do in SQL Server.
I know it's context when it is used on a query that it sets up an alias, however, I do not know what it means when it's used in stored procedures or User Defined Functions. Why is it needed before the begin keyword?
CREATE FUNCTION dbo.UDF_rangeoddeven( #start int, #end int, #oddeven nvarchar(4))
returns #table table
(
Number int
)
AS
BEGIN
IF #oddeven = 'even'
WHILE #start <= #end
BEGIN
IF #start%2 = 0
BEGIN
INSERT INTO #table
SELECT #start;
END
SET #start = #start + 1;
END
ELSE
WHILE #start <= #end
BEGIN
IF #start%2 = 1
BEGIN
INSERT INTO #table
SELECT #start;
END
SET #start = #start + 1;
END
RETURN
END
GO

Answer 1

It's a hunk of language syntax.
For stored functions / procedures (sps), common table expressions (CTEs), and views, AS works as part of the declaration syntax. Think of it this way: declare public_part AS private part.
The public part of an sp is its declaration: how to access it. The private part is the code. The same is true of a CTE:
WITH summary AS (
SELECT id, MAX(val) maxval, MIN(val) minval, SUM(val) sumval
FROM raw
GROUP BY ID
)
The grammar is reversed from the way it's used in SELECTs, in which it's declare expression AS name.
Why is it reversed? Maybe Dr. Stonebraker or Dr. Date know?
Some SQL dialects allow AS to be omitted in some contexts.