Optimizing SQL- swapping out functions for JOINS and confusion when variables exist - sql-server-2008

Question

I just started a new job in Dev Ops at a publishing company. My first task is to optimize a huge SQL query that is composed of functions. Functions are really slow. The guy that created the query was smart but didn't know SQL and used functions when he could have used JOINs instead. I am having trouble with converting the functions that have variables. For example, this is one of the functions. Here it is within the query and next is the associated function stored elsewhere.
dbo.rpt_get_isbn(b.bookkey, 21) AS
f_upc
Then the function...
ALTER FUNCTION [dbo].[rpt_get_isbn](
#i_bookkey INT,
#i_isbn_type INT)
/* Returns the identifier such as EAN,
ISBN, with or without dashes
PARAMETER #i_isbn_type
10 = ISBN10
13 = ISBN 13
16 = EAN
17 = EAN (no dashes)
18 = GTIN
19 = GTIN (no dashes)
20 = LCCN
21 = UPC
*/
RETURNS VARCHAR(50)
AS
BEGIN
DECLARE #RETURN VARCHAR(50)
DECLARE #v_desc VARCHAR(50)
IF #i_isbn_type = 10
BEGIN
SELECT #v_desc = isbn10
FROM isbn
WHERE bookkey = #i_bookkey
END
ELSE IF #i_isbn_type = 13
BEGIN
SELECT #v_desc = isbn
FROM isbn
WHERE bookkey = #i_bookkey
END
ELSE IF #i_isbn_type = 16
BEGIN
SELECT #v_desc = ean
FROM isbn
WHERE bookkey = #i_bookkey
END
ELSE IF #i_isbn_type = 17
BEGIN
SELECT #v_desc = ean13
FROM isbn
WHERE bookkey = #i_bookkey
END
ELSE IF #i_isbn_type = 18
BEGIN
SELECT #v_desc = gtin
FROM isbn
WHERE bookkey = #i_bookkey
END
ELSE IF #i_isbn_type = 19
BEGIN
SELECT #v_desc = gtin14
FROM isbn
WHERE bookkey = #i_bookkey
END
ELSE IF #i_isbn_type = 20
BEGIN
SELECT #v_desc = lccn
FROM isbn
WHERE bookkey = #i_bookkey
END
ELSE IF #i_isbn_type = 21
BEGIN
SELECT #v_desc = upc
FROM isbn
WHERE bookkey = #i_bookkey
END
IF LEN(#v_desc) > 0
BEGIN
SELECT #RETURN =
LTRIM(RTRIM(#v_desc))
END
ELSE
BEGIN
SELECT #RETURN = ''
END
RETURN #RETURN
END
So this function can return various different results based on the variable given as the second parameter. If that wasn't there, this would be easy. I would simply convert it with a solution like this one to retrieve the book's cover..
LTRIM(RTRIM(bo.ean13)) AS p_coverimagepath
and the JOIN needed...
LEFT JOIN Isbn bo WITH (NOLOCK) ON bo.bookkey = b.bookkey
But again, now I'm dealing with parameters and a function that uses If/elses to derive an answer. So do I need to add this if/else logic to my main query? I can't think of a ways that will yield as simple an answer. I look forward to figuring this out. Please let me know if I forgot any crucial elements to understand what I'm doing. Thanks!
UPDATE
Here's where the code is going
,pss8.dbo.xml_StripIllegalChars(dbo.rpt_get_series_volume(b.bookkey)) AS
p_seriesvol
,CASE
WHEN dbo.rpt_get_isbn(b.bookkey, 17) = ''
THEN (
SELECT ipg_id
FROM tmmdb.ipg_extra.dbo.vw_Pss8IsbnOrUpc bo
WHERE bo.bookkey = b.bookkey
)
ELSE dbo.rpt_get_isbn(b.bookkey, 17)
END AS p_coverimagepath
,CASE
WHEN dbo.rpt_get_isbn(b.bookkey, 17) = ''
THEN (
SELECT ipg_id
FROM tmmdb.ipg_extra.dbo.vw_Pss8IsbnOrUpc bo
WHERE bo.bookkey = b.bookkey
)
ELSE dbo.rpt_get_isbn(b.bookkey, 17)
END AS TSP_p_coverimagepath
,pss8.dbo.xml_StripIllegalChars(replace(dbo.rpt_get_title(b.bookkey,
'T'), '&', '&')) AS p_title /* 30OCT14 */
,pss8.dbo.xml_StripIllegalChars(replace(dbo.rpt_get_sub_title(b.bookkey),
'&', '&')) AS p_subtitle /* 20OCT14 */

Answer 1

Not a complete answer, but an example structure to help you dig your way out of this. I have made many assumptions because your question requires more detail. But hopefully you can appreciate this is way simpler.
SELECT
CASE
WHEN ISNULL(LTRIM(RTRIM(bo.ean13)),'') = '' THEN T2.ipg_id
ELSE LTRIM(RTRIM(bo.ean13))
END as p_coverimagepath
FROM MainTable b
LEFT JOIN Isbn bo
ON bo.bookkey = b.bookkey
LEFT JOIN tmmdb.ipg_extra.dbo.vw_Pss8IsbnOrUpc T2
ON T2.bookkey=b.bookkey
In your example code, two columns do exactly the same thing so I haven't repeated this.
A few things noted:
-Don't sprinkle NOLOCK in your code thinking it's a performance boost
-If you're using an outer join, you need to accommodate in case a NULL is returned
-It's easier to join once to tmmdb.ipg_extra.dbo.vw_Pss8IsbnOrUpc and use columns out of it rather than repeat the code over and over.