Convert DECODE into mapping / reference table - sql

Question

I want to make sure that this conversion of a DECODE function into a SELECT statement joining it to a mapping table would run properly and I'm not using coding or format that works in SQL Server but is different in Oracle SQL
About the code: it is using the DECODE function to map a series of four digit medical taxonomy codes into two digit provider specialty codes. The primary table is PRVDR.TXNMY_CD, the outcome would be a column PRFRM_PRVDR_SPCLTY_CD.
Original code:
SELECT
DECODE (SUBSTR(PRVDR.TXNMY_CD, 1, 4),
'261Q', '70','347E', '59','332H', '96','332B', 'A6','1711', 'Y9','2257', 'Y9','106H', '62','103K', '26','101Y', '26','367A', '42','207K', '03', '3416', '59','367H', '32','207L', '05','211D', '48','231H', '64','2376', '64','111N', '35','291U', '69','103G', '86','364S', '89','208C', '28', '172V', '60','251S'
) END AS PRFRM_PRVDR_SPCLTY_CD
FROM
NPS_CLM_HDR
My conversion attempt:
First, I'd separately create this table called MAPPING with the following columns
| TXNMY_CD_MAP | PRFRM_PRVDR_SPCLTY_CD |
| 1711 | Y9 |
| 2257 | Y9 |
| 106H | 62 |
| 367A | 42 |
etc.
Then I would use the following query:
SELECT PRFRM_PRVDR_SPCLTY_CD
FROM REF.MAPPING AS M
JOIN PRVDR.TXNMY_CD AS P ON P.TXNMY_CD = M.TXNMY_CD_MAP
Does this look correct or have I used terminology from SQL Server that does not work with Oracle SQL?

Answer 1

Hmmm . . . I am expecting the two columns to be:
TXNMY_CD4 PRFRM_PRVDR_SPCLTY_CD
'261Q' '70'
'347E' '59'
'332H' '96'
. . .
This may be what your table looks like, but these are the values at the beginning of the table.
And then:
SELECT m.PRFRM_PRVDR_SPCLTY_CD
FROM PRVDR.TXNMY_CD P LEFT JOIN
REF.MAPPING M
ON LEFT(P.TXNMY_CD, 4) = M.TXNMY_CD_MAP
Except for the LEFT() vs. SUBSTR(), this should work in both databases.
Note that this uses LEFT JOIN to ensure that no rows are lost, even if there are no matches.

Answer 2

decode() is an Oracle function, that is not available in SQL Server. If you wanted to translate your decode to SQL Server, you would use case:
case left(prvdr.txnmy_cd, 4)
when '261Q' then '70'
when '347E' then '59'
...
end as prfrm_prvdr_spclty_cd
from nps_clm_hdr
Note that substr() is not supported in SQL Server - here we cause left() instead.
That said, using a mapping table is a better approach: it scales better, and makes it easy to maintain the mapping (there is no need to modify the code of the query, just the data).
You would phrase the query as:
select prfrm_prvdr_spclty_cd
from prvdr.txnmy_cd as p
left join ref.mapping as m on left(p.txnmy_cd, 4) = m.txnmy_cd_map
The left join allows unmapped values.
Not all database support left(), nor substr() (it is sometimes called substring(), as in SQL Server). I think that the most portable approach uses like and concat():
left join ref.mapping as m on p.txnmy_cd like concat(m.txnmy_cd_map, '%')