Looking for a scalar function to find the last occurrence of a character in a string - sql

Question

Table FOO has a column FILEPATH of type VARCHAR(512). Its entries are absolute paths:
FILEPATH
------------------------------------------------------------
file://very/long/file/path/with/many/slashes/in/it/foo.xml
file://even/longer/file/path/with/more/slashes/in/it/baz.xml
file://something/completely/different/foo.xml
file://short/path/foobar.xml
There's ~50k records in this table and I want to know all distinct filenames, not the file paths:
foo.xml
baz.xml
foobar.xml
This looks easy, but I couldn't find a DB2 scalar function that allows me to search for the last occurrence of a character in a string. Am I overseeing something?
I could do this with a recursive query, but this appears to be overkill for such a simple task and (oh wonder) is extremely slow:
WITH PATHFRAGMENTS (POS, PATHFRAGMENT) AS (
SELECT
1,
FILEPATH
FROM FOO
UNION ALL
SELECT
POSITION('/', PATHFRAGMENT, OCTETS) AS POS,
SUBSTR(PATHFRAGMENT, POSITION('/', PATHFRAGMENT, OCTETS)+1) AS PATHFRAGMENT
FROM PATHFRAGMENTS
)
SELECT DISTINCT PATHFRAGMENT FROM PATHFRAGMENTS WHERE POS = 0

Answer 1

I think what you're looking for is the LOCATE_IN_STRING() scalar function. This is what Info Center has to say if you use a negative start value:
If the value of the integer is less than zero, the search begins at
LENGTH(source-string) + start + 1 and continues for each position to
the beginning of the string.
Combine that with the LENGTH() and RIGHT() scalar functions, and you can get what you want:
SELECT
RIGHT(
FILEPATH
,LENGTH(FILEPATH) - LOCATE_IN_STRING(FILEPATH,'/',-1)
)
FROM FOO

Answer 2

One way to do this is by taking advantage of the power of DB2s XQuery engine. The following worked for me (and fast):
SELECT DISTINCT XMLCAST(
XMLQuery('tokenize($P, ''/'')[last()]' PASSING FILEPATH AS "P")
AS VARCHAR(512) )
FROM FOO
Here I use tokenize to split the file path into a sequence of tokens and then select the last of these tokens. The rest is only conversion from SQL to XML types and back again.

Answer 3

I know that the problem from the OP was already solved but I decided to post the following anyway to hopefully help others like me that land here.
I came across this thread while searching for a solution to my similar problem which had the exact same requirement but was for a different kind of database that was also lacking the REVERSE function.
In my case this was for a OpenEdge (Progress) database, which has a slightly different syntax. This made the INSTR function available to me that most Oracle typed databases offer.
So I came up with the following code:
SELECT
SUBSTRING(
foo.filepath,
INSTR(foo.filepath, '/',1, LENGTH(foo.filepath) - LENGTH( REPLACE( foo.filepath, '/', '')))+1,
LENGTH(foo.filepath))
FROM foo
However, for my specific situation (being the OpenEdge (Progress) database) this did not result into the desired behaviour because replacing the character with an empty char gave the same length as the original string. This doesn't make much sense to me but I was able to bypass the problem with the code below:
SELECT
SUBSTRING(
foo.filepath,
INSTR(foo.filepath, '/',1, LENGTH( REPLACE( foo.filepath, '/', 'XX')) - LENGTH(foo.filepath))+1,
LENGTH(foo.filepath))
FROM foo
Now I understand that this code won't solve the problem for T-SQL because there is no alternative to the INSTR function that offers the Occurence property.
Just to be thorough I'll add the code needed to create this scalar function so it can be used the same way like I did in the above examples.
-- Drop the function if it already exists
IF OBJECT_ID('INSTR', 'FN') IS NOT NULL
DROP FUNCTION INSTR
GO
-- User-defined function to implement Oracle INSTR in SQL Server
CREATE FUNCTION INSTR (#str VARCHAR(8000), #substr VARCHAR(255), #start INT, #occurrence INT)
RETURNS INT
AS
BEGIN
DECLARE #found INT = #occurrence,
#pos INT = #start;
WHILE 1=1
BEGIN
-- Find the next occurrence
SET #pos = CHARINDEX(#substr, #str, #pos);
-- Nothing found
IF #pos IS NULL OR #pos = 0
RETURN #pos;
-- The required occurrence found
IF #found = 1
BREAK;
-- Prepare to find another one occurrence
SET #found = #found - 1;
SET #pos = #pos + 1;
END
RETURN #pos;
END
GO
To avoid the obvious, when the REVERSE function is available you do not need to create this scalar function and you can just get the required result like this:
SELECT
SUBSTRING(
foo.filepath,
LEN(foo.filepath) - CHARINDEX('\', REVERSE(foo.filepath))+2,
LEN(foo.filepath))
FROM foo

Answer 4

You could just do it in a single statement:
select distinct reverse(substring(reverse(FILEPATH), 1, charindex('/', reverse(FILEPATH))-1))
from filetable