Get single item from a list string in MySQL - mysql

Question

Given the following strings which represent possible lists, how may I get an item at a specified index n
1,2,3,4,5
word1 word2 word3
pipe|delimited|list
Possible reasons for this functionality are
Extraction of specific elements from GROUP_CONCAT output
Extraction of specific elements from a SET column output (when cast to string)
Extraction of specific elements from a poorly normalised table containing a comma-separated list
Use within an iteration procedure to loop over a list and perform an action on each element within

Answer 1

There is no native function for this. You can use two SUBSTRING_INDEX functions. And you need to check if that specific index item exists:
SET #string:='1,2,3,4,5';
SET #delimiter:=',';
SET #n:=6;
SELECT
CASE WHEN
CHAR_LENGTH(#string)-CHAR_LENGTH(REPLACE(#string, #delimiter, ''))>=
#n*CHAR_LENGTH(#delimiter)-1
THEN
SUBSTRING_INDEX(SUBSTRING_INDEX(#string, #delimiter, #n), #delimiter, -1)
END;
SUBSTRING_INDEX(#string, #delimiter, #n) returns the substring from string #string before #n occurrences of the #delimiter.
SUBSTRING_INDEX( ... , #delimiter, -1) returns everything to the right of the final delimiter
you need to check if delimiter #n exists. We can substract the length of the string with the delimiter, and the string with the delimiter removed - using REPLACE(#string, #delimiter, '') - and see if it is greater than #n*CHAR_LENGTH(#delimiter)-1

Answer 2

Pure SQL way of doing it:-
SELECT SUBSTRING_INDEX(SUBSTRING_INDEX(somefield, '|', 3), '|', -1)
FROM sometable a
If you want to return NULL (or some other fixed value) if there is no 3rd element (for example):-
SELECT IF((LENGTH(somefield) - LENGTH(REPLACE(somefield, '|', '')) + 1) >= 10, SUBSTRING_INDEX(SUBSTRING_INDEX(somefield, '|', 10), '|', -1), NULL)
FROM sometable a

Answer 3

update totally forgot the SUBSTRING_INDEX -1 thing (pointed out by #fthiella and #Kickstart), so updated the examples below
Solved by creating a stored function which is able to be used in-line. The stored function is able to accept an input string, any single-character delimiter, and an index of the desired item to extract.
The stored function definition is as follows
CREATE DEFINER = `user`#`%`
FUNCTION `getListElement`(
inString VARCHAR(255) ,
inDelimiter CHAR(1) ,
inIndexToReturn TINYINT UNSIGNED
) RETURNS varchar(255) READS SQL DATA DETERMINISTIC SQL SECURITY INVOKER
BEGIN
-- Takes in as argument a string, and then breaks out the desired string
DECLARE resultString VARCHAR(255) DEFAULT inString;
DECLARE numberOfListElements TINYINT UNSIGNED DEFAULT 0;
DECLARE errorMessage VARCHAR(255) DEFAULT 'Requested index was < 1 which was invalid';
-- First of all, additional processing only needed for element 2 upwards
IF inIndexToReturn = 1 THEN
RETURN SUBSTRING_INDEX( resultString , inDelimiter , inIndexToReturn);
ELSEIF inIndexToReturn > 1 THEN
-- Count the number of elements
-- This will count the missing delimiters based off the replace. A list of 4 will be missing 3 characters.
SET numberOfListElements = ( CHAR_LENGTH( resultString ) + 1 ) - CHAR_LENGTH( REPLACE( resultString , inDelimiter , '' ) );
IF numberOfListElements >= inIndexToReturn THEN
-- Make sure to only return the last of the elements returend by the first SUBSTRING_INDEX
RETURN SUBSTRING_INDEX( SUBSTRING_INDEX( inString , inDelimiter , inIndexToReturn ) , inDelimiter , -1 );
END IF;
SET errorMessage = CONCAT('List index ',inIndexToReturn,' was requested from a list with ',numberOfListElements,' elements');
END IF;
SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = errorMessage;
END
In the examples above, the following could be used to extract specific list elements
SELECT getListElement( '1,2,3,4,5' , ',' , 2 ) returns 2
SELECT getListElement( REPLACE( 'word1 word2 word3' ,' ', ',' ) , ',' , 3 ) returns word3 (see note below for reason on the REPLACE)
SELECT getListElement( 'pipe|delimited|list' , '|' , 1 ) returns pipe
It would also be possible to use this within an iterator to loop over elements in a list. First of all you need to count the items in the list (see How to count items in comma separated list MySQL) however once you have that, it's just a matter of iterating over them as in this fragment from a stored procedure
-- Reinitialise variables
SET #list = '1,2,3,4,5';
SET #delimiter = ',';
SET #listLength = (CHAR_LENGTH( #list ) + 1 ) - CHAR_LENGTH( REPLACE( #list , #delimiter , '' ) );
SET #currentElement = 1;
listLoop: REPEAT
-- Select the list element and do something with it
SELECT getListElement( #list , #delimiter , #currentElement );
-- Increment the current element
SET #currentElement = #currentElement + 1;
UNTIL #currentElement > #listLength
END REPEAT listLoop;
important space-delimited lists seem to cause issues for this procedure, and so prior to parsing a string into the function I would recommend doing a simple REPLACE to replace the spaces with another suitable single-character separator (i.e. , or | depending on the content in the string)