Can you split/explode a field in a MySQL query?

Question

I have to create a report on some student completions. The students each belong to one client. Here are the tables (simplified for this question).

CREATE TAB         


        

Answer 1

You can create a function for this:

/**
* Split a string by string (Similar to the php function explode())
*
* @param VARCHAR(12) delim The boundary string (delimiter).
* @param VARCHAR(255) str The input string.
* @param INT pos The index of the string to return
* @return VARCHAR(255) The (pos)th substring
* @return VARCHAR(255) Returns the [pos]th string created by splitting the str parameter on boundaries formed by the delimiter.
* @{@example
*     SELECT SPLIT_STRING('|', 'one|two|three|four', 1);
*     This query
* }
*/
DROP FUNCTION IF EXISTS SPLIT_STRING;
CREATE FUNCTION SPLIT_STRING(delim VARCHAR(12), str VARCHAR(255), pos INT)
RETURNS VARCHAR(255) DETERMINISTIC
RETURN
    REPLACE(
        SUBSTRING(
            SUBSTRING_INDEX(str, delim, pos),
            LENGTH(SUBSTRING_INDEX(str, delim, pos-1)) + 1
        ),
        delim, ''
    );

Converting the magical pseudocode to use this, you would have:

SELECT e.`studentId`, SPLIT_STRING(',', c.`courseNames`, e.`courseId`)
FROM...

Answer 2

There's an easier way, have a link table, i.e.:

Table 1: clients, client info, blah blah blah

Table 2: courses, course info, blah blah

Table 3: clientid, courseid

Then do a JOIN and you're off to the races.

Answer 3

You can do this with JSON in more recent MySQL versions. It's a blast. We will have a quick preparation to create a numbers table. Then first we create an intermediary table to convert the comma delimited strings into a json array then we will use json_extract to blast them apart. I am encapsulating the strings in quotes carefully escaping existing quotes because I had semicolon separated strings containing commas.

So to create the numbers table, hopefully you have more clients than courses, choose an adequately big table if not.

CREATE TABLE numbers (n int PRIMARY KEY);
INSERT INTO numbers 
SELECT @row := @row + 1
FROM clients JOIN (select @row:=0) t2;

Add LIMIT 50 if you know you only have 50 courses. Now, that was easy, wasn't it? Now on to the real work, honestly it's the quotes that make it uglier but at least it's more generic that way:

CREATE TABLE json_coursenames 
SELECT clientId,clientName,CONCAT('["', REPLACE(REPLACE(courseName,'"','\\"'), ',', '","'), '"]') AS a
FROM clients;

CREATE TABLE extracted
SELECT clientId,clientName,REPLACE(TRIM(TRIM('"' FROM JSON_EXTRACT(a, concat('$[', n, ']')))), '\\"', '"')
FROM json_coursenames
INNER JOIN numbers ON n < JSON_LENGTH(a);

Wheee!

The meat here are these two: the CONCAT('["', REPLACE(coursename, ',', '","'), '"]') (I dropped the second REPLACE to make it more visible) will convert foo,bar,bar into "foo","bar","baz". The other trick is JSON_EXTRACT(a, concat('$[', n, ']') will become JSON_EXTRACT(a, $[12]) and that's the 13th element in the array, see JSON Path syntax.

Answer 4

Based on Alex answer above (https://stackoverflow.com/a/11022431/1466341) I came up with even better solution. Solution which doesn't contain exact one record ID.

Assuming that the comma separated list is in table data.list, and it contains listing of codes from other table classification.code, you can do something like:

SELECT 
    d.id, d.list, c.code
FROM 
    classification c
    JOIN data d
        ON d.list REGEXP CONCAT('[[:<:]]', c.code, '[[:>:]]');

So if you have tables and data like this:

CLASSIFICATION (code varchar(4) unique): ('A'), ('B'), ('C'), ('D')
MY_DATA (id int, list varchar(255)): (100, 'C,A,B'), (150, 'B,A,D'), (200,'B')

above SELECT will return

(100, 'C,A,B', 'A'),
(100, 'C,A,B', 'B'),
(100, 'C,A,B', 'C'),
(150, 'B,A,D', 'A'),
(150, 'B,A,D', 'B'),
(150, 'B,A,D', 'D'),
(200, 'B', 'B'),

Answer 5

I've resolved this kind of problem with a regular expression pattern. They tend to be slower than regular queries but it's an easy way to retrieve data in a comma-delimited query column

SELECT * 
FROM `TABLE`
WHERE `field` REGEXP ',?[SEARCHED-VALUE],?';

the greedy question mark helps to search at the beggining or the end of the string.

Hope that helps for anyone in the future

Answer 6

I just had a similar issue with a field like that which I solved a different way. My use case was needing to take those ids in a comma separated list for use in a join.

I was able to solve it using a like, but it was made easier because in addition to the comma delimiter the ids were also quoted like so:

keys "1","2","6","12"

Because of that, I was able to do a LIKE

SELECT twwf.id, jtwi.id joined_id FROM table_with_weird_field twwf INNER JOIN join_table_with_ids jtwi ON twwf.delimited_field LIKE CONCAT("%\"", jtwi.id, "\"%")

This basically just looks to see if the id from the table you're trying to join appears in the set and at that point you can join on it easily enough and return your records. You could also just create a view from something like this.

It worked well for my use case where I was dealing with a Wordpress plugin that managed relations in the way described. The quotes really help though because otherwise you run the risk of partial matches (aka - id 1 within 18, etc).