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 TABLE  `clients` (
  `clientId` int(10) unsigned NOT NULL auto_increment,
  `clientName` varchar(100) NOT NULL default '',
  `courseNames` varchar(255) NOT NULL default ''
)

The courseNames field holds a comma-delimited string of course names, eg "AB01,AB02,AB03"

CREATE TABLE  `clientenrols` (
  `clientEnrolId` int(10) unsigned NOT NULL auto_increment,
  `studentId` int(10) unsigned NOT NULL default '0',
  `courseId` tinyint(3) unsigned NOT NULL default '0'
)

The courseId field here is the index of the course name in the clients.courseNames field. So, if the client's courseNames are "AB01,AB02,AB03", and the courseId of the enrolment is 2, then the student is in AB03.

Is there a way that I can do a single select on these tables that includes the course name? Keep in mind that there will be students from different clients (and hence have different course names, not all of which are sequential,eg: "NW01,NW03")

Basically, if I could split that field and return a single element from the resulting array, that would be what I'm looking for. Here's what I mean in magical pseudocode:

SELECT e.`studentId`, SPLIT(",", c.`courseNames`)[e.`courseId`]
FROM ...

Answer 1

Building on Alwin Kesler's solution, here's a bit of a more practical real world example.

Assuming that the comma separated list is in my_table.list, and it's a listing of ID's for my_other_table.id, you can do something like:

SELECT 
    * 
FROM 
    my_other_table 
WHERE 
    (SELECT list FROM my_table WHERE id = '1234') REGEXP CONCAT(',?', my_other_table.id, ',?');

Answer 2

I used the above logic but modified it slightly. My input is of format : "apple:100|pinapple:200|orange:300" stored in a variable @updtAdvanceKeyVal

Here is the function block :

set @res = "";

set @i = 1;
set @updtAdvanceKeyVal = updtAdvanceKeyVal;

REPEAT


 -- set r =  replace(SUBSTRING(SUBSTRING_INDEX(@updtAdvanceKeyVal, "|", @i),
 --  LENGTH(SUBSTRING_INDEX(@updtAdvanceKeyVal, "|", @i -1)) + 1),"|","");

-- wrapping the function in "replace" function as above causes to cut off a character from
 -- the 2nd splitted value if the value is more than 3 characters. Writing it in 2 lines causes no such problem and the output is as expected
-- sample output by executing the above function :
-- orange:100
-- pi apple:200    !!!!!!!!strange output!!!!!!!!
-- tomato:500

      set @r =  SUBSTRING(SUBSTRING_INDEX(@updtAdvanceKeyVal, "|", @i),
                  LENGTH(SUBSTRING_INDEX(@updtAdvanceKeyVal, "|", @i -1)) + 1);

      set @r = replace(@r,"|","");

      if @r <> "" then

              set @key = SUBSTRING_INDEX(@r, ":",1);
              set @val = SUBSTRING_INDEX(@r, ":",-1);

              select @key, @val;
      end if;

      set @i = @i + 1;

     until @r = ""
END REPEAT;

Answer 3

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 4

MySQL's only string-splitting function is SUBSTRING_INDEX(str, delim, count). You can use this, to, for example:

• Return the item before the first separator in a string:

  mysql> SELECT SUBSTRING_INDEX('foo#bar#baz#qux', '#', 1);
  +--------------------------------------------+
  | SUBSTRING_INDEX('foo#bar#baz#qux', '#', 1) |
  +--------------------------------------------+
  | foo                                        |
  +--------------------------------------------+
  1 row in set (0.00 sec)
  

• Return the item after the last separator in a string:

  mysql> SELECT SUBSTRING_INDEX('foo#bar#baz#qux', '#', -1);
  +---------------------------------------------+
  | SUBSTRING_INDEX('foo#bar#baz#qux', '#', -1) |
  +---------------------------------------------+
  | qux                                         |
  +---------------------------------------------+
  1 row in set (0.00 sec)
  

• Return everything before the third separator in a string:

  mysql> SELECT SUBSTRING_INDEX('foo#bar#baz#qux', '#', 3);
  +--------------------------------------------+
  | SUBSTRING_INDEX('foo#bar#baz#qux', '#', 3) |
  +--------------------------------------------+
  | foo#bar#baz                                |
  +--------------------------------------------+
  1 row in set (0.00 sec)
  

• Return the second item in a string, by chaining two calls:

  mysql> SELECT SUBSTRING_INDEX(SUBSTRING_INDEX('foo#bar#baz#qux', '#', 2), '#', -1);
  +----------------------------------------------------------------------+
  | SUBSTRING_INDEX(SUBSTRING_INDEX('foo#bar#baz#qux', '#', 2), '#', -1) |
  +----------------------------------------------------------------------+
  | bar                                                                  |
  +----------------------------------------------------------------------+
  1 row in set (0.00 sec)
  

In general, a simple way to get the nth element of a #-separated string (assuming that you know it definitely has at least n elements) is to do:

SUBSTRING_INDEX(SUBSTRING_INDEX(your_string, '#', n), '#', -1);

The inner SUBSTRING_INDEX call discards the nth separator and everything after it, and then the outer SUBSTRING_INDEX call discards everything except the final element that remains.

If you want a more robust solution that returns NULL if you ask for an element that doesn't exist (for instance, asking for the 5th element of 'a#b#c#d'), then you can count the delimiters using REPLACE and then conditionally return NULL using IF():

IF(
    LENGTH(your_string) - LENGTH(REPLACE(your_string, '#', '')) / LENGTH('#') < n - 1,
    NULL,
    SUBSTRING_INDEX(SUBSTRING_INDEX(your_string, '#', n), '#', -1)
)

Of course, this is pretty ugly and hard to understand! So you might want to wrap it in a function:

CREATE FUNCTION split(string TEXT, delimiter TEXT, n INT)
RETURNS TEXT DETERMINISTIC
RETURN IF(
    (LENGTH(string) - LENGTH(REPLACE(string, delimiter, ''))) / LENGTH(delimiter) < n - 1,
    NULL,
    SUBSTRING_INDEX(SUBSTRING_INDEX(string, delimiter, n), delimiter, -1)
);

You can then use the function like this:

mysql> SELECT SPLIT('foo,bar,baz,qux', ',', 3);
+----------------------------------+
| SPLIT('foo,bar,baz,qux', ',', 3) |
+----------------------------------+
| baz                              |
+----------------------------------+
1 row in set (0.00 sec)

mysql> SELECT SPLIT('foo,bar,baz,qux', ',', 5);
+----------------------------------+
| SPLIT('foo,bar,baz,qux', ',', 5) |
+----------------------------------+
| NULL                             |
+----------------------------------+
1 row in set (0.00 sec)

mysql> SELECT SPLIT('foo###bar###baz###qux', '###', 2);
+------------------------------------------+
| SPLIT('foo###bar###baz###qux', '###', 2) |
+------------------------------------------+
| bar                                      |
+------------------------------------------+
1 row in set (0.00 sec)

Answer 5

Here's how you do it for SQL Server. Someone else can translate it to MySQL. Parsing CSV Values Into Multiple Rows.

SELECT Author, 
NullIf(SubString(',' + Phrase + ',' , ID , CharIndex(',' , ',' + Phrase + ',' , ID) - ID) , '') AS Word 
FROM Tally, Quotes 
WHERE ID <= Len(',' + Phrase + ',') AND SubString(',' + Phrase + ',' , ID - 1, 1) = ',' 
AND CharIndex(',' , ',' + Phrase + ',' , ID) - ID > 0

The idea is to cross join to a predefined table Tally which contains integer 1 through 8000 (or whatever big enough number) and run SubString to find the right ,word, position.

Answer 6

If you need get table from string with delimiters:

SET @str = 'function1;function2;function3;function4;aaa;bbbb;nnnnn';
SET @delimeter = ';';
SET @sql_statement = CONCAT('SELECT '''
                ,REPLACE(@str, @delimeter, ''' UNION ALL SELECT ''')
                ,'''');
SELECT @sql_statement;
SELECT 'function1' UNION ALL SELECT 'function2' UNION ALL SELECT 'function3' UNION ALL SELECT 'function4' UNION ALL SELECT 'aaa' UNION ALL SELECT 'bbbb' UNION ALL SELECT 'nnnnn'