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

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)