Question

i have a mysql db with a table ‘difficulties’ with a few records. If i do “select * from difficulties” i get them back in the order they were added, ordered by primary key id:

mysql> select * from difficulties;
+----+-------+-----------+--------+----------+-----------+
| id | value | name      | letter | low_band | high_band |
+----+-------+-----------+--------+----------+-----------+
|  1 |     1 | very_easy | VE     |        1 |         1 | 
|  2 |     2 | easy      | E      |        2 |         5 | 
|  3 |     3 | medium    | M      |        6 |        10 | 
|  4 |     4 | hard      | H      |       11 |        12 | 
|  5 |     0 | na        | NA     |        0 |         0 | 
+----+-------+-----------+--------+----------+-----------+

However, if i do “select name from difficulties” i get them back in a different order:

mysql> select name from difficulties;
+-----------+
| name      |
+-----------+
| easy      | 
| hard      | 
| medium    | 
| na        | 
| very_easy | 
+-----------+

My question is: what determines this order? Is there any logic to it? Is it something like “the order the files representing the records happen to be in within the filesystem” or something else that is to all intents and purposes random?

thanks, max

Answer 1

This is correct and by design: if you don’t ask for sorting, the server doesn’t bother with sorting (sorting can be an expensive operation), and it will return the rows in whatever order it sees fit. Without a requested order, the way the records are ordered can even differ from one query to the next (although that’s not too likely).

The order is definitely not random – it’s just whatever way the rows come out of the query, and as you see, even minor modifications can change this un-order significantly. This “undefined” ordering is implementation dependent, unpredictable and should not be relied upon.

If you want the elements to be ordered, use the ORDER BY clause (that’s its purpose) – e.g.

SELECT name FROM difficulties ORDER BY name ASC;

That will always return the result sorted by name, in ascending order. Or, if you want them ordered by the primary key, last on top, use:

SELECT name FROM difficulties ORDER BY id DESC;

You can even sort by function – if you actually want random order, do this (caveat: horrible performance with largish tables):

SELECT name FROM difficulties ORDER BY RAND();

For more details see this tutorial and the documentation.