Maintaining order in MySQL "IN" query

As the other answer mentions: the query you posted has nothing about what order you'd like your results, just which results you'd like to get.

To order your results, I would use ORDER BY FIELD():

SELECT * FROM foo f where f.id IN (2, 3, 1)
ORDER BY FIELD(f.id, 2, 3, 1);

The argument list to FIELD can be variable length.

The values in an IN() predicate are considered to be a set, and the result returned by an SQL query has no way to automatically infer order from that set.

In general, the order of any SQL query is arbitrary unless you specify an order with an ORDER BY clause.

You can use a MySQL function FIND_IN_SET() to do what you want:

SELECT * FROM foo f where f.id IN (2, 3, 1)
ORDER BY FIND_IN_SET(f.id, '2,3,1');

Note that the list argument to FIND_IN_SET() isn't a variable length list like the arguments of IN(). It has to be a string literal or a SET.

Re questions about performance: I'm curious too, so I tried both FIND_IN_SET() and FIELD() methods against my copy of the StackOverflow data:

With no index on VoteTypeId:

SELECT * FROM Votes ORDER BY FIND_IN_SET(VoteTypeId, '13,1,12,2,11,3,10,4,9,5,8,6,7');

3618992 rows in set (31.26 sec)
3618992 rows in set (29.67 sec)
3618992 rows in set (28.52 sec)

SELECT * FROM Votes ORDER BY FIELD(VoteTypeId, 13,1,12,2,11,3,10,4,9,5,8,6,7);

3618992 rows in set (37.30 sec)
3618992 rows in set (49.65 sec)
3618992 rows in set (41.69 sec)

With an index on VoteTypeId:

SELECT * FROM Votes ORDER BY FIND_IN_SET(VoteTypeId, '13,1,12,2,11,3,10,4,9,5,8,6,7');

3618992 rows in set (14.71 sec)
3618992 rows in set (14.81 sec)
3618992 rows in set (25.80 sec)

SELECT * FROM Votes ORDER BY FIELD(VoteTypeId, 13,1,12,2,11,3,10,4,9,5,8,6,7);

3618992 rows in set (19.03 sec)
3618992 rows in set (14.59 sec)
3618992 rows in set (14.43 sec)

Conclusion: with limited testing, there is no great advantage to either method.