How do you write a case insensitive query for both MySQL and Postgres?

I'm running a MySQL database locally for development, but deploying to Heroku which uses Postgres. Heroku handles almost everything, but my case-insensitive Like statements become case sensitive. I could use iLike statements, but my local MySQL database can't handle that.

What is the best way to write a case insensitive query that is compatible with both MySQL and Postgres? Or do I need to write separate Like and iLike statements depending on the DB my app is talking to?

Answers


select * from foo where upper(bar) = upper(?);

If you set the parameter to upper case in the caller, you can avoid the second function call.


The moral of this story is: Don't use a different software stack for development and production. Never.

You'll just end up with bugs which you can't reproduce in dev; your testing will be worthless. Just don't do it.

Using a different database engine is out of the question - there will be FAR more cases where it behaves differently than just LIKE (also, have you checked the collations in use by the databases? Are they identical in EVERY CASE? If not, you can forget ORDER BY on varchar columns working the same)


Use Arel:

Author.where(Author.arel_table[:name].matches("%foo%"))

matches will use the ILIKE operator for Postgres, and LIKE for everything else.


In postgres, you can do this:

SELECT whatever FROM mytable WHERE something ILIKE 'match this';

I'm not sure if there is an equivalent for MySQL but you can always do this which is a bit ugly but should work in both MySQL and postgres:

SELECT whatever FROM mytable WHERE UPPER(something) = UPPER('match this');

There are several answers, none of which are very satisfactory.

  • LOWER(bar) = LOWER(?) will work on MySQL and Postgres, but is likely to perform terribly on MySQL: MySQL won't use its indexes because of the LOWER function. On Postgres you can add a functional index (on LOWER(bar)) but MySQL doesn't support this.
  • MySQL will (unless you have set a case-sensitive collation) do case-insensitive matching automatically, and use its indexes. (bar = ?).
  • From your code outside the database, maintain bar and bar_lower fields, where bar_lower contains the result of lower(bar). (This may be possible using database triggers, also). (See a discussion of this solution on Drupal). This is clumsy but does at least run the same way on pretty much every database.

REGEXP is case insensitive (unless used with BINARY), and can be used, like so...

    SELECT id FROM person WHERE name REGEXP 'john';

...to match 'John', 'JOHN', 'john', etc.


If you're using PostgreSQL 8.4 you can use the citext module to create case insensitive text fields.


use COLLATE.

http://dev.mysql.com/doc/refman/5.0/en/case-sensitivity.html


You might also consider checking out the searchlogic plugin, which does the LIKE/ILIKE switch for you.


You can also use ~* in postgres if you want to match a substring within a block. ~ matches case-sensitive substring, ~* case insensitive substring. Its a slow operation, but might I find it useful for searches.

Select * from table where column ~* 'UnEvEn TeXt';
Select * from table where column ~ 'Uneven text';

Both would hit on "Some Uneven text here" Only the former would hit on "Some UNEVEN TEXT here"


Converting to upper is best as it covers compatible syntax for the 3 most-used Rails database backends. PostgreSQL, MySQL and SQLite all support this syntax. It has the (minor) drawback that you have to uppercase your search string in your application or in your conditions string, making it a bit uglier, but I think the compatibility you gain makes it worthwile.

Both MySQL and SQLite3 have a case-insensitive LIKE operator. Only PostgreSQL has a case-sensitive LIKE operator and a PostgreSQL-specific (per the manual) ILIKE operator for case-insensitive searches. You might specify ILIKE insead of LIKE in your conditions on the Rails application, but be aware that the application will cease to work under MySQL or SQLite.

A third option might be to check which database engine you're using and modify the search string accordingly. This might be better done by hacking into / monkeypatching ActiveRecord's connection adapters and have the PostgreSQL adapter modify the query string to substitute "LIKE" for "ILIKE" prior to query execution. This solution is however the most convoluted and in light of easier ways like uppercasing both terms, I think this is not worh the effort (although you'd get plenty of brownie points for doing it this way).


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