Drupal - Using SQL functions in conditionals in Drupal 7 db_select()

change

$query->addExpression("LOWER(ttd.name) = $category");

to

$query->where('LOWER(ttd.name) = :category', array(':category' => $category));

Using LOWER() is considered slow in MySQL. It isn't necessary too because LIKE in Drupal's Database API (DBTNG) is case insensitive, at least when your MySQL table is configured to use one of the *_ci collations. A standard MySQL installation uses *utf8_general_ci* and so is Drupal.

So you just need to use a LIKE condition:

$query->condition('name', $category, 'LIKE');

See Conditional Clauses for a comprehensive explaination.

BTW: A DBTNG based database driver is responsible for implementing a case insensitive LIKE. PostgreSQL for instance uses ILIKE instead of LIKE which is handled in includes/database/pgsql/database.inc.