SelectQueryBuilder is a new fluent interface for constructing database queries, which has been merged to master for release in MediaWiki 1.35. Please consider using it in new code.
SELECT page_id FROM page WHERE page_namespace=$namespace AND page_title=$title
becomes
$id = $db->newSelectQueryBuilder() ->select( 'page_id' ) ->from( 'page' ) ->where( [ 'page_namespace' => $namespace, 'page_title' => $title, ] ) ->fetchField();
As explained on the design task T243051, SelectQueryBuilder was loosely based on the query builder in Doctrine, but I made an effort to respect existing MediaWiki conventions, to make migration easy.
SelectQueryBuilder is easy to use for simple cases, but has the most impact on readability when it is used for complex queries. That's why I chose to migrate the showIndirectLinks query in Special:WhatLinksHere as a pilot -- it was one of the gnarliest queries in core.
SelectQueryBuilder excels at building joins, including parenthesized (nested) joins and joins on subqueries.
SelectQueryBuilder can be used as a structured alternative to the "query info" pattern, in which the parameters to Database::select() are stored in an associative array. It can convert to and from such arrays. As a pilot of this functionality, I converted ApiQueryBase to use a SelectQueryBuilder to store accumulated query info.
Check it out!
-- Tim Starling