How to write a self join query in drupal 7 using db_select

Question

I want to write a self join query, However I dunno how to write it in drupal 7 using db_select().

The query will look like this

SELECT * FROM msg where sender = receiver

Both sender and receiver fields are part of the msg table.

Answer 1

A query using db_select is a lot slower than a 'vanilla' db_query call...for a query as simple as the one you're doing just run it through db_query:

$query = db_query('SELECT * FROM msg where sender = receiver');

Answer 2

Its possible with the following query,

db_select('msg')
  ->fields('msg')
  ->where('sender = receiver');