title: MySQL query for finding rows that are in one table but not another
body: <p>Let's say I have about 25,000 records in two tables and the data in each should be the same. If I need to find any rows that are in table A but NOT in table B, what's the most efficient way to do this.</p>
  
  <p>We've tried it as a subquery of one table and a <code>NOT IN</code> the result but this runs for over 10 minutes and almost crashes our site.</p>
  
  <p>There must be a better way. Maybe a <code>JOIN</code>?</p>