title: When to use LEFT JOIN and when to use INNER JOIN?
body: <p>I feel like I was always taught to use <code>LEFT JOIN</code>s and I often see them mixed with <code>INNER</code>s to accomplish the same type of query throughout several pieces of code that are supposed to do the same thing on different pages. Here goes:</p>
  
  <pre><code>SELECT ac.reac, pt.pt_name, soc.soc_name, pt.pt_soc_code
  FROM
    AECounts ac
    INNER JOIN 1_low_level_term llt on ac.reac = llt.llt_name
    LEFT JOIN 1_pref_term pt ON llt.pt_code = pt.pt_code
    LEFT JOIN 1_soc_term soc ON pt.pt_soc_code = soc.soc_code
  LIMIT 100,10000
  </code></pre>
  
  <p>Thats one I am working on:</p>
  
  <p>I see a lot like:</p>
  
  <pre><code>SELECT COUNT(DISTINCT p.`case`) as count
  FROM FDA_CaseReports cr
    INNER JOIN ae_indi i ON i.isr = cr.isr
    LEFT JOIN ae_case_profile p ON cr.isr = p.isr
  </code></pre>
  
  <p>This seems like the LEFT may as well be INNER is there any catch?</p>