Question

I have two tables: projects and user_licenses.

I’d like to grab the entire list of projects from the database, as well as the user’s current license state. The licenses table has a user ID field which I check against a $_SESSION variable for the value. The thing is, they might not have a license, or a non-logged in visitor may want to see the projects list. My question is this: How can I get the data from the left table always display, but only grab data for that row from the right table when certain conditions are met?

The query I have at the moment is this:

   SELECT Projects.*, 
          UserLicenses.* 
     FROM Projects 
LEFT JOIN UserLicenses ON Projects.id = UserLicenses.project_id 
    WHERE UserLicenses.user_id = 12 
 ORDER BY name ASC

Answer 1

Add any extra conditions to the on clause of the left join. They will only affect the joined table.

  SELECT Projects.*, 
          UserLicenses.* 
     FROM Projects 
LEFT JOIN UserLicenses 
       ON Projects.id = UserLicenses.project_id 
          and UserLicenses.user_id = 12 
          and UserLicences.Question = '6*7'
          and UserLicences.Answer = 42
 ORDER BY name ASC

This will return projects without matching licenses.