MySQL: Returning primary key as null

   SELECT pr.id as process_id, 
          lo.update_time as start_date, 
          REPLACE( sc.message, 'ip.ip.ip.ip', lo.variables ) as message, 
          sc.code 
     FROM logs AS lo 
LEFT JOIN pr开发者_高级运维ocesses AS pr ON lo.process_id = pr.id
LEFT JOIN status_codes AS sc ON lo.status_code_id = sc.code
 ORDER BY lo.id DESC 
    LIMIT 14

The issue with that query is that it's returning process_id as all NULL values, even though that's the auto-incremented primary key of that table without any NULL values.

Use SELECT lo.process_id not SELECT pr.id as process_id

The Rows in which pr.id is NULL are the ones for which no record exists in the processes pr table.

These are kept in because you are using a LEFT OUTER JOIN rather than an INNER JOIN

You're using a LEFT JOIN. Therefore, for every row that exists in either the logs or status_code tables that doesn't have a corresponding row in the logs table, you will get NULL values for the corresponding "row" of processes.