title: SELECT to return rows even if one of the tables is empty
body: <p>I'm trying to SELECT some columns from a table (joined with other tables).
  It must return all the rows where 'beactive_actividades.id' is 2 or where 'beactive_res_colaboradores.interv_id' is 2 (on the rows that 'act.id'='colab.actividade_id').</p>
  
  <p>My select is working, but only if there are any rows in beactive_res_colaboradores.
  If beactive_res_colaboradores is empty, it doesn't even return the rows where usr.id=2, which is opposed to what I need.</p>
  
  <p>I need all the rows WHERE there is either one condition OR the other.</p>
  
  <pre><code>SELECT DISTINCT act.id, act.descricao, act.detalhes, act.urgencia
  , est.id AS estado_id, est.descricao AS estado_desc
  , tipo.id AS tipo_id, tipo.descricao AS tipo_desc
  , cli.id AS cliente_id, cli.nome AS cliente_nome
  , usr.id AS user_id, usr.nome AS user_nome
  FROM beactive_actividades AS act
  LEFT JOIN beactive_act_estados AS est ON act.estado_id=est.id
  LEFT JOIN beactive_act_tipos AS tipo ON act.tipo_id=tipo.id
  LEFT JOIN beactive_clientes AS cli ON act.cliente_id=cli.id
  LEFT JOIN beactive_users AS usr ON act.interv_id=usr.id
  , beactive_res_colaboradores AS colab
  WHERE (usr.id=2 OR (colab.interv_id=2 AND act.id=colab.actividade_id))
  AND est.id&lt;6
  ORDER BY act.urgencia DESC, est.id DESC
  </code></pre>
  
  <p>I forgot to say that beactive_res_colaboradores is an helper table that links one user_id to an actividade_id, so only the pair user_id+actividade_id is unique.</p>