Question

I have three tables – node, content_type_product and share_content. There may be a 1:N relationship beetween node and share_content. I want to pull out only one record per id.
If there is multiple records in share_content, I want the latest one, that is the highest value of sc.auto_id

SELECT sc.uid, n.uid, n.nid, sc.message 
  FROM node n 
    LEFT JOIN content_type_product p ON n.nid = p.nid 
    LEFT JOIN share_content sc ON n.nid = sc.nid 
  WHERE n.nid = 40513 
  GROUP BY sc.nid
  ORDER BY sc.auto_id

Answer 1

Why are you joining to content_type_product ??
But that aside, try

 SELECT c.uid, n.uid, n.nid, c.message  
 FROM node n  
   LEFT JOIN share_content c 
      ON c.nid = n.nid  
          And c.auto_id
             = (Select Max(auto_id)
                From share_content 
                Where nid = p.nid ) 
 Where n.nid = 40513  
 ORDER BY c.auto_id