Question

Possible Duplicate:
SQL left join vs multiple tables on FROM line?

   SELECT messages.message_title, 
          messages.message_content, 
          messages.message_timestamp, 
          user_message_relations.sender_id
     FROM messages 
LEFT JOIN global_messages ON messages.message_id = global_messages.message_id
LEFT JOIN user_messages ON messages.message_id = user_messages.message_id, user_message_relations
    WHERE user_message_relations.receiver_id = 3

OR

SELECT messages.message_title, 
       messages.message_content, 
       messages.message_timestamp, 
       user_message_relations.sender_id
  FROM messages,  
       global_messages, 
       user_messages, 
       user_message_relations
 WHERE user_message_relations.receiver_id = 3

My main question is, what’s the point of using LEFT OUTER JOIN (or any kind of JOIN) if I can just call the table directly like the second query? Is there a benefit?

---

I see that the second method is not considered “best practice” … with that in mind, would this following query be correct if I wanted to populate the inbox of a user with the id of 3?

SELECT messages.message_id, 
    messages.message_title, 
    messages.message_content, 
    messages.message_timestamp, 
    user_messages.message_id, 
    user_message_relations.sender_id
FROM user_message_relations 
INNER JOIN user_messages ON user_message_relations.user_message_id = user_messages.user_message_id
INNER JOIN messages ON user_messages.message_id = messages.message_id
WHERE user_message_relations.receiver_id = 3

Answer 1

As OMG Ponies says, there’s no such thing as “calling tables” directly. In SQL, multiple tables can be be combined in three ways:

• outer join, where you match rows of the tables but allow for cases where one of the tables doesn’t have a matching row
• Cartesian product, where every row from the first table is combined with every row of the second table (hence “product”)
• an inner join, where rows are matched in both tables

As it happens, your three examples each has one of these, in the order I listed them. None is more “efficient” than the other; they return very different results!

The confusion comes because there are two different syntaxes for these; your example 2 is using the older SQL89 syntax in which table names are separated by commas, and the combination is a Cartesian product unless something in the WHERE clause says to join them.