LIMITing an SQL JOIN

You'll have to specify which expense item you want to get. The most expensive? The newest? Then join against a subquery that returns only that:

SELECT
    expense.*, transaction.*, user.*
FROM
    (SELECT * FROM expense WHERE ...) AS expense
INNER JOIN
    transaction ON expense_id = transaction_expense_id

So assuming we can exclude the user table, it could be rewritten as:

select * from expense, transaction where expense_id = transaction_expense_id

Now if you want to apply a limit, you could do it like this:

select * from expense, transaction where expense_id = transaction_expense_id and 
  expense_id in (select expense_id from expense limit 1)

Would that do what you wanted? Obviously you need to be cautious about what order your expense_ids are going to come back in, so you probably want to use ORDER BY whatever.

Edit: Given the MySQL limitation described in your comment below, maybe this will work:

select * from (select id from expense order by WHATEVER limit 1) as t1, transaction where expense_id=transaction_expense_id;

Ben