Mixing explicit and implicit joins fails with “There is an entry for table … but it cannot be referenced from this part of the query”

The SQL spec states that explicit joins are performed before implicit joins. This is an implicit join:

FROM table1 t1, table2 t2 WHERE t1.id=t2.t1id

This is an explicit join:

FROM table1 t1 JOIN table2 t2 ON (t1.id=t2.t1id)

This code bit:

categories c 
     LEFT JOIN photos p 
        ON p.referencekey = i.key 

is an explicit join and is run first. Note that at this point the table aliased as i hasn’t been looked at yet, so it can’t be joined yet. Note that MySQL fixed this behaviour in 5.2 I believe, and this query will no longer work there either.