The problem of specifying a condition in a LEFT OUTER JOIN has been discussed before. LEFT OUTER JOIN with ON condition or WHERE condition? (September 16, 2005) explains the difference.
Your query probably looked something like this:
SELECT t1.foo , t2.bar FROM table1 AS t1 LEFT OUTER JOIN table2 AS t2 ON t2.table1_id = t1.id WHERE t2.datefld = ( SELECT MIN(datefld) FROM table2 WHERE table1_id = t1.id )
The problem here is that the WHERE condition will surely filter out all unmatched rows from table1. If there is no matching row in table2, then the MIN will be NULL, and so the WHERE condition fails (nothing is equal to NULL).
SELECT t1.foo , t2.bar FROM table1 AS t1 LEFT OUTER JOIN table2 AS t2 ON t2.table1_id = t1.id AND t2.datefld = ( SELECT MIN(datefld) FROM table2 WHERE table1_id = t1.id )
Now, the MIN condition has been moved into the ON clause. In effect, the LEFT OUTER JOIN now says "get matching rows based on the keys and on the matching row being the MIN matching row." In other words, if there is no matching row in table2, the row from table1 is still returned.
This was first published in May 2008