The maximum number of join columns in a JoinDef is 50. When you use multiple columns in a join other than a Master/Detail join, every set of join columns must have common values in order for the query to return a row. For example, if one set of join columns contains last names, and a second set of join columns contains first names, a query returns only the rows in which the first and last names are both identical. Thus, if two joined tables contained the first name Thomas and last name Jefferson in a particular row, a query returns data for that row from both tables. But if one table contained the first name Thomas and last name Jefferson, and the other contained the first name Thomas and last name Becket, a query cannot return any rows because only the Firstname join columns had a name in common.