Unanswered: Adding na (not applicable) to column...
Does it make sense to add a value such as na (for not applicable) to a column where that value doesn't apply? In my example below, 'cat' (for category) is of the primary keys of the table 'tbl_scores' so NULL values are not permitted for the column.
I do not have a value of cat for Science and I'm wondering if it's bad design to insert value like 'na' into a mysql column.
member_id subject cat topic_id practice_id score date
2 English grammar 100 1 60 2004-02-12
2 English vocab 100 1 80 2004-02-12
2 English vocab 100 2 20 2004-02-12
2 Science na 102 1 60 2004-02-12