1. The existence of a null value (3-value-logic) 2. permitting duplicate rows 3. permitting duplicate columns. 4. "outer" joins. and.. other things, that you c

1. The existence of a null value (3-value-logic) 2. permitting duplicate rows 3. permitting duplicate columns. 4. "outer" joins. and.. other things, that you can enforce in your client application, but then that rather defeats the whole point of having a "relational database management system". you might as well just have a dumb key-value store, if you have to enforce so much of what a "relational database" is in your app. note that not even the SQL standards use the word "relational". anywhere. Years ago I couldn't believe this, but go ahead and look. It's not there.