3/31    What's the difference between KEY, PRIMARY KEY, UNIQUE, and INDEX?
        \_ 3, b, 6, 5
        \_ use SQL_book; import knowledge; do(homework);
        \_ I think PRIMARY KEY means both UNIQUE ande INDEX. INDEX does
           fast indexing with fast lookup but slow insert. I actually
           don't know the difference between KEY and PRIMARY KEY
        \_ PRIMARY KEY cannot be null but KEY and UNIQUE KEY can contain NULL