$14\mathrm{.}14.$ Does a relation with two or more columns always have an MVD$?$ Show with

an example.

$14\mathrm{.}15.$ Define fourth normal form. When is it violated? When is it typically applicable?

$14\mathrm{.}16.$ Define join dependency and fifth normal form.

$14\mathrm{.}17.$ Why is $5$NF also called project$-$join normal form $($PJNF$)?$

$14\mathrm{.}18.$ Why do practical database designs typically aim for BCNF and not aim for

higher normal forms?