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This is most easily done in two parts and then defaulting the rest to welcome:
q)T:update newCol:`hi from T where a=`F,b<c q)T:update newCol:`hi from T where a<>`F,c>d q)T:update`welcome^newCol from T q)T a b c d newCol ------------------ F 10 9 10 welcome R 20 21 22 welcome
You may find the reference on update statements useful: https://code.kx.com/q/ref/update/
If you want it done in one line (which doesn’t scale well for any additional clauses you might want):
q)update newCol:`welcome`hi any(all(a=`F;b<c);all(a<>`F;c>d))from T a b c d newCol ------------------ F 10 9 10 welcome R 20 21 22 welcome