Hello radha,
Sorry I knw I m too late bcz of some problems but now I again xplain u
Before AND query select the pnumbr whose dnum is 5 frm project
And after AND query select the whole table which essn=ssn and c.bpno=b.bpno frm works_on c and not exist means gives those tuple which are not in selected by this (after AND) query
Now as we knw result of AND is true only when both condition (after AND nd befor AND) is true
Now it will result those bpno whose dnum is 5 but it is not equal to b.bpno and its c.essn is not equal to ssn
Thanks