here what this question releted to :- joins
there are 3 table they want to join ok,
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here, something that present in the qustion
1) union
- join
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usually vertical 8 like symbol use for natuaral join but if they but some condtion with this then this convert into conditional cross product.
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now here in order to solve this question remember relational algebra always eliminate duplicatiy
so the order in this question is good to solve
do union------>eleminate duplicate tuple------>then aplpy condition on the table a union b------>and if some tuple satified this condition they will do cross product with right side table.
1) when you will do union and after eliminating duplcay you will get 5 tuple.
2) after that if you will look at this condition they are like union means if you will look at first table and if they satisfyed
a.id>40 then they will get cross product with c even with out checking condition becuse this is or and T or anything always true .
3) from second point if you will do then in aUb two tuple will satisfied condition of a.id>40 so these two tuple make pair
with c and tuple they will give is 4 (simply like cross product )
now there tuple left in aUb which doesnot satisfied condition of id>40 so they will go and check for c>15 so only one
tuple will satisfed from c tablle so these 3 tuple will get cross product with that only one tuple so totol 7 tuple will come
in the result.