Ace Test Series: DBMS - ER Diagram To Relational Table

Question

Answer given is 3

But my answer is 2 . I have combined the Department and Project relation since the participation of Project was total.

please someone explain why can't we combine Department and Project.

Comments

You can combine two table in one table only if both have total participation.

Here only project have total participation and not to department that is why department should have its separate table .

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Here are some key points which might help you.

For one to many and many to many relation:

• [A]--1--<R>--N--[B]    => 2 tables ( each for A and B) + 1FK (In table  B referring to A's PK).
• [A]--M--<R>--N--[B]    => 3 tables ( each for A & B  and one more for relation R) + 2FK ( In relation R referring to A and B PK)

Now for one to one relations

• [A]--1--<R>--1--[B]   => 2 tables ( each for A and B) + 1FK (any where in table A or B).
• [A]-----<R>===[B]    => 2 tables ( each for A and B) + 1FK (in table B referring to A's PK).
• [A]===<R>===[B]    => 1 tables (for both A and B) + No FK.

For recursive relations:

• [A]-----<R>--N---[A]    => 1 tables (for A ) + 1FK (in table A referring to A's PK).

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@Bhushan Laware

thankx sir it will surely help :)

Answer 1

on solving we get given ER diagram

(EMP) _____ (manages) ___1___(DEPARTMENT)___1___(SPONSORED)______[PROJECT]

PROJECT has total participation but we don't know the mapping of it bw DEPARTMENT . So  it can be either one to many or one to one

In case one to one , we required one table for (DEP)(SPONS)(PROJECT)

In case one to many , we required two table for (DEP)(SPONS)(PROJECT)  

so we take worst case

EMP and DEP can have many one or one to one mapping. Instead of whatever the mapping , it can take  two table

(EMP , MANAGES)

(DEP)

(PROJECT, SPONS)

TOTAL 3