ER diagram

Question

Comments

E1RE2 is one relation containing key (D)   // partial participation on E1 side we can combine.

R2E3 is one relation and key is(G).

two tables are sufficient.

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@santoshdevulapally......I've a doubt......

when we create a relational table....

i) for each entity

ii) for multivalued and composite attributes

iii) for weak entity and

iv) for m:n relationship......

in this question

atleast three tables required for 3 entities.....correct me if i'm wrong.....

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In the table there are no multivalued attributes and no m:n relationships all 1:1 relationships.

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but there are 3 entities...???

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@anmol

If there is partial participation we can merge into one table .

Key value is 'A' act like alternate key and AD act like primary key.

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When we have to merge the attributes and when to not in the relation......can you please explain me with an example.....????

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If there is one to one correspandance b/w two entities intermediate relation(R) can be merge with any of the two entites because there is one-one correspandance.

2)if there is (A-entity)1:m(B-entity)  R is mapped with B entity because In A one attribute mapped to many attributes.

3) if there is (A)m:1(B)  R is mapped with entity A same reason.

4) if any multivalued or weak entity set we cannot combine with any of entity this should be kept separate table.

5)If any total participation we can merge two tables and it satisfy the BCNF relation also.

6) if one side partial participation and one one correspandance b/w them we can merge R to participation entity side.

If u have any doubt refer std text book once.

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Thankyou......:-)

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@santosh, I agrre with you that E1 R E2 will form a single table. But you said AD is key?? Why is that so??? I think A as a key is sufficeient.

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@rahul

If null values are present in E1 we cannot map to any entity in E2.If A is primary key null values not allowed.

we cannot guarantee every entity need not participate with every other entity in E2.

So A act like alternate key and AD act like primary key.

It depends on table which we consider.

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But null values are not allowed in primary key right?? If E1 has 3 tuples and E2 has 5 tuples. Than E1 will habe null values right, that is what you wanted to say in previous comment if I am getting it right. Byt then can an attribute of compound primary key take a null value???

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primary key not allow null values.

kk got it .Here A is primary key so A is sufficient key for table E1RE2.

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@santosh i am not sure here, btw I was asking not concluding.

How will such relation be stored when E1 has 3 tuples and E2 has 5 tuples with E1 having total participation. I think D should be the key. And my doubt is that , Is attribute of compound PK allowed to take NULL???

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Even i have some doubt in this,now it is cleared.

E1(n attributes) and E2(m entities) [n<=m]and E1-E2 participation on E1 side

Then m acts like primary key.

In the given diagram D is key.

primary key may be simple or compound it is not allowed null values.

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Yeah. That should be the answer.

Cheers to discussions and gateoverflow☺

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Ha ha....

Answer 1

4 table

i.E1{a,b,c}

ii.E2{d,e,f}

iii.E3{g,h}

iv.R2{g,d}

Answer 2

in case of 1:1 relationship we can merge the two tables into a single table but here where is 1:1 ..is it b/w E2 and E3??
??