IMP. DBMS doubt !!

Question

What does different types of lines mean here? Provide some ref. if you can

Comments

2 relation i think

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Well you think right.

But how?

and what does lines mean?

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how are u saying its 2??

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@thor , previously due to some misconeptions I was having some different answer.
But now I've updated it, kindly check.

Answer 1

• Single Line - No constraint 
• Double Line - Participation Constraint 
• Single Line with an Arrow on one end - Key constraint.
• Double Line with an Arrow on one of the End - Key + Participation Constraint.

As we can see there is

• 1:1 relationship R1 between A and B.
• 1:1 relationship R3 between B and C.
• N:N relationship R2 between A and C.
  
  Here we can Merge
• 1) A_R1_B
  OR
• 2) B_R3_C 
  
  But not Both At the Same Time.As A and C are not in 1:1 Total Participation.
  
  So, Minimum Tables Required -  2.
• 1) A_R1_B  AND C
  OR
• 2) B_R3_C AND A

Comments

Bt ans is 2

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What does 

• Single Line with an Arrow on one end - Key constraint. 

mean ?

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Key-That means 0 or 1 Participation.

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Minimum Tables Rquired= Total Entities Already Present+ Additional Table Required To Handle N:N Relationship if Any.

Here , total 3 Entities are already present.

And to Handle R2 as it is many - to - many, we can't merge A with C.

so 1+1=2 min tables are required here. 

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@Jason,how are u meging them..i did not understand

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@Akriti , we merge two Entities if there is a Total Prticipation and Key Constraint.

Here with A and B , and B and C there is total Participation and key Constraint.

So we can Merge either A with B or B with C.

But can't merge all together as there is No Constraint(N:N) in Relationship between A and C.

So, we need to maintain one of the Seperate Table either for A(if B and C get merged) or

C(if A and B get merged).

Hope You are Clear.

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so we merge only when there is total participation and 1:1 relation??

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Exactly !!!

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thankyou jason:-)

any link for this?

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@Jason, 3 should be the answer then. You forget R2. It is in m:n relation will require a seperate table

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First kindly verify the above approach for the rest 2 Tables.

I will discuss with you the 3rd Table topic.

Answer 2

Double lines indicate total participation of an entity in a relationship set(ref: korth)

Arrow represents one to one relation.

single line represents many to many relation.

Comments

how is ans to above $2$

Answer 3

same kind of que was asked in gate check it out...
https://gateoverflow.in/87025/gate2008-83?show=87025#q87025

 here the only difference is that B and A is in one to one relation as well as B and C also, but there is no constrain on A and C so for creating a table we  just need columns of B and A together in one table and col. of the primary key in B and all cols of  C in another table..  so only 2 relation is required... bcz as there is no constrain on A and C no relation is needed for that

So answer is 2 relation required here.. i hope u r cleared..

Comments

Plz provide the table name and attributes that you have taken in each.

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As A and C entities are in M:N relationship (i.e with R2) a seperate table is required for R2 relationship set

Now R2 table involves primary key of A and C entities as attributes

next A and B are in total paticipation,key constraint(1:1 relationship) with the relationship set R1

so we can merge A and B in to single table, that is A_R1_B

similarly B and C are in total participation,keyconstraint(1:1 relationship) with  the relationship set R3

so we can merge B and C in to single table that is B_R3_C

 

finally As B has total participation with R1 and R2 relationship sets we can merge the above two tables in to one .that is A_R1_B_R3_C

therefore minimum tables required is 2 (R2  and  A_R1_B_R3_C )

Hope this is the answer,Thank you for your question :)