Ace _testseries

Question

I have doubt in  3rd statement second part 

Answer 1

alternate keys can have nul values.

primary keys don't have null values.

Comments

But unique key can have only one value as null in unique specified attribute(column). in question they specifying : unique key can hav null VALUES.

 ( what is difference between alternate key ,unique key & primary key,??)

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Hello ravi , please try to answer in detailed way , and answer each and every portion.

i don't think the way you provided answer would help anyone.

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Super Key is the set of one or more column (ie attributes) which uniquely identifies a record.

(unique key)Candidate key is a minimal Super key.(it mean we cant remove any attributes from it otherwise it will not remain Superkey anymore).

Primary Key is a arbitrary selected Candidate key that don't allow any null values. There must be only and only One primary key. We can choose any candidate key as a Primary key. Other candidate keys which are not chosen as Primary are called Alternate Keys

Answer 2

1)for BCNF , dependencies will be of type    super key->any attribute , so we don't care what's there in right side , left side must be super key. if i talk more specific then if prime attributes are transitively dependent on key then relation will be in 3NF and every 3NF is BCNF.

2) this is must condition for 2NF , not for 3NF as even if this is satisfied then still there would be redundancy caused by non-prime->non-prime , which is eliminated in 3NF.

3) unique key is different from primary key.It can be NULL.

https://en.wikipedia.org/wiki/Unique_key

https://stackoverflow.com/questions/9565996/difference-between-primary-key-and-unique-key

Comments

thank you sir...

Answer 3

a)false

b)false

c)true