BDD_QCM

Null value:

• The boolean expression ((A<1) or (A>=1)) always evaluates to true

• The boolean expression (A=null) never evaluates to true

• The boolean expression (A=null) never evaluates to false

• Null can only be used attributes of some numeric type

Which of the following word pairs are synonims:

• table and relation

• table and tuple

• column and tuple

• attribute and row

Which of the following statements are true regarding the comparison between primary key and unique key ?

• A unique key attribute can be null, a primary key attribute cannot

• There can be more than one unique key in a table, but there can only be one primary key

• A primary key attribute can be null, a unique key attribute cannot

• There can be mutliple primary keys in a table, but there can only be one unique key

Which of the following SQL statements allows you to drop table T ?

• drop table T

• delete from T

• delete from T where true

• truncate table T

Consider table R(A_,B_, C). The query "select A, B from R" returns the same result as:

• select distinct A, B from R

• select A, B from R where A <> null

• select distinct A, B from R where B <> null

• select A, B from R where A <> null and B <> null

Consider the table T (A: int_, B: int). Which query below can produce duplicates?

• select * from T

• select B from T

• select * from T where A = 0

• select * from T where B =0

Consider tables R(A, B) and S(B, C). The query "select A from R natural join S" returns the same result as :

• select A from R cross join S

• select A from R, S

• select A from R, S where R.B = S.B

• select A from R cross join S where R.B = S.B

Consider tables R(A, B, C) and S(B, D). Then :

• "R natural inner join S" contains more tuples than "R natural join S"

• "R natural outer join S" contains at least as many tuples as "R natural join S"

• "R natural outer join S" contains at least as many tuples as "R natural left outer join S"

• "R natural left outer join S" contains at least as many tuples as "R natural right outer join S"

• None

The query "select avg(distinct b), min(c) from R" returns:

• Tuple (2,1)

• Tuple (3,1)

• Tuple (2.66,1)

• Tuple (2,null)

The query "select count (distinct B), max(distinct B) from R" returns:

• Tuple (null, null)

• Tuple (2,4)

• Tuple (3,4)

• an error