Stuck on some multichoice questions for MYSQL -

i doing past year test on mysql questions , not sure of following questions. can me out?

my picks following questions follows i'm not sure if right:

4c 

4c

add column , you'll error

column count doesn't match value count @ row 1

a) doesn't matter long don't want insert duplicate keys

b) data truncated , warning produced, not fail

d) not true, can specify whatever value want (as long it's not duplicate if column primary key or has unique index on it)

e) why should break. don't have have pk, though i'd recommend have one.

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6a

c, d , e not unique, of course. difference between , b is, in b district not needed make unique. have district in automatically created index primary keys? no. if want improve performance of queries on table, 1 can assume every field of table in clause. therefore want have compound index , in this, want have columns first, filter rows. , neither country, nor district.

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21b

not that, it's pretty clear.

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12d

the difficult part question how interpret sentences given. sure thing though, foreign key defined makes sure, can not insert value a.x when there's no corresponding value in b.y. makes b parent table, i'd go "from b a". , it's 1:n relation, that's clear.