confusion in key and FD

Question

can i say A is a key here , here elements of A repeating , i  read primary key should be unique , what about otherr

Answer 1

A is a key without doubt since for every value of A we get a unique tuple (row).

but this table is not a relational database as relational database is a set  of tuples.

and elements of a set are unique

Comments

so bro if i say x  ----->   y  then  x will be a key ?

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yes if relation contains only two attributes i-e x and y

see key means through which you can uniquely identify a row.

if i give you a key x_i  and ask to give all unique tuples(rows
) where key value is x_i then output must be only and exactly one tuple.

for better insight https://stackoverflow.com/questions/5735592/determine-keys-from-functional-dependencies