Answer Posted / priya reddy

primary key- if a column is declared as priamry then there
wont be any repeated values in that column throughout that
table, as well as it wont take any null values and we can
use primary key only once in a table. but we can declare
combination of two columns as primary key.
unique- if a column is declared as unique then again there
wont be any repeated values in that column, but this will
take null values and even we can declare more thatn one
unique column in a table.
foreign key-if a column is declared with this key then we
have to use a reference ie we need to give the column which
will have the same values as this column and the table that
column belongs to.. this referced column of other table
will be the primary key of that table.

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