what is primary key,unique key, foreign key? can u teach me
in simple language?
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Answer Posted / subin
PRIMARY KEY - is a constraint(restriction) placed on one or a group of more than one columns of a table by the database admin (who does so while creating the table) that the value(s) of this column(s) will never be duplicate ensuring that every record(row) of the table is a unique one.
# There can only be a single primary key for a table. Hence the name primary key.
# This is the key used by foreign keys of other tables
to identify a unique record of this table.
# null value cannot be given to columns of primary key as
it will not ensure uniqueness of the record.
UNIQUE KEY - is a constraint on one or group of columns that
disallows duplication of records. It differs from primary key in that
# no restrictions to number of unique keys for a table.
# depending on different database software being used
one or many records can have null values.
FOREIGN KEY - constraint placed on certain column(s) of a table ensuring that the set of value(s) in theese column(s) will uniquely identify a record in a mentioned foreign table (mentioned while defining foreign key)that should be aptly associated with this record of this table.This is achieved with the help of primary key of the foreign table.
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