Foreign key refer to unique constraint????

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  • KULJEET
    #1

    Foreign key refer to unique constraint????

    foreign key also refer to unique constraint.
    (GREAT...)

    1.then table that containt unique constraint act as master table?????
    2.IS unique constraint will replace with primary key??
    3.Is unique constraint gives all fuctionality as primary key constraint???

    thanks
    kuljeet pal singh
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    • Carlos
      #2
      Re: Foreign key refer to unique constraint????

      foreign key also refer to unique constraint.
      (GREAT...)
      >
      1.then table that containt unique constraint act as master table?????
      Yes.
      2.IS unique constraint will replace with primary key??
      3.Is unique constraint gives all fuctionality as primary key constraint???
      The only diference between PK and UK (besides built-in indexes) is
      that UK can contain NULLs, whereas PK cannot. A given table could
      contain many rows with a NULL in the UK column (this is, the
      'uniqueness' affects only NOT NULL columns).

      Regards.

      Carlos.

        Comment

        • Carlos
          #3
          Re: Foreign key refer to unique constraint????

          foreign key also refer to unique constraint.
          (GREAT...)
          >
          1.then table that containt unique constraint act as master table?????
          Yes.
          2.IS unique constraint will replace with primary key??
          3.Is unique constraint gives all fuctionality as primary key constraint???
          The only diference between PK and UK (besides built-in indexes) is
          that UK can contain NULLs, whereas PK cannot. A given table could
          contain many rows with a NULL in the UK column (this is, the
          'uniqueness' affects only NOT NULL columns).

          Regards.

          Carlos.

            Comment

            • Mark D Powell
              #4
              Re: Foreign key refer to unique constraint????

              kuljeet_twtpl@h otmail.com (KULJEET) wrote in message news:<febbed51. 0401200544.7256 a5a2@posting.go ogle.com>...
              foreign key also refer to unique constraint.
              (GREAT...)
              >
              1.then table that containt unique constraint act as master table?????
              2.IS unique constraint will replace with primary key??
              3.Is unique constraint gives all fuctionality as primary key constraint???
              >
              thanks
              kuljeet pal singh
              Yes, a FK can refer to a UK or a PK. This is desirable as some table
              will have both a PK and UK on them and it may be necessary to validate
              against both.

              If this bothers you
              What is the difference between a unique index and a unique constraint?
              http://www.jlcomp.demon.co.uk/faq/uk_idx_con.html


              HTH -- Mark D Powell --

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