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Foriegn key and null values

  • October 31, 2007 at 1:22 am

    #63109

    Hi Experts,

    How a table which contains foreign key relationship accepts more than one NULL value ? What is the reason ? which logic is applied here ?

    For example,

    Table A - Master Table , Eno - Primary key

    Eno Name

    1 aa

    2 bb

    3 cc

    4 dd

    5 ee

    Table B - Child Table, Eno - Foreign Key

    Eno Age

    1 21

    2 25

    3 55

    Null 34

    Null 52

    Null 42

    Null 48

    Null 31

    How it accepts null value ? What concept is applied here ?

    Regards:cool:

    Karthik

    karthik

  • October 31, 2007 at 2:49 am

    #746949

    Hi Karthik,

    nulls are handled differently in foreign keys. A quote from books online:

    "if any column of a composite FOREIGN KEY constraint contains null values, verification of all values that make up the FOREIGN KEY constraint is skipped."

    If you want to avoid this, you can specify "NOT NULL" for the referencing columns.

    You can read more in Books Online: http://msdn2.microsoft.com/en-us/library/ms175464.aspx

    Regards,

    Andras

    Andras Belokosztolszki, MCPD, PhD
    GoldenGate Software

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