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  1. #1
    SitePoint Zealot Jafar's Avatar
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    Unique & PRIMARY- what's the difference?

    Hi,
    I want to know what's the difference between Unique and primary key in mysql?
    for example what's difference between using Unique for Username Field (in user table ) or primary?
    Thanks
    regards

  2. #2
    SQL Consultant gold trophysilver trophybronze trophy
    r937's Avatar
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    in mysql? nothing
    rudy.ca | @rudydotca
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  3. #3
    SitePoint Guru
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    Columns in a unique constraint may be null, which is not allowed for primary key columns.

  4. #4
    reads the ********* Crier silver trophybronze trophy longneck's Avatar
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    also, the auto_increment feature requires a primary index.

  5. #5
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    More than one field in a table may be unique for a record. One can be the primary key, for the other you'd use the unique constraint.
    membership_no, name, address, telno, locker_no
    membership_no is unique and the primary key
    locker_no is also unique in the table, as no-one shares lockers. (assumes old members who used the locker last year are deleted)

  6. #6
    SitePoint Zealot Jafar's Avatar
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    thnx

    Thanks All,
    very useful tips.


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