unique=true in hibernate

I would like to know more about unique=true in hibernate. Where and how and why to use it. How to use it for many-many, one-many, many-one, one-one Any ideas, resources, links, sample code highly appreciated. thanks in advance.

Getting more confused especially with one-one where for example many students can have same address in this scenario STUDENT_ADDRESS column under student table is set to unique in STUDENT.hbm.xml file.

similarly in one-many example where one student can have many phone numbers the phone id kept as unique within set collection of many-many tag.
please advise.
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Frankly, the example about students and phones does not seem very starightforward to me either (maybe someone else will eplain it to us),
however the concept of unique=true is really simple.
In this way you communicate to hibernate that according to your design this field cannot have the same
value in two different rows - this would allow to apply unique constraint to the corrresponding column
which stores this field in the table. This in many cases can make many database operations
more effective and would allow databse to do the check on insert
which would catcth possible mistkes long before they can create real problems for your data.
Sathish David Kumar NArchitectCommented:
unique is the one of  the property element  used by Hibernate's DDL generation .
To create the unique index in database.. You will not do anything with that hibernate will take care while ur using unique =true ! then Hiberante will increase the index value .
Sathish David Kumar NArchitectCommented:
Sorry i am getting your other question please tell bit clear !!
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Sathish David Kumar NArchitectCommented:
sorry i am leaking little bit in english...
gudii9Author Commented:
I was confused on how to define unique inside mapping file for many-many, many-one, one-one etc. please advise
Sathish David Kumar NArchitectCommented:
This parameter are you can use in u can define like that !! what ever you need !

Sathish David Kumar NArchitectCommented:
(1)name: The name of the property.  
(2) column (optional): The name of the foreign key column. This may also be specified by nested <column> element(s).  
(3) class (optional - defaults to the property type determined by reflection): The name of the associated class.  
(4) cascade (optional): Specifies which operations should be cascaded from the parent object to the associated object.  
(5) join (optional - defaults to select): Chooses between outer-join fetching or sequential select fetching.  
(6) update, insert (optional - defaults to true) specifies that the mapped columns should be included in SQL UPDATE and/or INSERT statements. Setting both to false allows a pure "derived" association whose value is initialized from some other property that maps to the same colum(s) or by a trigger or other application.  
(7) property-ref: (optional) The name of a property of the associated class that is joined to this foreign key. If not specified, the primary key of the associated class is used.  
(8) access (optional - defaults to property): The strategy Hibernate should use for accessing the property value.  
(9) unique (optional): Enable the DDL generation of a unique constraint for the foreign-key column. Also, allow this to be the target of a property-ref. This makes the association multiplicity effectively one to one.  
(10) not-null (optional): Enable the DDL generation of a nullability constraint for the foreign key columns.  
(11) optimistic-lock (optional - defaults to true): Specifies that updates to this property do or do not require acquisition of the optimistic lock. In other words, dertermines if a version increment should occur when this property is dirty.  
(12) lazy (optional - defaults to false): Specifies that this property should be fetched  
Sathish David Kumar NArchitectCommented:
Thats key element is common for many-to-many,one-to -one,many-to-one
gudii9Author Commented:
>>>unique (optional): Enable the DDL generation of a unique constraint for the foreign-key column. Also, allow this to be the target of a property-ref. This makes the association multiplicity effectively one to one.  

please elaborate on this. I was not clear.
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