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multiple unique columns in a table

Posted on 2011-09-16
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can we have multiple unique columns in a table. I was not clear on this concept. Please advise. thanks in advance
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Question by:gudii9
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8 Comments
 
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by:for_yan
ID: 36552810
you can have multiple unique columns in a table - there is nothing that may prevent it
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for_yan earned 500 total points
ID: 36552821
Let's say you have the table of patients of the hospital - they may have their own id for each patient , but they may have a column for insusrance id and those
may be also be unique - certainly no problem with that
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by:momi_sabag
momi_sabag earned 500 total points
ID: 36552830
you can have as many unique constraint you want on a table
a unique constraint verify that a column or set of columns have unique values in every row in a table
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by:Amitkumar Panchal
Amitkumar Panchal earned 500 total points
ID: 36553738
Yes, You can have unique constraint on the all fields of the table. Only thing is that if the column having unique constraint must contains a unique value in that field across the table. However, the field contains a null (no value) value in it, so multiple null values are allowed in the field having a  unique constraint.
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by:Kent Olsen
Kent Olsen earned 500 total points
ID: 36553894
Hi gudi,

The easiest way to enforce multiple unique columns in a table is to put a unique index on each of the columns.  

In fact, if you have a significant amount of data in the table it will be almost imperative to have these columns indexed as every insert or update will need to verify that the new data is unique within the column.  Without indexes on these column a full table scan will be required just to insert a new row!


Good Luck,
Kent
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by:gudii9
ID: 36718886
How to put  unique index on each column
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Expert Comment

by:for_yan
ID: 36718899
you can create index for any column or you can impose unique constraint on each colimn -
thes are different things
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LVL 45

Expert Comment

by:Kent Olsen
ID: 36718913
Hi gudi,

You'll need to add the index or constraint for each column that must be unique.

CREATE UNIQUE INDEX idx0001 ON mytable (column1);
CREATE UNIQUE INDEX idx0002 ON mytable (column2);
etc....



Kent
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