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Need SQL to add a Key to an Existing Table

Posted on 2006-06-14
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Last Modified: 2011-10-03
Can someone please show the SQL to add an key an existing table.  I need to add a primary key to a table but to not have access to enterprise manager only query analyzer.  
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Question by:Hojoformo
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by:Anthony Perkins
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ALTER TABLE YourTableName ADD CONSTRAINT PK_YourTableName PRIMARY KEY CLUSTERED  (YourColumnName)
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MikeWalsh earned 250 total points
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There is no primary key in this table already? Do you plan on adding data for the primary key column for the existing records? Adding a primary key after the fact with data in a table can cause some issues which need working around.. but the syntax for adding a key is, or is this is an existing column?:


-- To add a column
ALTER TABLE tableName
ADD columnName dataType

-- To add a primary key
ALTER TABLE tableName
ADD Constraint
[PK_KeyName] PRIMARY KEY  CLUSTERED
      ([ColumnName])

This is the basic syntax. There are other options you can add. you may already have a clustered index, you can make this a nonclustered index. it has to be an index of one of those sorts.

You have query analyzer so I would look up ALTER TABLE in the index of books online (SQL Server Help) to see the full syntax.. Type ALTER TABLE, highlight it and press Shift + F1 and you will see more on this.

Also search for Primary Key and select creating.
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by:MikeWalsh
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you don't need to add the column if it already exists. If it doesn't exist you will need to add the column, fill it with values and then assign the primary key constraint. Hopefully you already have the column and are just asking to identify an existing column(or columns) as a primary key.
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by:jrb1
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First make sure the column you want to be the primary key is defined as NOT NULL.  Then:

ALTER TABLE Customer ADD PRIMARY KEY (SID);
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by:Racim BOUDJAKDJI
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Just a little additional comment concerning UNIQUENESS on a relational standpoint which states that all rows must be unique.

Adding a numerical key defining a counter only is equivalent to implement a surrogate arbitrary key (It is not sufficient to define UNIQUENESS on a relational standpoint)...You need to adress the issue of candidate keys defining uniqueness on the row then create an additional constraint of uniqueness based on one candidate key or concatenation of candidate keys.  If you don't adresse such issue there is little usefulness into defining a primary key whose primary role is to identify uniquely the row and avoid redundance.

For instance if Table1 is made of Field1, Field2, Field3, Field4 (all being int for instance) and the combination of Field1 and Field4 is what makes the row unique then you can create an additional column using both candidate keys to make a concatenated primary key...


alter table tableName add primary_key as (Field1 + Field4) int
alter table tableName add constraint [PK_KeyName] primary key clustered  ([primary_key])

or you can create a surrogate key as stated above but you will still need to adresse the issue of uniqueness...

Of course, if the script above refuses to run ...that means you will have to deal with dupplicate records first...

Hope this helps...
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