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putane.sanjay
Yak Posting Veteran
77 Posts |
 Posted - 2007-03-09 : 03:09:43
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| Hi Iam learning tsql .HOW TO CREATE A CANDIDATE KEY, COMPOSITE KEYS ON A COLUMN IN A TABLE? Please give me an example with the code . |
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snSQL
Master Smack Fu Yak Hacker
1837 Posts |
Posted - 2007-03-09 : 12:43:33
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| You don't create candidate keys, they are a logical concept. Any way that you can uniquely identify ever row in a table is a candidate key, there can be many and each one could be based on a single column or a group of columns (that's a composite key).What's important is that you select one of the candidate keys as the key that you will use to uniquely identify every row in the table, and that is the primary key.See the CREATE TABLE statement in Books Online for the syntax and code samples to create tables with a primary key. |
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rlaubert
Yak Posting Veteran
96 Posts |
Posted - 2007-03-09 : 13:10:13
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| A candidate key is any column in the table that you think may make a good index column. IE the column will be used in joins or the where clause.Composite Keys are two or more columns that are used as part of an index. Normally composite keys are a good idea when the join or where clause has multiple column to check OR when the composite index 'satisfies' the query. For example, if you have a composite index that contains the first and last name fields and your select statement returns the first and last names, a composite index will be used by SQL to return the data rather than reading the data from the table itself. This reduces IO operations and can improve performance in some cases.Raymond LaubertMCDBA, MCITP:Administration, MCT |
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create cadidate key