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SQL Server FAQ - UNIQUE Constraint Creating Default Index

By: FYIcenter.com

(Continued from previous topic...)

Does the UNIQUE Constraint Create an Index?

If you add the UNIQUE constraint on a column, SQL Server will automatically add a non-clustered index for that column. The tutorial exercise below shows you the index created as part of the UNIQUE column, "id", of "fyi_links":

USE FyiCenterData;

-- Drop the old table, if needed
DROP TABLE fyi_links;

-- Create a table with a UNIQUE constraint
CREATE TABLE fyi_links (
  notes VARCHAR(1024),
  counts INT,
  created DATETIME NOT NULL DEFAULT(getdate())

-- Create an index for column "url"
CREATE INDEX fyi_links_url ON fyi_links (url);

-- View indexes
EXEC SP_HELP fyi_links;
index_name               index_description           keys
-----------------------  --------------------------  ----
fyi_links_url            nonclustered located        url
                         on PRIMARY

UQ__fyi_links__4222D4EF  nonclustered, unique
                         key located on PRIMARY      id

Notice that the index created as part of the UNIQUE constraint is named by SQL Server as "UQ__fyi_links__4222D4EF".

(Continued on next topic...)

  1. What Are Indexes?
  2. How To Create an Index on an Existing Table?
  3. How To View Existing Indexes on an Given Table using SP_HELP?
  4. How To View Existing Indexes on an Given Table using sys.indexes?
  5. How To Drop Existing Indexes?
  6. Is the PRIMARY KEY Column of a Table an Index?
  7. Does the UNIQUE Constraint Create an Index?
  8. What Is the Difference Between Clustered and Non-Clustered Indexes?
  9. How To Create a Clustered Index?
  10. How To Create an Index for Multiple Columns?
  11. How To Create a Large Table with Random Data for Index Testing?
  12. How To Measure Performance of INSERT Statements?
  13. Does Index Slows Down INSERT Statements?
  14. Does Index Speed Up SELECT Statements?
  15. What Happens If You Add a New Index to Large Table?
  16. What Is the Impact on Other User Sessions When Creating Indexes?
  17. What Is Index Fragmentation?
  18. What Causes Index Fragmentation?
  19. How To Defragment Table Indexes?
  20. How To Defragment Indexes with ALTER INDEX ... REORGANIZE?
  21. How To Rebuild Indexes with ALTER INDEX ... REBUILD?
  22. How To Rebuild All Indexes on a Single Table?
  23. How To Recreate an Existing Index?

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