How can I create a unique constraint on my column (SQL Server 2008 R2)?

Using the SQL Server Management Studio (SSMS)

Would you rather use an interactive interface than write SQL? Here are the steps for SQL Server Management Studio (SSMS):

1. Fire up SSMS and find your server and database.
2. Identify your table under the tables list, right-click on it, then choose "Design".
3. Select the column to apply the constraint to, right-click and pick "Indexes/Keys" from the context menu.
4. Hit the "Add" button in the "Indexes and keys" dialogue box.
5. Choose "Unique Key" as the "Type".
6. Confirm the column in the columns section.
7. Give your constraint a meaningful name in the properties section.
8. Close it up and hit "Save" to apply the changes.

Don't forget: unique constraint violations will occur during the process if the column data is not already unique.

Diving deeper: Understanding unique constraints and unique indexes

You've got two choices in SQL Server to enforce uniqueness: unique constraints and unique indexes.

Unique Constraints

• A UNIQUE CONSTRAINT guarantees that all column data is unique.
• It's like a declaration that we mean business.
• It'll flash an error message when someone tries to duplicate a value.

Unique Indexes

• A UNIQUE INDEX also ensures uniqueness, but it optimizes queries as an added bonus!
• It's a low-level technique for the speed demons among us.
• Provides the same duplication protection as a unique constraint.

In SQL Server, an index in the background underpins both methods, but your choice may depend on specific design needs or performance requirements.

Troubleshooting: Overcoming common issues

Implementing unique constraints may seem straightforward until you encounter these common issues:

Handling Null Entries

If you've got NULLs in your column (a SQL Server unique constraint does consider NULL a unique value), and want to allow multiple NULLs:

CREATE UNIQUE NONCLUSTERED INDEX idx_col_nullable ON tbl(col)
WHERE col IS NOT NULL;  -- NULLs? No problem! 😉


Renaming a Unique Constraint

Gave your constraint a forgettable name? Rename it:

EXEC sp_rename 'old_constraint_thingy', 'new_catchy_name', 'OBJECT';  -- Whoops! Fixed it...
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Applying Constraints to Existing Tables

Wish to add a constraint to a table already brimming with data? First, check for duplications:

SELECT col, COUNT(*)
FROM tbl
GROUP BY col
HAVING COUNT(*) > 1;  -- Hmmm... spot any twins? 👬👭


Then resolve the duplicates, and you're all set to add the unique constraint!

Naming Constraints Well

When creating unique constraints, dig deep and come up with meaningful names. A uniform naming system like uq_tablename_columnname can help you easily identify constraints later on.

Efficient Constraint Checks

For a quick overview of table constraints, use this handy system procedure:

EXEC sp_help 'tbl';  -- SQL's version of a quick "Google search"


The resulting list highlights existing constraints against the 'constraint_name' column and describes their effect in the 'definition' column.

Integrity of Data

Unique constraints ensure data integrity. This can prove vital in systems where data accuracy is strictly enforced.

GUI or T-SQL?

The GUI in SSMS is definitely user-friendly, but when handling multiple objects or performing tasks repetitively, using T-SQL scripts can be more efficient.