How to create a nonclustered index in SQL Server

Slow queries in SQL Server almost always share the same cause: a missing index. A nonclustered index is the fastest way to speed up lookups on columns that are not part of the primary key.

Prerequisites

• SQL Server (2016 or later) and SQL Server Management Studio (SSMS) or Azure Data Studio.
• ALTER permission on the table where you will create the index.
• A table with enough data to see the difference (a few thousand rows).

Step 1: Identify the column to index

Pick the column that appears often in WHERE or JOIN clauses. In this example we will index the Email column of the Clientes table.

Step 2: Create the nonclustered index

Use the CREATE NONCLUSTERED INDEX statement, giving the index a descriptive name:

CREATE NONCLUSTERED INDEX IX_Clientes_Email
    ON dbo.Clientes (Email);

If your queries return more columns, add them with INCLUDE to build a covering index:

CREATE NONCLUSTERED INDEX IX_Clientes_Email_Inc
    ON dbo.Clientes (Email)
    INCLUDE (Nome, Telefone);

Step 3: Confirm the query uses the index

Turn on the actual execution plan in SSMS (Ctrl+M) and run the query:

SELECT Nome, Telefone
FROM dbo.Clientes
WHERE Email = 'ana@exemplo.pt';

Verify the result

In the execution plan you should see an Index Seek on IX_Clientes_Email instead of a Table Scan. An Index Seek means SQL Server jumps straight to the right rows — much faster.

Conclusion

Creating the right index is the highest-return optimization in SQL Server. Index the columns you actually filter on and avoid indexing everything — each index speeds up reads but makes writes slightly slower. Which of your slowest queries would benefit from an index today?