How to fix Error 10601 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: Error 10601 – Cannot Specify Included Columns for a Clustered Index

Why This Error Happens

SQL Server Error 10601 occurs because the INCLUDE clause is only valid for nonclustered indexes, not clustered indexes. A clustered index already physically contains all columns of the table in its leaf level, so there is no concept of a “included” (non-key) column to add separately. This error typically surfaces when a CREATE INDEX or CREATE TABLE script incorrectly applies the INCLUDE syntax to a CLUSTERED index definition, often due to copy-pasted T-SQL templates or auto-generated scripts from migration tools.

Step-by-Step Resolution Guide

Step 1: Identify the Offending T-SQL Statement

  • Locate the exact CREATE INDEX or ALTER TABLE statement that triggered Error 10601 in SSMS or your deployment log.
  • Check whether the index definition contains both the CLUSTERED keyword and an INCLUDE (...) clause in the same statement.
  • Example of invalid syntax:
  CREATE CLUSTERED INDEX IX_Orders_Date
  ON dbo.Orders (OrderDate)
  INCLUDE (CustomerID);

Step 2: Decide on the Correct Index Strategy

  • Determine if you actually need a clustered index (defines physical row order) or a nonclustered index (separate structure with optional included columns).
  • If you need included columns for query performance (covering index), switch the index type to NONCLUSTERED.
  • If you truly need a clustered index, remove the INCLUDE clause entirely since all columns are already “included” by definition.

Step 3: Rewrite the Index Definition

  • For a covering index with included columns, use:
  CREATE NONCLUSTERED INDEX IX_Orders_Date
  ON dbo.Orders (OrderDate)
  INCLUDE (CustomerID);
  • For a pure clustered index without included columns, use:
  CREATE CLUSTERED INDEX IX_Orders_Date
  ON dbo.Orders (OrderDate);
  • Save and re-run the corrected script in SSMS or your deployment pipeline (SSDT/DACPAC, Flyway, etc.).

Step 4: Audit Related Scripts and Templates

  • Search your source control repository (Git, TFVC) for similar CREATE CLUSTERED INDEX ... INCLUDE patterns using a text search across .sql files.
  • Update any shared index-generation templates or ORM migration scripts (Entity Framework, Liquibase) that may have produced the invalid syntax.
  • Re-validate automated database build pipelines on Windows Server 2025 to ensure the corrected scripts are used going forward.

Alternative or Verification Steps

  • Verify the fix by re-executing the corrected CREATE INDEX statement in SSMS connected to your SQL Server 2025 instance — it should complete without Error 10601, and you can confirm the index type via sys.indexes (type_desc column should show CLUSTERED or NONCLUSTERED as expected).
  • If you are unsure whether columns should be key columns or included columns, as an alternative, create the clustered index on the key columns only, then add a separate nonclustered covering index with the INCLUDE clause for the additional columns needed by your queries.