How to fix Error 456 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 456 – Collation Conflict on Implicit Conversion

Why This Error Happens

This error occurs when a query compares, joins, or concatenates two character-based values (columns, variables, or literals) that have been assigned different collations, and SQL Server cannot determine which collation should take precedence during implicit conversion. This typically arises when tables or databases are created with different default collations, or when a column-level COLLATE clause conflicts with the database or session collation. It is also common after restoring a database from a server with a different default collation, or when linking objects across databases with mismatched collation settings.

Step-by-Step Resolution Guide

Step 1: Identify the Conflicting Objects and Their Collations

  • Run the failing query in SQL Server Management Studio (SSMS) and note the exact object names mentioned in the error (%ls placeholders will show actual column/table names).
  • Query sys.columns and sys.databases to check collation values:
  SELECT name, collation_name FROM sys.columns WHERE object_id = OBJECT_ID('YourTableName');
  SELECT name, collation_name FROM sys.databases;
  • Confirm the server-level default collation using:
  SELECT SERVERPROPERTY('Collation');

Step 2: Apply an Explicit COLLATE Clause in the Query

  • Modify the query to force both sides of the comparison/join to use the same collation:
  SELECT * FROM TableA a
  JOIN TableB b
    ON a.ColumnX COLLATE DATABASE_DEFAULT = b.ColumnY COLLATE DATABASE_DEFAULT;
  • Alternatively, explicitly specify a known collation (e.g., SQL_Latin1_General_CP1_CI_AS) if DATABASE_DEFAULT does not resolve the conflict.
  • Save and re-run the query to confirm Error 456 no longer appears.

Step 3: Standardize Collation at the Column or Database Level (Permanent Fix)

  • For a permanent fix, alter the column collation to match the rest of the schema:
  ALTER TABLE YourTableName
  ALTER COLUMN ColumnX VARCHAR(100) COLLATE SQL_Latin1_General_CP1_CI_AS;
  • If the conflict is database-wide (common after cross-server restores), consider migrating data into a database with a consistent collation using sp_rename and staging tables, since ALTER DATABASE ... COLLATE only affects new objects, not existing ones.
  • Restart the SQL Server 2025 instance service via Services.msc (Windows Server 2025/Windows 11) if collation changes were made at the instance level, to ensure metadata caches refresh.

Step 4: Validate Application Connection Strings

  • Check that ODBC/OLEDB connection strings or sqlcmd sessions are not overriding the collation via connection-level settings.
  • Ensure client applications aren’t passing literals with an unexpected collation (e.g., Unicode N'...' strings vs. non-Unicode columns).

Alternative or Verification Steps

  • Re-run the original query or stored procedure that triggered Error 456 — if it executes without the collation conflict message, the fix is confirmed.
  • As a quick alternative without schema changes, wrap the specific problematic columns with COLLATE DATABASE_DEFAULT inline in every affected query, though this is a short-term workaround rather than a permanent solution.