Why Does My Database Throw an 'Invalid Index Key Column' Error, and How Can I Fix It?

Invalid Index Key Column: Understanding and Solving the Error

The error message "Column 'key' in table 'misc_info' is of a type that is invalid for use as a key column in an index" indicates an issue with the data type of a column being used as a key in an index. While a quick search reveals that the index length limit is 450 characters, this alone doesn't provide a solution to the error.

In this case, the error pertains to a column named "key" with the data type nvarchar(max). To resolve this, the key column's data type must be modified to a valid type for an index key. Unique constraints on index keys have a maximum size limit of 8000 bytes per row, with only the first 900 bytes being utilized. Therefore, to ensure maximum compatibility and avoid potential errors, the key column should have a data type that allows for a maximum length of 450 characters.

The following revised SQL statement resolves the error by modifying the key column's data type to nvarchar(450):

By using nvarchar(450) for the key column, you can create a unique index without encountering the "invalid type for key column" error. Note that if possible, using varchar instead of nvarchar may allow you to increase the key column length to 900 characters.

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