Why Can't I Create a Unique Index on My nvarchar(max) Column in SQL Server?

Invalid Index Key Column Type: "is of a type that is invalid for use as a key column in an index"

In the context of relational database management systems, an error message indicating "Column 'key' in table 'misc_info' is of a type that is invalid for use as a key column in an index" arises when attempting to create a unique index on a column of a specific data type.

Problem Context:

The provided error message highlights that an attempt to create a unique index on a column named "key" in the table "misc_info" has failed. The column's data type, nvarchar(max), is incompatible with being used as an index key, leading to the error.

Solution:

Unique indexes impose limitations on the size of values they can store as keys. For the nvarchar data type, which can hold a variable length string, the maximum length allowed for index keys is 450 characters. This limitation is a safeguard against exceeding the 8000-byte row size constraint imposed by SQL Server.

To resolve the error, reduce the maximum length of the "key" column to 450 characters. This can be achieved by modifying the table definition as follows:

ALTER TABLE misc_info ALTER COLUMN key nvarchar(450) UNIQUE NOT NULL;

Alternatively, if the requirement is to store strings longer than 450 characters, consider using the varchar data type instead of nvarchar. Varchar has a maximum length limit of 8000 characters, providing more flexibility for storing longer keys while still maintaining the unique index constraint.

ALTER TABLE misc_info ALTER COLUMN key varchar(8000) UNIQUE NOT NULL;

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