Why is my SQL Index Failing Due to Key Column Length Limitations?

Key Column Index Length Limitation in SQL

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 that the length of key column exceeds the maximum allowed for an index.

The maximum length of an index column depends on the data type used. For nvarchar data type, the maximum length is 450 characters. This is because a unique constraint on nvarchar columns can't exceed 8000 bytes per row, and only the first 900 bytes are used.

To resolve this issue, reduce the length of the key column to a maximum of 450 characters. The following modified SQL statement creates a table with a key column of nvarchar(450):

create table [misc_info]
(
    [id] INTEGER PRIMARY KEY IDENTITY NOT NULL,
    [key] nvarchar(450) UNIQUE NOT NULL,
    [value] nvarchar(max) NOT NULL
);

If possible, consider switching to varchar instead of nvarchar, which can increase the maximum length to 900 characters.

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