SQL Server Unique Constraints for Large Text Columns

In SQL Server, an unique constraint is enforced using an unique index on the column. But the index key size has a limitation of 900 bytes and this is why we can't enforce the unique constraint on columns over 900 bytes. This limitation not only applies to indexes, but also for foreign keys and primary keys as well. This 900 byte limitation is documented in the Maximum Capacity Specifications for SQL Server.

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