U

Unique constraint

A database object (data integrity mechanism) that ensures that each value within a column or combination of columns is unique.

Uniquely identifies each row of a table and can contain a null.

Uniqueidentifier column

An internally used 4-byte column that SQL Server’s database engine automatically adds to a row to make each index key unique.

Unstructured data

Data that has no metadata, such as text files.

User-defined database role

A database role created by the DBA.

User-defined function

A custom function written by developers in SQL Server.