An operator that can combine two SELECT statements into one large rowset.
UNIQUE constraint
Constraints that enforce entity integrity on a non-Primary Key. UNIQUE constraints ensure that no duplicate values are entered and that an index is created to enhance performance.
UNIQUE index
An index in which no two rows are permitted to have the same index value, thus prohibiting duplicate index or key values. The system checks for duplicate key values when the index is created and checks each time data is added with an INSERT or UPDATE statement.
UPDATE
The act of modifying one or more data values in an existing row or rows, typically by using the UPDATE statement. Sometimes, the term update refers to any data modification, including INSERT, UPDATE, and DELETE operations.
UPDATE STATISTICS
A command that updates statistical information for an index. Index statistics need to be up to date for the optimizer to decide upon the fastest route of access.
UPDATE TRigger
UPDATE triggers are FOR or AFTER triggers that can be used to evaluate UPDATE statements issued against a table to modify existing data. They can be used to allow or reject data modification attempts, to log the attempt, or to generate an alert. They fire automatically when an UPDATE statement is executed against the table.
updateable Subscribers
Subscribers that are capable of updating and modifying data when it is replicated. This option can be used with snapshot replication and transactional replication. A transactional or snapshot publication may allow updateable Subscribers. Changes made on the Subscriber's replica are propagated to the Publisher either in real time via DTC, or near real time via a queue.
user
A user is a database-wide security context.
user-defined function
A collection of T-SQL statements with a well-defined set of input parameters, but only one outputwhich can be a scalar value or a table. User-defined functions allow the encapsulation of various logical and database operations, but cannot be used to affect changes to a database.
