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SQL UNIQUE Constraint. The UNIQUE constraint ensures that all values in a column are different. Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has a UNIQUE constraint.
20 gru 2023 · You can create a unique constraint in SQL Server by using SQL Server Management Studio or Transact-SQL to ensure no duplicate values are entered in specific columns that don't participate in a primary key. Creating a unique constraint automatically creates a corresponding unique index.
5 lip 2011 · Here is the syntax for creating a unique CONSTRAINT as opposed to a unique INDEX. ALTER TABLE publishers ADD CONSTRAINT uqc_pub_name UNIQUE (pub_name) It is important to note that there are subtle differences dependent on which method you use to enfore the uniqueness of a column.
4 lut 2021 · The UNIQUE constraint is very commonly used in SQL. It makes sure that a column or a combination of columns does not contain duplicate values. Its implementation is quite straightforward, and it’s very handy across a range of use cases.
2 sty 2024 · A UNIQUE constraint can be referenced by a FOREIGN KEY constraint. When a UNIQUE constraint is added to an existing column or columns in the table, by default, the Database Engine examines the existing data in the columns to make sure all values are unique.
20 gru 2013 · CREATE TABLE UniqueTable ( ID int IDENTITY, TableCode varchar (20), CONSTRAINT UQ_TableCode UNIQUE CLUSTERED (TableCode)) To, który indeks opatrzyć atrybutem CLUSTERED zależy od zapytań i tego, które kolumny są częściej wykorzystywane w tych zapytaniach (głównie w sekcji WHERE i predykatach złączeń).
SQL Server UNIQUE constraints allow you to ensure that the data stored in a column, or a group of columns, is unique among the rows in a table. The following statement creates a table whose data in the email column is unique among the rows in the hr.persons table: CREATE TABLE hr.persons( person_id INT IDENTITY PRIMARY KEY,
