Syntax

CREATE TABLE table_name
(column1 datatype null/not null,
column2 datatype null/not null,
...
CONSTRAINT constraint_name UNIQUE (column1, column2, . column_n)
);

Popis

Príkaz jazyka SQL
A unique constraint designates a column as a unique key. A composite unique key designates a combination of columns as the unique key. When you define a unique constraint inline, you need only the UNIQUE keyword. When you define a unique constraint out of line, you must also specify one or more columns. You must define a composite unique key out of line.

To satisfy a unique constraint, no two rows in the table can have the same value for the unique key. However, the unique key made up of a single column can contain nulls. To satisfy a composite unique key, no two rows in the table or view can have the same combination of values in the key columns. Any row that contains nulls in all key columns aically satisfies the constraint. However, two rows that contain nulls for one or more key columns and the same combination of values for the other key columns violate the constraint.

When you specify a unique constraint on one or more columns, Oracle implicitly creates an index on the unique key. If you are defining uniqueness for purposes of query performance, then Oracle recommends that you instead create the unique index explicitly using a CREATE UNIQUE INDEX statement. You can also use the CREATE UNIQUE INDEX statement to create a unique function-based index that defines a conditional unique constraint.

Príklad

Vytvorenie UNIQUE KEY pre svoju tabuľku

Táto stránka Vám vytvorí UNIQUE KEY pre Vašu tabuľku a priamo na tabuľke vytvorí trigger, ktorý bude pri porušení UNIQUE KEY vypisovať užívateľovi slovenskú hlášku.
Prejsť na stránku. 

Pozri aj

PRIMARY KEY, CONSTRAINT