However, two rows that contain nulls for one or more key columns and the sameĬombination of values for the other key columns violate the constraint. Any row thatĬontains nulls in all key columns automatically satisfies the constraint. Must use table_constraint syntax rather than column_constraintĪ constraint that designates a composite unique key, no two rows in the tableĬan have the same combination of values in the key columns.
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Oracle creates an index on the columns of a unique key, so a composite unique DefiningĬomposite unique key is a unique key made up of a combination of columns. Statement also uses the USING INDEX option to specify storage characteristicsįor the index that Oracle creates to enable the constraint. Syntax: CREATE TABLE dept (deptno NUMBER(2), dname VARCHAR2(9), loc VARCHAR2(10), CONSTRAINT unq_dname UNIQUE (dname) USING INDEX PCTFREE 20 TABLESPACE user_x STORAGE (INITIAL 8K NEXT 6K) ) You can define and enable this constraint with the table_constraint However, the constraint does allow departments without names. ThisĬonstraint ensures that no two departments in the table have the same name. On the DNAME column: CREATE TABLE dept (deptno NUMBER(2), dname VARCHAR2(9) CONSTRAINT unq_dname UNIQUE, loc VARCHAR2(10) ) Ĭonstraint UNQ_DNAME identifies the DNAME column as a unique key. Exampleįollowing statement creates the DEPT table and defines and enables a unique key Definingĭefine a unique key on a single column with column_constraint syntax. However, you can designate the sameĬolumn or combination of columns as both a unique key and a foreign key. Same column or combination of columns as both a unique key and a primary key orĪs both a unique key and a cluster key.
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Key column cannot be of datatype LONG or LONG RAW. However, the unique key made up of a single column can Satisfy a UNIQUE constraint, no two rows in the table can have the same valueįor the unique key. Constraint designates a column or combination of columns as a unique key.