Adding a UNIQUE Constraint

The following ALTER TABLE statement adds a UNIQUE constraint to the emp.empno column:


SQL> CREATE TABLE EMP (EMPNO NUMBER(4) NOT NULL,
  2                    ENAME VARCHAR2(10),
  3                    JOB VARCHAR2(9),
  4                    SAL NUMBER(7, 2),
  5                    DEPTNO NUMBER(2));

Table created.

SQL>
SQL> ALTER TABLE emp
  2  ADD CONSTRAINT empno_uq UNIQUE (empno);

Table altered.

SQL>

Any existing or new rows must always have a unique value in the status column.

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Constraints:
  1. Adding a Constraint with CHECK
  2. Adding a NOT NULL Constraint
  3. Adding a FOREIGN KEY Constraint
  4. ON DELETE CASCADE
  5. ON DELETE SET NULL
  6. Adding a UNIQUE Constraint
  7. CHECK constraint
  8. Multiple Constraints
  9. Dropping a Constraint
  10. Disabling a Constraint
  11. Enabling a Constraint
  12. Deferred Constraints
  13. Getting Information on Constraints:user_constraints and all_constraints
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