A unique constraint is a uniqueness constraint that repeats a divergence value for an application that requires a column type. You can add a separate column to the unique constraint. Ability to add multiple columns to a unique constraint (). Assuming that a unique constraint is added to multiple columns, it is only necessary to ensure that the values of these columns are not all the same.
When you create a table, add a unique constraint to the column. Forms such as the following:
column_name data_type [constraint constraint_name] unique or
CREATE TABLE table_name (
column_name data_type,[,...]
[Constraint constraint_name] Unique (column_name)
[,...]
)
It is also possible to constrain Lietina in a table that has already been created, and you will need to use alter TALBE. Add statement. Forms such as the following:
ALTER TABLE table_name ADD [constraint constraint_name] Unique (column_name);
Delete a unique constraint
To delete a unique constraint on a column, you can use the ALTER TABLE...DROP statement in the form such as the following:
ALTER TABLE table_name drop unique (column_name)
Assume that the constraint has a name. You can also delete the constraint using the specified name, such as the following statement form:
ALTER TABLE table drop constraint constraint_name;
Detailed actions such as the following:
sql> CREATE TABLE person
2 (PID number (4) is not null unique,
3 pname varchar2 (Ten) Unique,
4 Sex char (2)
5);
The table is created.
Create a person table above and specify a unique constraint for both PID and PName
Sql> INSERT into the person values (1, ' aaa ', ' female ');
1 rows have been created.
Sql> INSERT into the person values (2, ' aaa ', ' female ');
INSERT into person values (2, ' aaa ', ' female ')
*
An error occurred on line 1th:
ORA-00001: Violates the unique constraint (SYSTEM. sys_c0010079)
Sql> INSERT into the person values (2, ' abcd ', ' female ');
1 rows have been created.
As seen above, when adding the name of the same times the wrong.
Sql> INSERT into the person values (2, ' abcd ', ' female ');
1 rows have been created
Change under PName. There is no error at this time.
Another table is created.
Sql> CREATE TABLE P2 (
2 PID Number (4),
3 pname varchar2 (10),
4 Psex char (2),
5 constraint P2_unique unique (pid,pname)
6);
The table is created.
Sql> ALTER TABLE p2 add constraint unique_p2sex unique (psex); --Join unique for Psex
The table has changed.
sql> ALTER TABLE P2 drop unique (psex); --delete unique UNIQUE constraint
The table has changed.
sql> ALTER TABLE P2 drop constraint p2_unique;
Table changes.
Sql>
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Unique constraints for Oracle
