Oracle Unique | Unique - oracle tutorial - sql tutorial
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What is unique constraint in oracle ?
- A unique constraint is a single field or combination of fields that uniquely defines a record.
- Some of the fields can contain null values as long as the combination of values is unique.
Difference Between Primary Key and Unique Key:
Primary Key:
- None of the fields that are part of the primary key can contain a null value.
Unique Key:
- Some of the fields that are part of the unique constraint can contain null values as long as the combination of values is unique.
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Syntax
CREATE TABLE table_name
(
column1 datatype [ NULL | NOT NULL ],
column2 datatype [ NULL | NOT NULL ] ...
CONSTRAINT constraint_name UNIQUE (uc_col1, uc_col2, ... uc_col_n)
);
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Parameters or Arguments
table_name
- The name of the table that you wish to create.
column1, column2
- The columns that you wish to create in the table
constraint_name
- The name of the unique constraint.
uc_col1, uc_col2, ... uc_col_n
- The columns that make up the unique constraint.
Example
CREATE TABLE wikitechy(
Emp_Id int NOT NULL,
Emp_Name varchar(20) NOT NULL,
Position varchar(20),
City varchar(20) ,
CONSTRAINT wikitechy_unique UNIQUE ( Emp_Id) );
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- Insert the values in the wikitechy_emp table by using INSERT Statement.
- .
- Click Run button to execute the query.
- We can see the row which has been successfully inserted in the wikitech_emp table
Disable:
Syntax
- ALTER TABLE table_name DISABLE CONSTRAINT constraint_name;
Example
ALTER TABLE wikitechy DISABLE CONSTRAINT wikitechy_unique;
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Enable:
Syntax
ALTER TABLE table_name ENABLE CONSTRAINT constraint_name;
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Example
ALTER TABLE wikitechy ENABLE CONSTRAINT wikitechy_unique;
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Drop:
Syntax
ALTER TABLE table_name DROP CONSTRAINT constraint_name;
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Example
ALTER TABLE wikitechy DROP CONSTRAINT wikitechy_unique;
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