Oracle Unique | Unique - oracle tutorial - sql tutorial




Oracle unique

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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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 unique table
  1. Insert the values in the wikitechy_emp table by using INSERT Statement.
  2. .
  3. Click Run button to execute the query.
  4. 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 table

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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 enable table

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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 drop table

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