Preamble
PostgreSQL Constraints: You’ll learn how to create, add, and remove unique constraints in PostgreSQL with syntax and examples.
What is a unique constraint in PostgreSQL Constraints?
A unique constraint is a single field or a combination of fields that uniquely define a record. Some fields may contain zero values if the combination of values is unique.
What is the difference between a unique constraint and a primary key?
Primary key
|
Unique Constraint
|
---|---|
None of the fields, which are part of the primary key, can contain zero value.
|
Some fields that are part of the uniqueness constraint may contain zero values if the combination of values is unique.
|
Create a unique Constraint use the CREATE TABLE operator
Syntax to create a unique constraint using the CREATE TABLE operator in PostgreSQL:
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)
);
- table_name – The name of the table you want to create.
- column1, column2 – The columns you want to create in the table.
- constraint_name – The name of a unique constraint.
- uc_col1, uc_col2,… uc_col_n – The columns that make up the unique constraint.
Consider an example of how to create a unique limitation in PostgreSQL using the CREATE TABLE statement.
CREATE TABLE order_details
( order_detail_id integer CONSTRAINT order_details_pk PRIMARY KEY,
order_id integer NOT NULL,
order_date date,
size integer,
notes varchar(200),
CONSTRAINT order_unique UNIQUE (order_id)
);
In this example, we created a unique restriction for the order_details table called order_unique. It consists of only one field, order_id.
We can also create a unique constraint with more than one field, as in the example below:
CREATE TABLE order_details
( order_detail_id integer CONSTRAINT order_details_pk PRIMARY KEY,
order_id integer NOT NULL,
order_date date,
size integer,
notes varchar(200),
CONSTRAINT order_date_unique UNIQUE (order_id, order_date)
)
Create a unique Constraint using the ALTER TABLE operator
Syntax to create a unique constraint using ALTER TABLE in PostgreSQL:
ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column1, column2,... column_n);
- table_name – Name of the table to change. This is the table to which you want to add a unique constraint.
- constraint_name – The name of the unique constraint.
- column1, column2,… column_n – The columns that make up the unique constraint.
Let’s consider an example of how to add a unique limitation to an existing table in PostgreSQL using the ALTER TABLE operator.
ALTER TABLE order_details
ADD CONSTRAINT order_unique UNIQUE (order_id);
In this example, we have created a unique restriction for an existing order_details table with the name order_unique. It consists of a field with the name order_id.
We can also create a unique constraint with more than one field, as in the example below:
ALTER TABLE order_details
ADD CONSTRAINT order_date_unique UNIQUE (order_id, order_date);
Delete unique Constraint
Syntax to remove the unique restriction in PostgreSQL:
ALTER TABLE table_name
DROP CONSTRAINT constraint_name;
- table_name – The name of the table to change. This is the table from which you want to remove the unique constraint.
- constraint_name – The name of the unique constraint to be removed.
Let’s consider an example of how to remove a unique limitation from a table in PostgreSQL.
ALTER TABLE order_details
DROP CONSTRAINT order_unique;
In this example, we discard a unique restriction on an order_details table named order_unique.
PostgreSQL: Creating Tables with Constraints | Course
About Enteros
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