Constraints in Postgresql (Advanced Topics)

How to add jointly unique constraints?

To add jointly unique constraints in PostgreSQL, you can specify multiple columns in the unique constraint definition. This ensures that the combination of values across those columns is unique. Here's an example of how to add jointly unique constraints:

CREATE TABLE my_table (
  id SERIAL PRIMARY KEY,
  first_name VARCHAR(255),
  last_name VARCHAR(255),
  email VARCHAR(255),
  ...
);

-- Add jointly unique constraint on first_name and last_name
ALTER TABLE my_table ADD CONSTRAINT uq_name UNIQUE (first_name, last_name);

In this example, a jointly unique constraint named uq_name is added to the my_table table, covering the first_name and last_name columns. This constraint ensures that no two rows can have the same combination of values for first_name and last_name.

You can also define jointly unique constraints during table creation. Here's an example:

CREATE TABLE my_table (
  id SERIAL PRIMARY KEY,
  first_name VARCHAR(255),
  last_name VARCHAR(255),
  email VARCHAR(255),
  ...
  CONSTRAINT uq_name UNIQUE (first_name, last_name)
);

In this case, the jointly unique constraint uq_name is defined inline with the table creation statement, covering the first_name and last_name columns.

Jointly unique constraints are useful when you want to ensure the uniqueness of combinations of values across multiple columns. They allow you to enforce business rules or data integrity requirements that involve multiple fields in your database schema.

Altering Constraints

To alter check constraints and unique constraints in PostgreSQL, you can use the ALTER TABLE statement along with the ADD, DROP, or ALTER clauses. Here are the steps for altering check constraints and unique constraints:

1. Altering Check Constraints:

To alter a check constraint, you need to drop and re-create it with the desired changes. Follow these steps:

a. Find the name of the check constraint:
You can query the pg_constraint system catalog table to retrieve the name of the check constraint associated with the table. Here's an example:

​​SELECT conname
​​FROM pg_constraint
​​WHERE conrelid like 'user_table_%';

b. Drop the existing check constraint:
Once you have the name of the check constraint, you can drop it using the ALTER TABLE statement with the DROP CONSTRAINT clause. Here's an example:

​​ALTER TABLE your_table
​​DROP CONSTRAINT your_constraint_name;

c. Add the altered check constraint:
After dropping the existing check constraint, you can add the altered check constraint using the ALTER TABLE statement with the ADD CONSTRAINT clause. Here's an example:

​​ALTER TABLE your_table
​​ADD CONSTRAINT your_new_constraint_name CHECK (new_condition);

Replace your_table with the name of your table, your_constraint_name with the name of the existing check constraint, and your_new_constraint_name with the desired name for the altered check constraint. Also, modify new_condition with the updated condition for the check constraint.

2. Altering Unique Constraints:

To alter a unique constraint, you can use the ALTER TABLE statement with the ALTER CONSTRAINT clause. Follow these steps:

a. Find the name of the unique constraint:
Similar to check constraints, you can query the pg_constraint system catalog table to retrieve the name of the unique constraint associated with the table. Here's an example:

​​SELECT conname
​​FROM pg_constraint
​​WHERE conrelid like 'your_table_%'::regclass;

b. Alter the unique constraint:
Once you have the name of the unique constraint, you can alter it using the ALTER TABLE statement with the ALTER CONSTRAINT clause. Here's an example:

​​ALTER TABLE your_table
​​ALTER CONSTRAINT your_constraint_name [RENAME TO your_new_constraint_name];

In this example, you can optionally use the RENAME TO clause to change the name of the unique constraint. If you don't want to change the name, you can omit the RENAME TO clause.

Replace your_table with the name of your table, your_constraint_name with the name of the existing unique constraint, and your_new_constraint_name with the desired new name for the unique constraint.

Remember to adjust the table and constraint names according to your specific scenario. By following these steps, you can alter check constraints and unique constraints in PostgreSQL.

Drop Constraints

To drop constraints in PostgreSQL, you can use the ALTER TABLE statement with the DROP CONSTRAINT clause. Here's the general syntax for dropping constraints:

ALTER TABLE table_name
DROP CONSTRAINT constraint_name;

You need to replace table_name with the name of the table that contains the constraint and constraint_name with the name of the constraint you want to drop.