What are SQL constraints and how do they work?

What are SQL constraints and how do they work?

SQL constraints are rules applied to columns in a table to ensure the integrity of the data. They define the conditions that data must meet to be valid.

Can you give me examples of different types of SQL constraints?

Sure! Common types of SQL constraints include NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, and CHECK constraints.

What is the difference between PRIMARY KEY and UNIQUE constraints?

A PRIMARY KEY uniquely identifies each record in a table and cannot be NULL, while a UNIQUE constraint also ensures uniqueness but can allow NULL values.

How do I add a constraint to an existing table?

You can add a constraint using the ALTER TABLE statement. For example: ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column_name);