All 7 Constraints (Primary Key, Foreign Key, Check & Unique) questions
More in Constraints (Primary Key, Foreign Key, Check & Unique)
What is a foreign key constraint, and how does it help maintain data integrity between two related tables? What is a check constraint, and how do you use one to enforce a specific business rule at the database level? How is a unique constraint different from a primary key constraint, and when would you use one instead of the other? How do you temporarily disable a constraint to perform a bulk data load, and how do you safely re-enable it afterward? How do cascading actions work with foreign key constraints, such as ON DELETE CASCADE, and what risks come with using them? How would you add a new constraint to an existing table that already contains data, especially if some existing rows might violate that constraint?
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SQL Server › Constraints (Primary Key, Foreign Key, Check & Unique)

What is a primary key constraint, and why is it important for a database table?

Beginner

Answer

A primary key constraint uniquely identifies each row in a table, ensuring no two rows can have the same value in that column, and it also automatically prevents that column from containing NULL values, making it the reliable way other tables can reference a specific row.
CREATE TABLE Customers (
  CustomerId INT PRIMARY KEY,
  Name VARCHAR(100)
);
Real-world example An online store uses CustomerId as the primary key for its customers table, guaranteeing that every customer record can always be uniquely identified and referenced by other tables like orders.

Common follow-ups: Can a table have more than one primary key column?;What happens if you try to insert a duplicate primary key value?

Data Types & Schema Design;Normalization
All 7 Constraints (Primary Key, Foreign Key, Check & Unique) questions