CompTIA DataSys+ (DS0-001)Database FundamentalsEasy
A developer is writing a script to insert new customer records into a `Customers` table. Each customer record must include a unique `CustomerID`. Which SQL Data Definition Language (DDL) constraint ensures that no two customer records can have the same `CustomerID`?
- ANOT NULL
- BFOREIGN KEY
- CPRIMARY KEY
- DCHECK
Show answer & explanationAnswer & explanation
Correct answer: C. PRIMARY KEY
A PRIMARY KEY constraint uniquely identifies each record in a table and automatically enforces a NOT NULL constraint. This ensures that every customer will have a unique and non-empty CustomerID.
Why the other options are wrong
- A. NOT NULL ensures that a column cannot contain a NULL value but does not enforce uniqueness.
- B. FOREIGN KEY establishes a link between two tables, enforcing referential integrity.
- D. CHECK enforces that all values in a column satisfy a specific condition.
PRIMARY KEY
A column or set of columns in a table that uniquely identifies each row in that table.
- Must contain unique values.
- Cannot contain NULL values (implicitly NOT NULL).
- A table can have only one primary key.
Memory trick: Primary keys are the unique ID badges of your data.