SQL Database Constraint Rules Quiz

A signup page requires every user to provide an email. The database column storing emails must never allow empty or missing entries. Which constraint ensures this rule?

A company ensures that no two employees share the same work email. Which constraint enforces this uniqueness across all rows?

A salary column must always hold a positive number. Which constraint ensures only valid values are stored?

A new row should automatically receive today’s date when no creation date is provided. Which constraint supplies this automatic value?

A product table uses an ID column that uniquely identifies each product. It may not be NULL or duplicated. Which concept describes this requirement?

A foreign key prevents an order from referencing a non-existing customer. What concept describes this protection?

A system demands that age values be at least 18 for certain accounts. What type of rule enforces this?

A table definition requires every product code to be unique so that scanning devices identify items correctly. What constraint supports this?

A system wants to prevent negative stock values. Which constraint is appropriate?

A reminders app must always store a timestamp for each reminder. Which constraint ensures the timestamp cannot be omitted?

A price value should always be non-negative and not null. Which combination correctly applies both rules?

A DEFAULT value for a status field sets new entries to 'pending' unless overridden. What advantage does this bring?

A foreign key constraint is added to ensure product reviews always refer to a valid product. This prevents what?

A UNIQUE constraint on a username field ensures what important behavior?

A CHECK constraint validates that a temperature reading stays within realistic limits. What role does this play?

A NOT NULL column is added to an existing table. What challenge can arise when applying this constraint to existing data?

CHECK constraints help enforce rules like 'quantity must be positive.' What kind of data issue does this prevent?

A user table includes a UNIQUE constraint on phone numbers. What does this allow the system to do reliably?

A DEFAULT of true is added to an 'is_active' flag so newly created accounts begin active. What advantage does this provide?

A uniqueness rule on (first_name, last_name, birthday) is applied to ID verification. What type of key does this combination form?

If a column has a DEFAULT value but also allows NULL, what happens when a user explicitly inserts NULL?

A database uses a CHECK constraint requiring delivery dates to be on or after order dates. What does this prevent?

A NOT NULL column paired with DEFAULT ensures what behavior for new rows?

A UNIQUE constraint still allows NULL for a nullable column. Why?

A CHECK constraint on a role-level column ensures only allowed values such as 'admin', 'viewer', or 'editor' appear. What does this enforce?

This definition enforces that names cannot be empty. Which rule applies?

CREATE TABLE people (
  id SERIAL PRIMARY KEY,
  name TEXT NOT NULL
);

This rule ensures no two users share the same email. What does it enforce?

CREATE TABLE users (
  email TEXT UNIQUE
);

This constraint enforces that an item's price cannot be zero. What rule is applied?

CREATE TABLE items (
  price DECIMAL CHECK (price > 0)
);

This column receives a fallback value. What effect does this DEFAULT create?

CREATE TABLE tasks (
  status TEXT DEFAULT 'pending'
);

This definition adds a NOT NULL rule to a birthdate column. What must inserts do?

CREATE TABLE employees (
  birthdate DATE NOT NULL
);

This constraint prevents invalid rating values. What does the CHECK enforce?

CREATE TABLE ratings (
  score INT CHECK (score BETWEEN 1 AND 5)
);

This column enforces uniqueness on user names. What happens if a duplicate is inserted?

CREATE TABLE accounts (
  username TEXT UNIQUE
);

This rule ensures stock values cannot become negative. What logic is used?

CREATE TABLE inventory (
  stock INT CHECK (stock >= 0)
);

This DEFAULT assigns a current timestamp to new rows. What outcome results?

CREATE TABLE logs (
  created_at TIMESTAMP DEFAULT NOW()
);

This line defines a foreign key. What requirement does it enforce?

project_id INT REFERENCES projects(id)

This check ensures email addresses contain an '@'. What logic ensures this?

CHECK (email LIKE '%@%')

This constraint requires start_date to be before end_date. What effect does it produce?

CHECK (start_date < end_date)

This rule ensures that two values together must remain unique. What constraint is used?

UNIQUE (student_id, course_id)

This rule sets a default quantity. What happens on new inserts?

quantity INT DEFAULT 1

This definition blocks deleting referenced customers. What behavior occurs?

customer_id INT REFERENCES customers(id) ON DELETE RESTRICT

This command inserts a NULL into a NOT NULL field. What occurs?

INSERT INTO people (name) VALUES (NULL);

This CHECK rule ensures numeric values stay below a limit. What does it guarantee?

CHECK (score <= 100)

This DEFAULT uses NOW(). What behavior does it introduce?

created_at TIMESTAMP DEFAULT NOW()

This constraint ties a contact to a valid user. What rule does it define?

user_id INT REFERENCES users(id)

This CHECK ensures discount percentages stay within 0–50. What rule is applied?

CHECK (discount >= 0 AND discount <= 50)

This rule stops duplicate category names. What happens if two inserts attempt the same name?

category_name TEXT UNIQUE

This rule blocks invalid email formats. What does the CHECK accomplish?

CHECK (email LIKE '%@%')

This default assigns the value 'draft' to new posts. What result does this have?

status TEXT DEFAULT 'draft'

This CHECK ensures ages are realistic. What is being enforced?

CHECK (age BETWEEN 0 AND 120)

This rule allows a NULL foreign key when a project is removed. Which behavior applies?

project_id INT REFERENCES projects(id) ON DELETE SET NULL