Test Your MySQL UNIQUE Constraint Knowledge with This Quiz

Progress

True / False

In MySQL, a UNIQUE constraint ensures that all values in a column are different.

True False

The UNIQUE constraint in MySQL ensures that all values in a specified column are unique, meaning no two rows can have the same value in that column.

True / False

MySQL allows NULL values in a column with a UNIQUE constraint.

True False

In MySQL, a column with a UNIQUE constraint can have multiple NULL values, because NULL is considered different from any other NULL value.

True / False

You cannot add a UNIQUE constraint to an existing table in MySQL.

True False

You can add a UNIQUE constraint to an existing table in MySQL using the ALTER TABLE statement.

True / False

In MySQL, a table can have only one UNIQUE constraint.

True False

MySQL allows multiple UNIQUE constraints in a single table, each applying to different columns or sets of columns.

True / False

Dropping a UNIQUE constraint in MySQL requires the use of the DROP CONSTRAINT clause.

True False

In MySQL, the correct syntax to drop a UNIQUE constraint involves using the DROP INDEX statement because UNIQUE constraints are implemented using unique indexes.

True / False

In MySQL, UNIQUE constraints can be defined on multiple columns to ensure that the combination of column values is unique.

True False

MySQL allows UNIQUE constraints on multiple columns, ensuring that the combined values of the columns are unique across the table.

True / False

MySQL's UNIQUE constraint can be temporarily disabled and re-enabled for maintenance purposes.

True False

MySQL does not support temporarily disabling and re-enabling UNIQUE constraints. Constraints must be dropped and re-created if modifications are needed.

True / False

Adding a UNIQUE constraint on a column with existing duplicate values will fail in MySQL.

True False

If you try to add a UNIQUE constraint to a column with existing duplicate values, MySQL will reject the operation because it violates the uniqueness requirement.

True / False

In MySQL, a UNIQUE constraint always creates a unique index automatically.

True False

MySQL enforces the UNIQUE constraint by automatically creating a unique index on the specified column(s).

Q10

True / False

In MySQL certification exams, knowledge of using the UNIQUE constraint in conjunction with foreign keys is essential.

True False

Understanding how to use the UNIQUE constraint along with foreign keys is an important aspect of MySQL database design and is often tested in certification exams.

Q11

Single Choice

Which of the following best describes the UNIQUE constraint in MySQL?

Allows duplicate values in a column Ensures that all values in a column are different Forces a column to be emptyAllows NULL values only

The UNIQUE constraint ensures that all values in a column are distinct, preventing duplicate entries.

Q12

Single Choice

How do you add a UNIQUE constraint to an existing column in MySQL?

ALTER TABLE table_name ADD UNIQUE (column_name); CREATE UNIQUE TABLE table_name (column_name); INSERT UNIQUE INTO table_name (column_name);DELETE UNIQUE FROM table_name (column_name);

The correct syntax to add a UNIQUE constraint to an existing column is ALTER TABLE table_name ADD UNIQUE (column_name);.

Q13

Single Choice

What happens if you try to insert a duplicate value into a column with a UNIQUE constraint in MySQL?

The duplicate value is inserted without error The existing value is replaced with the new one An error is raised and the insert failsA warning is issued but the insert succeeds

Inserting a duplicate value into a column with a UNIQUE constraint will raise an error and the insertion will fail.

Q14

Single Choice

How can you define a UNIQUE constraint on multiple columns in a MySQL table?

CREATE UNIQUE INDEX ON table_name (column1, column2); ALTER TABLE table_name ADD UNIQUE (column1, column2); INSERT UNIQUE INTO table_name (column1, column2);DELETE UNIQUE FROM table_name (column1, column2);

To define a UNIQUE constraint on multiple columns, use the syntax ALTER TABLE table_name ADD UNIQUE (column1, column2);.

Q15

Single Choice

Which SQL keyword is used to remove a UNIQUE constraint in MySQL?

DELETE DROP REMOVECLEAR

The DROP keyword is used to remove a UNIQUE constraint from a table in MySQL.

Q16

Single Choice

Can a column with a UNIQUE constraint in MySQL contain NULL values?

Yes, but only one NULL value No, it cannot contain NULL values Yes, it can contain multiple NULL valuesIt depends on the MySQL version

A column with a UNIQUE constraint can contain NULL values, but only one NULL value is allowed.

Q17

Single Choice

How do you create a UNIQUE constraint on an existing table for a combination of columns in MySQL?

ALTER TABLE table_name ADD CONSTRAINT unique_key UNIQUE (column1, column2); CREATE UNIQUE INDEX unique_index ON table_name (column1, column2); ALTER TABLE table_name ADD UNIQUE INDEX (column1, column2);ALTER TABLE table_name ADD PRIMARY KEY (column1, column2);

The correct way to create a UNIQUE constraint on a combination of columns is ALTER TABLE table_name ADD CONSTRAINT unique_key UNIQUE (column1, column2);.

Q18

Single Choice

What is the maximum number of UNIQUE constraints you can have on a MySQL table?

1 16 3264

MySQL allows up to 64 UNIQUE constraints on a single table.

Q19

Single Choice

Which of the following is true about the UNIQUE constraint and primary keys in MySQL?

UNIQUE constraints allow duplicate values but primary keys do not A table can have multiple UNIQUE constraints but only one primary key Both UNIQUE constraints and primary keys can have multiple NULL valuesUNIQUE constraints and primary keys are identical in functionality

A table can have multiple UNIQUE constraints but only one primary key, and both enforce uniqueness of values.

Q20

Single Choice

Which of the following MySQL statements correctly removes a UNIQUE constraint named unique_key from the table employees?

ALTER TABLE employees DROP INDEX unique_key; ALTER TABLE employees DROP UNIQUE unique_key; ALTER TABLE employees DROP CONSTRAINT unique_key;ALTER TABLE employees DELETE UNIQUE unique_key;

The correct syntax to remove a UNIQUE constraint in MySQL is ALTER TABLE employees DROP INDEX unique_key;.

Q21

Multiple Choice

Identify the correct SQL statement to create a table with a UNIQUE constraint on a single column.

SQL Code

CREATE TABLE users (
 user_id INT PRIMARY KEY,
 email VARCHAR(100) UNIQUE
);

CREATE TABLE users ( user_id INT PRIMARY KEY, email VARCHAR(100) UNIQUE ); CREATE TABLE users ( user_id INT PRIMARY KEY, email VARCHAR(100) NOT NULL ); CREATE TABLE users ( user_id INT PRIMARY KEY, email VARCHAR(100) UNIQUE NOT NULL );CREATE TABLE users ( user_id INT PRIMARY KEY, email VARCHAR(100) );

The UNIQUE constraint ensures that all values in the email column are distinct, preventing duplicate entries.

Q22

Multiple Choice

Choose the correct SQL statement to add a UNIQUE constraint to an existing column.

SQL Code

ALTER TABLE employees
ADD CONSTRAINT unique_ssn UNIQUE(ssn);

ALTER TABLE employees ADD CONSTRAINT unique_ssn UNIQUE(ssn); ALTER TABLE employees MODIFY COLUMN ssn UNIQUE; ALTER TABLE employees ADD UNIQUE (ssn);ALTER TABLE employees ALTER COLUMN ssn UNIQUE;

The ALTER TABLE statement with ADD CONSTRAINT is used to add a UNIQUE constraint to an existing column, ensuring that all SSN values are unique.

Q23

Multiple Choice

Determine the correct SQL statement to create a table with a composite UNIQUE constraint.

SQL Code

CREATE TABLE orders (
 order_id INT,
 product_id INT,
 customer_id INT,
 UNIQUE (order_id, product_id)
);

CREATE TABLE orders ( order_id INT, product_id INT, customer_id INT, UNIQUE (order_id, product_id) ); CREATE TABLE orders ( order_id INT, product_id INT, customer_id INT, UNIQUE (product_id, customer_id) ); CREATE TABLE orders ( order_id INT, product_id INT, customer_id INT, UNIQUE (order_id, customer_id) );CREATE TABLE orders ( order_id INT, product_id INT, customer_id INT, UNIQUE (order_id) );

A composite UNIQUE constraint ensures that the combination of values in order_id and product_id columns is unique across the table.

Q24

Multiple Choice

Which SQL statement correctly drops a UNIQUE constraint from a table?

SQL Code

ALTER TABLE employees
DROP INDEX unique_email;

ALTER TABLE employees DROP INDEX unique_email; ALTER TABLE employees DROP UNIQUE unique_email; ALTER TABLE employees REMOVE CONSTRAINT unique_email;ALTER TABLE employees DROP CONSTRAINT unique_email;

In MySQL, UNIQUE constraints are implemented as unique indexes. To drop a UNIQUE constraint, you use the DROP INDEX statement.

Q25

Multiple Choice

Select the correct SQL statement to create a table where a column has both NOT NULL and UNIQUE constraints.

SQL Code

CREATE TABLE products (
 product_id INT PRIMARY KEY,
 product_code VARCHAR(50) NOT NULL UNIQUE
);

CREATE TABLE products ( product_id INT PRIMARY KEY, product_code VARCHAR(50) NOT NULL UNIQUE ); CREATE TABLE products ( product_id INT PRIMARY KEY, product_code VARCHAR(50) UNIQUE ); CREATE TABLE products ( product_id INT PRIMARY KEY, product_code VARCHAR(50) UNIQUE NOT NULL );CREATE TABLE products ( product_id INT PRIMARY KEY, product_code VARCHAR(50) NOT NULL );

This SQL statement ensures that the product_code column is unique and cannot contain NULL values, providing data integrity.

Q26

Multiple Choice

Identify the correct SQL statement to enforce a UNIQUE constraint across multiple columns in an existing table.

SQL Code

ALTER TABLE customers
ADD CONSTRAINT unique_name_address UNIQUE (first_name, last_name, address);

ALTER TABLE customers ADD CONSTRAINT unique_name_address UNIQUE (first_name, last_name, address); ALTER TABLE customers ADD UNIQUE (first_name, last_name, address); ALTER TABLE customers MODIFY COLUMN first_name UNIQUE; MODIFY COLUMN last_name UNIQUE; MODIFY COLUMN address UNIQUE;ALTER TABLE customers ADD CONSTRAINT unique_name_address (first_name, last_name, address);

This SQL statement adds a composite UNIQUE constraint across the first_name, last_name, and address columns, ensuring no duplicate entries for these combinations.

Q27

Multiple Choice

Which SQL statement correctly creates a table with a UNIQUE constraint that allows for NULL values?

SQL Code

CREATE TABLE inventory (
 inventory_id INT PRIMARY KEY,
 serial_number VARCHAR(50) UNIQUE
);

CREATE TABLE inventory ( inventory_id INT PRIMARY KEY, serial_number VARCHAR(50) UNIQUE ); CREATE TABLE inventory ( inventory_id INT PRIMARY KEY, serial_number VARCHAR(50) NOT NULL UNIQUE ); CREATE TABLE inventory ( inventory_id INT PRIMARY KEY, serial_number VARCHAR(50) UNIQUE NOT NULL );CREATE TABLE inventory ( inventory_id INT PRIMARY KEY, serial_number VARCHAR(50) );

In MySQL, a UNIQUE constraint allows multiple NULL values in a column because NULL is considered distinct.

Q28

Multiple Choice

Determine the correct SQL statement to add a UNIQUE constraint and ensure no existing duplicate values cause errors.

SQL Code

ALTER IGNORE TABLE employees
ADD UNIQUE (email);

ALTER IGNORE TABLE employees ADD UNIQUE (email); ALTER TABLE employees ADD CONSTRAINT unique_email UNIQUE (email); ALTER TABLE employees ADD UNIQUE (email); DELETE FROM employees WHERE email IS NULL;ALTER TABLE employees ADD UNIQUE (email) IGNORE NULL;

The IGNORE keyword allows the addition of the UNIQUE constraint by automatically removing any duplicate values from the column.

Q29

Multiple Choice

Select the correct SQL statement to enforce a UNIQUE constraint on a column while creating an index for faster lookups.

SQL Code

CREATE UNIQUE INDEX idx_employee_ssn
ON employees (ssn);

CREATE UNIQUE INDEX idx_employee_ssn ON employees (ssn); ALTER TABLE employees ADD INDEX idx_employee_ssn UNIQUE (ssn); ALTER TABLE employees ADD CONSTRAINT unique_ssn UNIQUE (ssn);CREATE INDEX idx_employee_ssn ON employees (ssn) UNIQUE;

This SQL statement creates a unique index on the SSN column, enforcing uniqueness and improving query performance.

Q30

Multiple Choice

Which SQL statement correctly enforces a UNIQUE constraint on a column in an existing table with existing data?

SQL Code

ALTER TABLE customers
ADD CONSTRAINT unique_phone_number UNIQUE (phone_number);

ALTER TABLE customers ADD CONSTRAINT unique_phone_number UNIQUE (phone_number); ALTER TABLE customers MODIFY COLUMN phone_number UNIQUE; ALTER TABLE customers ADD UNIQUE (phone_number);ALTER TABLE customers CHANGE phone_number UNIQUE;

Before enforcing the UNIQUE constraint, the existing data should be reviewed to ensure no duplicate values exist in the phone_number column.

MySQL Database Quiz Questions