Primary key declaration in MySQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a primary key on the 'id' column.

MySQL

CREATE TABLE users (id INT [1], name VARCHAR(100));

Drag options to blanks, or click blank then click option'

AINDEX

BFOREIGN KEY

CUNIQUE

DPRIMARY KEY

Attempts:

3 left

💡 Hint

Common Mistakes

Using FOREIGN KEY instead of PRIMARY KEY

Using UNIQUE without PRIMARY KEY

Forgetting to specify any key constraint

The PRIMARY KEY constraint uniquely identifies each record in the table.

2fill in blank

medium

Complete the code to declare a primary key on the 'order_id' column in this table.

MySQL

CREATE TABLE orders (order_id INT [1], order_date DATE);

Drag options to blanks, or click blank then click option'

AUNIQUE

BPRIMARY KEY

CCHECK

DDEFAULT

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNIQUE instead of PRIMARY KEY

Using CHECK constraint incorrectly

Not specifying any key constraint

The PRIMARY KEY constraint ensures each order_id is unique and not null.

3fill in blank

hard

Fix the error in this table creation by correctly declaring the primary key.

MySQL

CREATE TABLE products (product_id INT, name VARCHAR(50), [1] (product_id));

Drag options to blanks, or click blank then click option'

APRIMARY KEY

BFOREIGN KEY

CUNIQUE

DCHECK

Attempts:

3 left

💡 Hint

Common Mistakes

Using FOREIGN KEY instead of PRIMARY KEY

Using UNIQUE constraint without PRIMARY KEY

Using CHECK constraint incorrectly

The PRIMARY KEY constraint must be declared to uniquely identify product_id.

4fill in blank

hard

Fill both blanks to declare a composite primary key on 'student_id' and 'course_id'.

MySQL

CREATE TABLE enrollments (student_id INT, course_id INT, [1] (student_id, [2]));

Drag options to blanks, or click blank then click option'

APRIMARY KEY

Bcourse_id

CFOREIGN KEY

DUNIQUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using FOREIGN KEY instead of PRIMARY KEY

Listing only one column in the key

Using UNIQUE instead of PRIMARY KEY

A composite primary key uses PRIMARY KEY and lists both columns inside parentheses.

5fill in blank

hard

Fill all three blanks to declare a primary key on 'id' and add a unique constraint on 'email'.

MySQL

CREATE TABLE users (id INT [1], email VARCHAR(100) [2], [3] (email));

Drag options to blanks, or click blank then click option'

APRIMARY KEY

BUNIQUE

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing PRIMARY KEY and UNIQUE keywords incorrectly

Forgetting to declare unique constraint on email

Using FOREIGN KEY instead of UNIQUE

The 'id' column is declared as PRIMARY KEY, 'email' column has a UNIQUE constraint, and the table-level unique constraint is declared on 'email'.

Practice

1. What is the main purpose of a PRIMARY KEY in a MySQL table?

easy

A. To uniquely identify each row and prevent duplicates

B. To allow NULL values in a column

C. To speed up SELECT queries only

D. To store large text data

Solution

1. Step 1: Understand the role of PRIMARY KEY

   A PRIMARY KEY uniquely identifies each row in a table and ensures no duplicate or NULL values exist in that column.

2. Step 2: Compare options with PRIMARY KEY purpose

   Only To uniquely identify each row and prevent duplicates correctly states the unique identification and prevention of duplicates, which is the main purpose.

3. Final Answer:

   To uniquely identify each row and prevent duplicates -> Option A

4. Quick Check:

   Primary key = unique row ID [OK]

Hint: Primary key means unique and no duplicates [OK]

Common Mistakes:

• Thinking primary key allows NULL values
• Confusing primary key with indexing only
• Assuming primary key stores large text

2. Which of the following is the correct syntax to declare a primary key inline in a MySQL CREATE TABLE statement?

easy

A. CREATE TABLE users (id INT, name VARCHAR(50 PRIMARY KEY));

B. CREATE TABLE users (id INT, name VARCHAR(50), PRIMARY KEY id);

C. CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50));

D. CREATE TABLE users (id INT, PRIMARY KEY, name VARCHAR(50));

Solution

1. Step 1: Review inline primary key syntax

   Inline primary key declaration is done by adding PRIMARY KEY right after the column type, like id INT PRIMARY KEY.

2. Step 2: Check each option

   CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50)); correctly places PRIMARY KEY inline after id INT. Others have syntax errors or misplaced keywords.

3. Final Answer:

   CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50)); -> Option C

4. Quick Check:

   Inline primary key = column type + PRIMARY KEY [OK]

Hint: Inline primary key goes right after column type [OK]

Common Mistakes:

• Placing PRIMARY KEY after column list without parentheses
• Using PRIMARY KEY without column name
• Putting PRIMARY KEY after another column

3. Given the table creation below, what will be the result of inserting two rows with the same id?

CREATE TABLE products (
  id INT PRIMARY KEY,
  name VARCHAR(100)
);
INSERT INTO products (id, name) VALUES (1, 'Pen');
INSERT INTO products (id, name) VALUES (1, 'Pencil');

medium

A. Both rows inserted successfully

B. Error due to duplicate primary key value

C. Second row overwrites the first row

D. NULL inserted for the second row's id

Solution

1. Step 1: Understand primary key uniqueness

   The primary key column id must have unique values. Duplicate values cause errors.

2. Step 2: Analyze the insert statements

   The first insert adds id=1. The second insert tries to add the same id=1, violating uniqueness.

3. Final Answer:

   Error due to duplicate primary key value -> Option B

4. Quick Check:

   Duplicate primary key = error [OK]

Hint: Primary key duplicates cause insert errors [OK]

Common Mistakes:

• Assuming duplicate inserts overwrite existing rows
• Thinking NULL can be inserted in primary key
• Believing both rows insert without error

4. Identify the error in the following MySQL table creation statement:

CREATE TABLE orders (
  order_id INT,
  customer_id INT,
  PRIMARY KEY order_id
);

medium

A. No error, statement is correct

B. Primary key cannot be declared separately

C. order_id must be VARCHAR type

D. Missing parentheses around the primary key column

Solution

1. Step 1: Check syntax for separate primary key declaration

   When declaring a primary key separately, the column name(s) must be inside parentheses, like PRIMARY KEY (order_id).

2. Step 2: Identify the error in the statement

   The statement uses PRIMARY KEY order_id without parentheses, causing a syntax error.

3. Final Answer:

   Missing parentheses around the primary key column -> Option D

4. Quick Check:

   Separate primary key needs parentheses [OK]

Hint: Separate primary key needs ( ) around columns [OK]

Common Mistakes:

• Omitting parentheses in separate primary key
• Thinking primary key must be inline only
• Assuming INT type is invalid for primary key

5. You want to create a table employees with a composite primary key consisting of department_id and employee_number. Which is the correct way to declare this primary key?

hard

A. CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY (department_id, employee_number));

B. CREATE TABLE employees (department_id INT PRIMARY KEY, employee_number INT PRIMARY KEY);

C. CREATE TABLE employees (department_id INT, employee_number INT PRIMARY KEY);

D. CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY department_id, employee_number);

Solution

1. Step 1: Understand composite primary key syntax

   A composite primary key is declared by listing multiple columns inside parentheses after PRIMARY KEY.

2. Step 2: Evaluate each option

   CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY (department_id, employee_number)); correctly declares both columns together as a composite primary key. CREATE TABLE employees (department_id INT PRIMARY KEY, employee_number INT PRIMARY KEY); tries to assign two primary keys separately, which is invalid. CREATE TABLE employees (department_id INT, employee_number INT PRIMARY KEY); declares only one column as primary key. CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY department_id, employee_number); has syntax errors missing parentheses.

3. Final Answer:

   CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY (department_id, employee_number)); -> Option A

4. Quick Check:

   Composite key = PRIMARY KEY (col1, col2) [OK]

Hint: Composite keys list columns inside parentheses [OK]

Common Mistakes:

• Declaring multiple primary keys separately
• Missing parentheses for composite keys
• Assigning primary key to only one column