DEFAULT values in SQL - Mini Project: Build & Apply

Using DEFAULT Values in SQL Tables

📖 Scenario: You are creating a simple database table to store information about books in a library. Some books may not have all details provided when they are added.

🎯 Goal: Create a table called Books with columns for BookID, Title, Author, and Available. Use a DEFAULT value for the Available column so that if no value is given, it is set to TRUE.

📋 What You'll Learn

Create a table named Books

Include columns BookID (integer), Title (text), Author (text), and Available (boolean)

Set the Available column to have a DEFAULT value of TRUE

Use standard SQL syntax

💡 Why This Matters

🌍 Real World

DEFAULT values are used in databases to automatically fill in missing data, making data entry easier and more reliable.

💼 Career

Knowing how to use DEFAULT values is important for database design and helps ensure data quality in real applications.

1

Create the Books table with basic columns

Write a SQL statement to create a table called Books with columns BookID as an integer, Title as text, and Author as text.

SQL

-- Your code here to create the Books table with BookID, Title, and Author columns

Hint

Use CREATE TABLE Books (column_name data_type, ...); syntax.

2

Add the Available column with DEFAULT value

Modify the Books table creation statement to add a column called Available of type boolean with a DEFAULT value of TRUE.

SQL

CREATE TABLE Books (
    BookID INTEGER,
    Title TEXT,
    Author TEXT,
    -- Add Available column with DEFAULT TRUE here
);

Hint

Add Available BOOLEAN DEFAULT TRUE as a new column.

3

Insert a book without specifying Available

Write a SQL INSERT statement to add a book with BookID 1, Title 'The Great Gatsby', and Author 'F. Scott Fitzgerald' without specifying the Available column.

SQL

-- Your INSERT statement here to add the book without Available column

Hint

Use INSERT INTO Books (BookID, Title, Author) VALUES (...); without Available.

4

Insert a book specifying Available as FALSE

Write a SQL INSERT statement to add a book with BookID 2, Title '1984', Author 'George Orwell', and set Available explicitly to FALSE.

SQL

-- Your INSERT statement here to add the book with Available set to FALSE

Hint

Include Available in the column list and set it to FALSE.

Practice

1. What does the DEFAULT keyword do in a SQL table column definition?

easy

A. Deletes the column if no value is provided

B. Prevents any value from being inserted into the column

C. Sets a value automatically if no value is provided during insert

D. Requires the user to always provide a value

Solution

1. Step 1: Understand the purpose of DEFAULT

   The DEFAULT keyword assigns a preset value to a column when no value is given during data insertion.

2. Step 2: Compare options with this purpose

   Only Sets a value automatically if no value is provided during insert correctly describes this automatic assignment behavior.

3. Final Answer:

   Sets a value automatically if no value is provided during insert -> Option C

4. Quick Check:

   DEFAULT = automatic value assignment [OK]

Hint: DEFAULT fills missing values automatically [OK]

Common Mistakes:

• Thinking DEFAULT deletes or restricts data
• Confusing DEFAULT with NOT NULL constraint
• Assuming DEFAULT requires manual input

2. Which of the following is the correct syntax to set a default value of 100 for a column named score in a table?

easy

A. score INT DEFAULT 100

B. score INT SET DEFAULT 100

C. score INT DEFAULT = 100

D. score INT DEFAULT TO 100

Solution

1. Step 1: Recall standard SQL syntax for DEFAULT

   The correct syntax is: column_name data_type DEFAULT value, without extra keywords or symbols.

2. Step 2: Check each option

   score INT DEFAULT 100 matches the correct syntax exactly. Options B, C, and D add incorrect keywords or symbols.

3. Final Answer:

   score INT DEFAULT 100 -> Option A

4. Quick Check:

   DEFAULT syntax = column type DEFAULT value [OK]

Hint: DEFAULT syntax: column type DEFAULT value [OK]

Common Mistakes:

• Adding extra keywords like SET or TO
• Using equals sign (=) after DEFAULT
• Placing DEFAULT before data type

3. Given the table users with column status VARCHAR(10) DEFAULT 'active', what will be the result of this query?

INSERT INTO users (id, name) VALUES (1, 'Alice');
SELECT id, name, status FROM users WHERE id = 1;

medium

A. [1, 'Alice', NULL]

B. [1, 'Alice', 'active']

C. [1, 'Alice', 'inactive']

D. Syntax error

Solution

1. Step 1: Understand DEFAULT behavior on insert

   Since status has a default of 'active' and no value was provided, SQL inserts 'active' automatically.

2. Step 2: Check the SELECT output

   The SELECT query will show id=1, name='Alice', and status='active' as inserted or defaulted.

3. Final Answer:

   [1, 'Alice', 'active'] -> Option B

4. Quick Check:

   DEFAULT fills missing column with 'active' [OK]

Hint: Missing column uses DEFAULT value automatically [OK]

Common Mistakes:

• Assuming missing columns get NULL instead of DEFAULT
• Thinking query causes syntax error
• Confusing default value with another string

4. Consider this table creation statement:

CREATE TABLE orders (
  id INT PRIMARY KEY,
  quantity INT DEFAULT 1,
  price DECIMAL(5,2) DEFAULT 0.00
);

Which of the following INSERT statements will cause an error?

medium

A. INSERT INTO orders (id, price) VALUES (12, 'free');

B. INSERT INTO orders (id, quantity) VALUES (11, DEFAULT);

C. INSERT INTO orders (id) VALUES (10);

D. INSERT INTO orders (id, quantity, price) VALUES (13, 5, 9.99);

Solution

1. Step 1: Analyze each INSERT statement

   INSERT INTO orders (id) VALUES (10); inserts id only; quantity and price get defaults (valid). INSERT INTO orders (id, quantity) VALUES (11, DEFAULT); uses DEFAULT keyword explicitly (valid). INSERT INTO orders (id, price) VALUES (12, 'free'); tries to insert 'free' into price (decimal), causing type error. INSERT INTO orders (id, quantity, price) VALUES (13, 5, 9.99); provides all valid values.

2. Step 2: Identify the error

   Only INSERT INTO orders (id, price) VALUES (12, 'free'); has a data type mismatch causing an error.

3. Final Answer:

   INSERT INTO orders (id, price) VALUES (12, 'free'); -> Option A

4. Quick Check:

   Wrong data type in INSERT causes error [OK]

Hint: Check data types when inserting values [OK]

Common Mistakes:

• Assuming DEFAULT keyword is invalid in VALUES
• Ignoring data type mismatch errors
• Thinking missing columns cause errors

5. You have a table products with columns id INT, name VARCHAR(50), and stock INT DEFAULT 0. You want to insert multiple rows but only specify id and name. Which SQL statement correctly inserts two products using DEFAULT for stock?

hard

A. INSERT INTO products (id, name, stock) VALUES (1, 'Pen', 0), (2, 'Pencil', 0);

B. INSERT INTO products VALUES (1, 'Pen'), (2, 'Pencil');

C. INSERT INTO products (id, name, stock) VALUES (1, 'Pen', DEFAULT), (2, 'Pencil', DEFAULT);

D. INSERT INTO products (id, name) VALUES (1, 'Pen'), (2, 'Pencil');

Solution

1. Step 1: Understand requirement for column specification

   To only specify id and name, list only those columns; omitted columns like stock use their DEFAULT value automatically.

2. Step 2: Evaluate each option

   INSERT INTO products (id, name) VALUES (1, 'Pen'), (2, 'Pencil'); specifies only id and name, stock uses DEFAULT (correct).
   INSERT INTO products VALUES (1, 'Pen'), (2, 'Pencil'); no column list with 2 values (vs 3 columns) causes mismatch error.
   INSERT INTO products (id, name, stock) VALUES (1, 'Pen', DEFAULT), (2, 'Pencil', DEFAULT); specifies stock column (violates 'only id and name').
   INSERT INTO products (id, name, stock) VALUES (1, 'Pen', 0), (2, 'Pencil', 0); specifies stock explicitly with value (not relying on DEFAULT).

3. Final Answer:

   INSERT INTO products (id, name) VALUES (1, 'Pen'), (2, 'Pencil'); -> Option D

4. Quick Check:

   Omit column with DEFAULT = auto-use [OK]

Hint: Omit columns with DEFAULT to use them automatically [OK]

Common Mistakes:

• No column list + fewer values = mismatch error
• Including stock column even with DEFAULT keyword
• Hardcoding default value instead of omitting column