DEFAULT values in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the DEFAULT value in a SQL table column?

The DEFAULT value sets a preset value for a column when no value is provided during data insertion. It helps avoid NULLs and ensures consistent data.

beginner

How do you define a DEFAULT value for a column in SQL?

You use the DEFAULT keyword followed by the value when creating or altering a table column, for example: age INT DEFAULT 18.

intermediate

Can a DEFAULT value be a function in SQL? Give an example.

Yes, a DEFAULT value can be a function. For example, created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP sets the current time automatically.

beginner

What happens if you insert a row without specifying a column that has a DEFAULT value?

The column automatically gets the DEFAULT value specified in the table definition instead of NULL or an error.

intermediate

Is it possible to change the DEFAULT value of an existing column? How?

Yes, you can change it using the ALTER TABLE statement with ALTER COLUMN and SET DEFAULT, for example: ALTER TABLE users ALTER COLUMN age SET DEFAULT 21;.

What does the DEFAULT keyword do in SQL?

A Sets a preset value for a column when no value is provided

B Deletes a column from a table

C Changes the data type of a column

D Creates a new table

DEFAULT sets a preset value for a column if no value is given during insertion.

Which of the following is a valid DEFAULT value for a TIMESTAMP column?

A DEFAULT 0

B DEFAULT TRUE

C DEFAULT CURRENT_TIMESTAMP

D DEFAULT 'hello'

CURRENT_TIMESTAMP is a function that returns the current date and time, valid as a DEFAULT for TIMESTAMP.

If a column has DEFAULT 100 and you insert a row without specifying that column, what value will it have?

A 100

B 0

C NULL

D Error

The column will automatically get the DEFAULT value 100.

How do you change the DEFAULT value of an existing column in SQL?

A You cannot change DEFAULT values

B DROP COLUMN and add a new one

C UPDATE the table

D ALTER TABLE ... ALTER COLUMN ... SET DEFAULT ...

You use ALTER TABLE with ALTER COLUMN and SET DEFAULT to change the default value.

Can DEFAULT values be expressions or functions?

A No, only constants

B Yes, for example CURRENT_TIMESTAMP

C Only strings

D Only numbers

DEFAULT values can be functions like CURRENT_TIMESTAMP to set dynamic defaults.

Explain what DEFAULT values are in SQL and why they are useful.

Think about what happens if you forget to give a value for a column.

Describe how to set and change DEFAULT values for columns in SQL tables.

Consider both table creation and modifying existing tables.

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