DEFAULT values and expressions in PostgreSQL

Introduction

DEFAULT values let you set a starting value for a column when no value is given. This helps keep data complete and consistent without extra work.

When you want a column to have a common starting value automatically.

When inserting new rows but some columns can be left empty and still get a value.

When you want to use a simple calculation or function as a starting value.

When you want to avoid errors from missing data in certain columns.

When you want to save time by not typing the same value repeatedly.

Syntax

PostgreSQL

CREATE TABLE table_name (
  column_name data_type DEFAULT default_value
);

The DEFAULT value can be a constant, an expression, or a function call.

If you don't provide a value for the column during insert, the DEFAULT is used.

Examples

This sets the default name to 'Guest' if no name is given.

PostgreSQL

CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  name TEXT DEFAULT 'Guest'
);

This uses the current date and time as the default order date.

PostgreSQL

CREATE TABLE orders (
  order_id SERIAL PRIMARY KEY,
  order_date TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

This sets the default price to 0.0 if no price is provided.

PostgreSQL

CREATE TABLE products (
  product_id SERIAL PRIMARY KEY,
  price NUMERIC DEFAULT 0.0
);

Sample Program

This creates an employees table with default start_date as today and default salary as 30000. We insert Alice without salary, so she gets the default. Bob has a salary given.

PostgreSQL

CREATE TABLE employees (
  employee_id SERIAL PRIMARY KEY,
  name TEXT NOT NULL,
  start_date DATE DEFAULT CURRENT_DATE,
  salary NUMERIC DEFAULT 30000
);

INSERT INTO employees (name) VALUES ('Alice');
INSERT INTO employees (name, salary) VALUES ('Bob', 40000);

SELECT employee_id, name, start_date, salary FROM employees ORDER BY employee_id;

Important Notes

DEFAULT values help avoid NULLs and keep data clean.

You can use functions like CURRENT_DATE or expressions like 100 + 50 as defaults.

DEFAULT only applies when you omit the column in INSERT, not when you insert NULL explicitly.

Summary

DEFAULT sets a value automatically when none is provided.

It can be a fixed value, expression, or function.

Using DEFAULT saves time and prevents missing data.

Practice

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

easy

A. Sets a value automatically when no value is provided during insert

B. Deletes the column if no value is provided

C. Requires the user to always provide a value

D. Creates an index on the column

Solution

1. Step 1: Understand the purpose of DEFAULT

   The DEFAULT keyword assigns a value automatically if no value is given during an insert operation.

2. Step 2: Compare options with this behavior

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

3. Final Answer:

   Sets a value automatically when no value is provided during insert -> Option A

4. Quick Check:

   DEFAULT = automatic value assignment [OK]

Hint: DEFAULT fills missing values automatically [OK]

Common Mistakes:

• Thinking DEFAULT deletes or modifies columns
• 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 PostgreSQL?

easy

A. ALTER TABLE mytable ALTER COLUMN score SET DEFAULT 100;

B. ALTER TABLE mytable SET DEFAULT score = 100;

C. ALTER TABLE mytable CHANGE score DEFAULT 100;

D. ALTER TABLE mytable ADD DEFAULT score = 100;

Solution

1. Step 1: Recall ALTER TABLE syntax for setting default

   In PostgreSQL, to set a default value, use: ALTER TABLE table ALTER COLUMN column SET DEFAULT value;

2. Step 2: Match syntax with options

   Only ALTER TABLE mytable ALTER COLUMN score SET DEFAULT 100; matches the correct syntax exactly.

3. Final Answer:

   ALTER TABLE mytable ALTER COLUMN score SET DEFAULT 100; -> Option A

4. Quick Check:

   ALTER COLUMN ... SET DEFAULT = correct syntax [OK]

Hint: Use ALTER COLUMN ... SET DEFAULT to set defaults [OK]

Common Mistakes:

• Using SET DEFAULT before column name
• Using CHANGE instead of ALTER COLUMN
• Trying to ADD DEFAULT as a separate clause

3. Given the table definition:

CREATE TABLE orders (
  id SERIAL PRIMARY KEY,
  order_date DATE DEFAULT CURRENT_DATE,
  status TEXT DEFAULT 'pending'
);

What will be the result of this query?

INSERT INTO orders (status) VALUES ('shipped') RETURNING *;

medium

A. A new row with default status 'pending' and NULL order_date

B. A new row with NULL order_date and status 'shipped'

C. A new row with current date as order_date and status 'shipped'

D. Error because order_date is missing

Solution

1. Step 1: Understand DEFAULT behavior on insert

   Since order_date has DEFAULT CURRENT_DATE, if not provided, it uses today's date automatically.

2. Step 2: Analyze the insert statement

   The insert provides status = 'shipped', so it overrides the default 'pending'. The order_date is omitted, so default applies.

3. Final Answer:

   A new row with current date as order_date and status 'shipped' -> Option C

4. Quick Check:

   DEFAULT fills missing columns, provided columns override [OK]

Hint: Defaults fill missing columns, provided values override [OK]

Common Mistakes:

• Assuming missing columns become NULL instead of default
• Expecting error when default exists
• Confusing default value with NOT NULL constraint

4. Consider this table:

CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  username TEXT NOT NULL,
  created_at TIMESTAMP DEFAULT NOW()
);

Which of the following INSERT statements will cause an error?

medium

A. INSERT INTO users (id, username, created_at) VALUES (DEFAULT, 'alice', DEFAULT);

B. INSERT INTO users (username, created_at) VALUES ('bob', DEFAULT);

C. INSERT INTO users (username) VALUES ('alice');

D. INSERT INTO users (created_at) VALUES (NOW());

Solution

1. Step 1: Check NOT NULL column requirements

   The username column is NOT NULL and must be provided in inserts.

2. Step 2: Analyze each insert

   INSERT INTO users (created_at) VALUES (NOW()); inserts only created_at but omits username, causing a NOT NULL violation error.

3. Final Answer:

   INSERT INTO users (created_at) VALUES (NOW()); -> Option D

4. Quick Check:

   NOT NULL columns must be provided [OK]

Hint: NOT NULL columns require values, even with defaults [OK]

Common Mistakes:

• Assuming DEFAULT VALUES inserts NULL for NOT NULL columns
• Thinking DEFAULT can replace NOT NULL columns
• Ignoring NOT NULL constraints

5. You want to create a table products where the created_at column defaults to the current timestamp, and the price column defaults to 9.99 if no price is given. Which of the following CREATE TABLE statements correctly implements this?

hard

A. CREATE TABLE products (
id SERIAL PRIMARY KEY,
name TEXT NOT NULL,
price NUMERIC DEFAULT 9.99,
created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP()
);

B. CREATE TABLE products (
id SERIAL PRIMARY KEY,
name TEXT NOT NULL,
price NUMERIC DEFAULT 9.99,
created_at TIMESTAMP DEFAULT NOW()
);

C. CREATE TABLE products (
id SERIAL PRIMARY KEY,
name TEXT NOT NULL,
price NUMERIC DEFAULT '9.99',
created_at TIMESTAMP DEFAULT NOW
);

D. CREATE TABLE products (
id SERIAL PRIMARY KEY,
name TEXT NOT NULL,
price NUMERIC DEFAULT 9.99,
created_at TIMESTAMP DEFAULT CURRENT_DATE
);

Solution

1. Step 1: Check default for price column

   Price should default to numeric 9.99 without quotes (numeric literal).

2. Step 2: Check default for created_at column

   Use NOW() function call for current timestamp, not CURRENT_TIMESTAMP or CURRENT_DATE (date only).

3. Step 3: Validate syntax correctness

   CREATE TABLE products ( id SERIAL PRIMARY KEY, name TEXT NOT NULL, price NUMERIC DEFAULT 9.99, created_at TIMESTAMP DEFAULT NOW() ); uses price NUMERIC DEFAULT 9.99 and created_at TIMESTAMP DEFAULT NOW(), which is correct syntax and behavior.

4. Final Answer:

   CREATE TABLE products ( id SERIAL PRIMARY KEY, name TEXT NOT NULL, price NUMERIC DEFAULT 9.99, created_at TIMESTAMP DEFAULT NOW() ); -> Option B

5. Quick Check:

   Use NOW() for timestamp, numeric literals without quotes [OK]

Hint: Use NOW() for timestamp defaults, numeric literals without quotes [OK]

Common Mistakes:

• Using quotes around numeric default values
• Using CURRENT_DATE instead of CURRENT_TIMESTAMP for timestamps
• Omitting parentheses in NOW() function