Why DEFAULT values and expressions in PostgreSQL? - Purpose & Use Cases

The Big Idea

What if your database could fill in missing details all by itself, saving you hours of cleanup?

The Scenario

Imagine you have a form where users enter their information, including their country and signup date. Without defaults, every time someone forgets to fill these fields, you have to manually check and fill them in later.

The Problem

Manually checking and filling missing data is slow and error-prone. You might miss some entries or enter inconsistent values, causing confusion and extra work to fix later.

The Solution

Using DEFAULT values and expressions in your database means the system automatically fills in missing data with sensible values, like the current date or a default country, saving you time and avoiding mistakes.

Before vs After

✗ Before

INSERT INTO users (name, country, signup_date) VALUES ('Alice', NULL, NULL);
-- Then update missing fields later

✓ After

CREATE TABLE users (
  name TEXT,
  country TEXT DEFAULT 'USA',
  signup_date DATE DEFAULT CURRENT_DATE
);
INSERT INTO users (name) VALUES ('Alice');

What It Enables

It enables your database to handle missing data smartly and consistently without extra manual work.

Real Life Example

An online store automatically records the signup date of new customers and sets their country to 'USA' if they don't specify one, ensuring clean and complete data.

Key Takeaways

DEFAULT values fill in missing data automatically.

This reduces manual updates and errors.

Expressions like CURRENT_DATE keep data fresh and accurate.

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