DEFAULT values and expressions in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the default value inserted?

Consider the table users with a column created_at TIMESTAMP DEFAULT now(). What will be the value of created_at after running INSERT INTO users DEFAULT VALUES RETURNING created_at;?

PostgreSQL

CREATE TABLE users (id SERIAL PRIMARY KEY, created_at TIMESTAMP DEFAULT now());
INSERT INTO users DEFAULT VALUES RETURNING created_at;

A The timestamp 1970-01-01 00:00:00

B NULL

C The current timestamp at insertion time (e.g., 2024-06-01 12:00:00)

D An error because no value was provided

💡 Hint

The DEFAULT clause with now() sets the current time automatically.

The DEFAULT value now() is a function that returns the current timestamp when the row is inserted. So the inserted row gets the current time automatically.

📝 Syntax

intermediate

Which DEFAULT expression is valid?

Which of the following DEFAULT expressions for a column score INTEGER is syntactically valid in PostgreSQL?

A DEFAULT 10 + 5

B DEFAULT '10 + 5'

C DEFAULT 10 + '5'

D DEFAULT (10 + 5)

💡 Hint

DEFAULT expressions can be arithmetic expressions but must be enclosed in parentheses if complex.

Option D uses parentheses to enclose the arithmetic expression, which is valid. Option D is invalid syntax. Option D sets a string, which is invalid for INTEGER. Option D mixes integer and string causing error.

❓ optimization

advanced

Optimizing DEFAULT expressions with volatile functions

You have a column random_val INTEGER DEFAULT floor(random() * 100). What is a potential downside of this DEFAULT expression when inserting multiple rows in a single statement?

A Each row gets a different random value because DEFAULT is evaluated per row

B All rows get the same random value because DEFAULT is evaluated once per statement

C The query will fail because random() cannot be used in DEFAULT

D The random values are always zero due to floor()

💡 Hint

DEFAULT expressions with volatile functions are evaluated once per statement, not per row.

In PostgreSQL, DEFAULT expressions are evaluated once per statement, so all rows inserted in one statement get the same random value. To get different values per row, use triggers or generate values in the insert query.

🔧 Debug

advanced

Why does this DEFAULT expression cause an error?

Given the column definition price NUMERIC DEFAULT 'abc', what error will occur when inserting a row without specifying price?

PostgreSQL

CREATE TABLE products (id SERIAL PRIMARY KEY, price NUMERIC DEFAULT 'abc');
INSERT INTO products DEFAULT VALUES;

A ERROR: invalid input syntax for type numeric: "abc"

B ERROR: column price does not exist

C ERROR: syntax error at or near 'abc'

D No error, price is set to 0

💡 Hint

The DEFAULT value must be compatible with the column type.

The DEFAULT value 'abc' is a string that cannot be cast to numeric, so PostgreSQL raises an invalid input syntax error.

🧠 Conceptual

expert

How does PostgreSQL handle DEFAULT expressions with volatile functions in multi-row inserts?

Consider a table with a column val INTEGER DEFAULT (floor(random() * 10)). You run INSERT INTO table DEFAULT VALUES, DEFAULT VALUES, DEFAULT VALUES;. What will be the values of val in the inserted rows?

A All three rows have the same random value because DEFAULT is evaluated once per statement

B Each row has a different random value because DEFAULT is evaluated per row

C The insert fails because random() cannot be used in DEFAULT

D The values are NULL because DEFAULT cannot use functions

💡 Hint

Think about how PostgreSQL evaluates DEFAULT expressions with volatile functions in multi-row inserts.

PostgreSQL evaluates DEFAULT expressions once per statement, so all rows inserted in one statement get the same value from the volatile function random().

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