DEFAULT values and expressions in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the DEFAULT keyword in PostgreSQL?

The DEFAULT keyword sets a predefined value for a column when no value is provided during an INSERT operation.

intermediate

How can you use an expression as a DEFAULT value in PostgreSQL?

You can use expressions like functions or calculations as DEFAULT values, for example: DEFAULT CURRENT_TIMESTAMP or DEFAULT (10 * 0.9).

beginner

True or False: DEFAULT values are applied when the INSERT statement omits the column.

True. DEFAULT values are used if the INSERT statement does not specify a value for that column.

beginner

How do you define a DEFAULT value for a column in a CREATE TABLE statement?

Use the syntax: column_name data_type DEFAULT default_value, for example: created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP.

intermediate

Can DEFAULT values be changed after table creation? If yes, how?

Yes, use ALTER TABLE to change DEFAULT values, for example: ALTER TABLE table_name ALTER COLUMN column_name SET DEFAULT new_value;

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

A The column value becomes NULL.

B The DEFAULT value is used for that column.

C The insert fails with an error.

D The column value is set to zero.

If a column has a DEFAULT value and no value is provided during insert, PostgreSQL uses the DEFAULT value.

Which of the following is a valid DEFAULT expression in PostgreSQL?

A DEFAULT NULL

B DEFAULT 'today'

C DEFAULT NOW() + '5'

D DEFAULT CURRENT_DATE

CURRENT_DATE is a valid function used as a DEFAULT expression. NOW() + '5' is invalid because you cannot add text directly to timestamp without casting.

How do you remove a DEFAULT value from a column?

A ALTER TABLE table_name ALTER COLUMN column_name DROP DEFAULT;

B ALTER TABLE table_name DROP COLUMN column_name;

C UPDATE table_name SET column_name = NULL;

D DELETE DEFAULT FROM column_name;

To remove a DEFAULT value, use ALTER TABLE ... ALTER COLUMN ... DROP DEFAULT.

Can a DEFAULT value be a subquery in PostgreSQL?

A Yes, any subquery can be used.

B Only if the subquery returns a single value.

C No, DEFAULT cannot be a subquery.

D Only if the subquery is in a function.

DEFAULT values cannot be subqueries; they must be constant expressions or functions.

Which statement correctly sets a DEFAULT value for a column after table creation?

A ALTER TABLE mytable ALTER COLUMN mycol SET DEFAULT 100;

B ALTER TABLE mytable SET DEFAULT mycol = 100;

C UPDATE mytable SET DEFAULT mycol = 100;

D ALTER TABLE mytable ADD DEFAULT mycol 100;

The correct syntax to set a DEFAULT value after creation is ALTER TABLE ... ALTER COLUMN ... SET DEFAULT.

Explain how DEFAULT values work in PostgreSQL and give an example of using an expression as a DEFAULT.

Think about what happens when you insert a row without specifying all columns.

Describe how to change or remove a DEFAULT value on an existing table column.

Focus on ALTER TABLE syntax for modifying columns.

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