Format function for safe formatting in PostgreSQL

Introduction

The format function helps you build text safely by inserting values into a string without mistakes or errors.

When you want to create a message that includes numbers or names safely.

When you need to build a query or command with values inserted in the right place.

When you want to avoid errors from mixing text and numbers in one string.

When you want to keep your text clear and easy to read with placeholders.

When you want to prevent mistakes from manual string joining.

Syntax

PostgreSQL

format(format_string, value1, value2, ...)

The format_string uses placeholders like %s for strings, %I for identifiers (like table or column names), and %L for literals (values).

Values are safely inserted in place of placeholders, avoiding errors or injection risks.

Examples

Inserts the string 'Alice' into the placeholder %s.

PostgreSQL

SELECT format('Hello, %s!', 'Alice');

Safely inserts table name and value into a query string.

PostgreSQL

SELECT format('SELECT * FROM %I WHERE id = %L', 'users', 42);

Inserts numbers as strings into the message.

PostgreSQL

SELECT format('Price: %s, Quantity: %s', 9.99, 5);

Sample Program

This query creates a message showing a user's name and points safely formatted.

PostgreSQL

SELECT format('User %s has %s points.', 'Bob', 150);

Important Notes

Use %I for identifiers to avoid SQL injection when inserting table or column names.

Use %L for literal values to safely quote strings or numbers.

The format function returns text, so you can use it anywhere text is expected.

Summary

The format function helps build text safely with placeholders.

Use %s for strings, %I for identifiers, and %L for literals.

It prevents errors and injection by inserting values correctly.

Practice

1. What is the main purpose of the format() function in PostgreSQL?

easy

A. To safely build text strings with placeholders for values

B. To execute SQL queries directly

C. To create new tables in the database

D. To delete rows from a table

Solution

1. Step 1: Understand the function's role

   The format() function is designed to build text strings safely by inserting values into placeholders.

2. Step 2: Compare with other options

   Options A, B, and C describe other SQL operations, not string formatting.

3. Final Answer:

   To safely build text strings with placeholders for values -> Option A

4. Quick Check:

   format() = safe string building [OK]

Hint: Remember: format() builds safe strings with placeholders [OK]

Common Mistakes:

• Thinking format() executes queries
• Confusing format() with table operations
• Ignoring the safety aspect of format()

2. Which placeholder should you use with format() to safely insert a table name (identifier)?

easy

A. %L

B. %s

C. %I

D. %d

Solution

1. Step 1: Identify placeholder types

   In format(), %I is used for identifiers like table or column names.

2. Step 2: Eliminate other placeholders

   %s is for strings, %L for literals, and %d is not a valid placeholder in PostgreSQL's format.

3. Final Answer:

   %I -> Option C

4. Quick Check:

   Identifier placeholder = %I [OK]

Hint: Use %I for identifiers like table names [OK]

Common Mistakes:

• Using %s for identifiers
• Confusing %L with %I
• Using invalid placeholders like %d

3. What is the output of this query?

SELECT format('SELECT * FROM %I WHERE name = %L', 'users', 'Alice');

medium

A. SELECT * FROM users WHERE name = Alice

B. SELECT * FROM users WHERE name = 'Alice'

C. SELECT * FROM 'users' WHERE name = 'Alice'

D. SELECT * FROM users WHERE name = Alice;

Solution

1. Step 1: Understand placeholders used

   %I inserts the identifier without quotes, so 'users' becomes users. %L inserts the literal with quotes, so 'Alice' becomes 'Alice'.

2. Step 2: Construct the output string

   The output is: SELECT * FROM users WHERE name = 'Alice'

3. Final Answer:

   SELECT * FROM users WHERE name = 'Alice' -> Option B

4. Quick Check:

   Identifier no quotes, literal with quotes = SELECT * FROM users WHERE name = 'Alice' [OK]

Hint: Remember %I no quotes, %L adds quotes around literals [OK]

Common Mistakes:

• Adding quotes around identifiers
• Missing quotes around literals
• Confusing %s with %I or %L

4. Identify the error in this usage of format():

SELECT format('INSERT INTO %I VALUES (%s)', 'products', 100);

medium

A. Using %s for a numeric value causes a syntax error

B. Missing quotes around the table name

C. No error, the query is correct

D. Should use %L for the numeric value to be safe

Solution

1. Step 1: Check placeholder for numeric value

   %s inserts the value as a string without quotes, which can cause errors or injection risks.

2. Step 2: Use %L for safe literal insertion

   Using %L safely quotes and escapes the numeric literal, preventing errors.

3. Final Answer:

   Should use %L for the numeric value to be safe -> Option D

4. Quick Check:

   Numeric literals need %L for safety [OK]

Hint: Use %L for all literal values, even numbers [OK]

Common Mistakes:

• Using %s for literals without quotes
• Assuming %s works for all types
• Ignoring injection risks with %s

5. You want to build a dynamic query to update a user's email safely. Which format() call is correct?

1) format('UPDATE %I SET email = %s WHERE id = %L', 'users', '[e-mail]', 42)
2) format('UPDATE %I SET email = %L WHERE id = %L', 'users', '[e-mail]', 42)
3) format('UPDATE %s SET email = %L WHERE id = %s', 'users', '[e-mail]', 42)
4) format('UPDATE %I SET email = %I WHERE id = %L', 'users', '[e-mail]', 42)

hard

A. Option 2

B. Option 1

C. Option 3

D. Option 4

Solution

1. Step 1: Check placeholders for each part

   Table name is an identifier, so %I is correct. Email is a string literal, so %L is needed. ID is a literal number, so %L is safe.

2. Step 2: Evaluate each option

   Option 2 uses %I for table, %L for email and id, which is correct. Option 1 uses %s for email (unsafe). Option 3 uses %s for table and id (wrong). Option 4 uses %I for email (wrong).

3. Final Answer:

   Option 2 -> Option A

4. Quick Check:

   Identifiers %I, literals %L = Option 2 [OK]

Hint: Use %I for identifiers, %L for all literal values [OK]

Common Mistakes:

• Using %s for literals
• Using %I for string literals
• Mixing placeholders incorrectly