Function creation syntax in PostgreSQL - Mini Project: Build & Apply

Create a Simple PostgreSQL Function

📖 Scenario: You are working as a database assistant for a small bookstore. The store wants to have a quick way to calculate the total price of books including tax.

🎯 Goal: Build a PostgreSQL function named calculate_total_price that takes a book price as input and returns the price including a fixed tax rate.

📋 What You'll Learn

Create a function named calculate_total_price

The function should accept one parameter named price of type numeric

The function should return a numeric value

Inside the function, calculate the total price by adding 10% tax to the input price

Use the LANGUAGE plpgsql syntax for the function

💡 Why This Matters

🌍 Real World

Functions in databases help automate calculations like pricing, taxes, and discounts, saving time and reducing errors.

💼 Career

Knowing how to write database functions is useful for database administrators, backend developers, and data analysts to implement business logic inside the database.

1

Create the function header

Write the first line to create a function named calculate_total_price that accepts one parameter called price of type numeric and returns numeric.

PostgreSQL

-- Write the function header here
-- Your code here

Hint

Use CREATE FUNCTION function_name(parameter_name parameter_type) RETURNS return_type syntax.

2

Add the function body start

Add the AS $$ line and start the BEGIN block after the function header.

PostgreSQL

CREATE FUNCTION calculate_total_price(price numeric) RETURNS numeric
AS $$
BEGIN
-- Your code here

Hint

Use AS $$ to start the function body and BEGIN to start the block.

3

Write the calculation and return statement

Inside the BEGIN block, write a RETURN statement that returns the price plus 10% tax (price * 1.10).

PostgreSQL

CREATE FUNCTION calculate_total_price(price numeric) RETURNS numeric
AS $$
BEGIN
-- Your code here

Hint

Use RETURN price * 1.10; to add 10% tax.

4

Close the function body and specify language

Add the END; line, the closing $$, and specify LANGUAGE plpgsql; to complete the function.

PostgreSQL

CREATE FUNCTION calculate_total_price(price numeric) RETURNS numeric
AS $$
BEGIN
    RETURN price * 1.10;
-- Your code here

Hint

Close the block with END;, then close the body with $$ and specify the language.

Practice

1. What is the purpose of the CREATE FUNCTION statement in PostgreSQL?

easy

A. To delete rows from a table

B. To create a new table in the database

C. To insert data into an existing table

D. To define a reusable block of code that can be called later

Solution

1. Step 1: Understand the role of functions in PostgreSQL

   Functions store reusable code inside the database to perform tasks repeatedly.

2. Step 2: Identify what CREATE FUNCTION does

   This statement defines a new function with parameters, return type, and body.

3. Final Answer:

   To define a reusable block of code that can be called later -> Option D

4. Quick Check:

   CREATE FUNCTION defines reusable code [OK]

Hint: Functions store reusable code blocks in the database [OK]

Common Mistakes:

• Confusing function creation with table creation
• Thinking it inserts or deletes data directly
• Mixing up functions with SQL commands like SELECT or DELETE

2. Which of the following is the correct basic syntax to create a function in PostgreSQL that returns an integer?

easy

A. CREATE FUNCTION myfunc() RETURNS int AS $$ BEGIN RETURN 1; END; $$ LANGUAGE plpgsql;

B. CREATE FUNCTION myfunc RETURNS int AS $$ RETURN 1; $$ LANGUAGE plpgsql;

C. CREATE FUNCTION myfunc() RETURNS integer BEGIN RETURN 1; END LANGUAGE plpgsql;

D. CREATE FUNCTION myfunc() RETURNS int AS BEGIN RETURN 1; END LANGUAGE plpgsql;

Solution

1. Step 1: Check the correct syntax for function creation

   The syntax requires parentheses after the function name, the RETURNS clause, the function body enclosed in $$, and the LANGUAGE specified.

2. Step 2: Validate each option

   CREATE FUNCTION myfunc() RETURNS int AS $$ BEGIN RETURN 1; END; $$ LANGUAGE plpgsql; correctly uses parentheses, RETURNS int, AS $$ ... $$, and LANGUAGE plpgsql. Others miss parentheses, AS $$, or semicolons.

3. Final Answer:

   CREATE FUNCTION myfunc() RETURNS int AS $$ BEGIN RETURN 1; END; $$ LANGUAGE plpgsql; -> Option A

4. Quick Check:

   Correct syntax includes parentheses, RETURNS, AS $$, LANGUAGE [OK]

Hint: Always use () after function name and AS $$ for body [OK]

Common Mistakes:

• Omitting parentheses after function name
• Missing AS $$ ... $$ around function body
• Not specifying LANGUAGE plpgsql
• Forgetting semicolons inside function body

3. Given the function below, what will be the output of SELECT add_one(5);?

CREATE FUNCTION add_one(x integer) RETURNS integer AS $$ BEGIN RETURN x + 1; END; $$ LANGUAGE plpgsql;

medium

A. 6

B. Syntax error

C. 5

D. NULL

Solution

1. Step 1: Understand the function logic

   The function takes an integer input x and returns x + 1.

2. Step 2: Apply the input value 5

   Calling add_one(5) returns 5 + 1 = 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Input 5 plus 1 equals 6 [OK]

Hint: Function adds 1 to input, so 5 becomes 6 [OK]

Common Mistakes:

• Confusing input and output values
• Expecting syntax error due to unfamiliarity
• Assuming function returns NULL without reason

4. Identify the error in the following function definition:

CREATE FUNCTION multiply_by_two(x integer) RETURNS integer AS $$ BEGIN RETURN x * 2 END; $$ LANGUAGE plpgsql;

medium

A. Missing RETURNS clause

B. Incorrect function name syntax

C. Missing semicolon after RETURN statement

D. LANGUAGE plpgsql is not allowed

Solution

1. Step 1: Review function body syntax

   In PL/pgSQL, each statement inside the function body must end with a semicolon.

2. Step 2: Locate missing semicolon

   The RETURN statement lacks a semicolon after x * 2, causing a syntax error.

3. Final Answer:

   Missing semicolon after RETURN statement -> Option C

4. Quick Check:

   Statements inside function need semicolons [OK]

Hint: Check for semicolons after each statement inside function [OK]

Common Mistakes:

• Forgetting semicolon after RETURN
• Misplacing LANGUAGE clause
• Omitting RETURNS clause
• Using invalid function names

5. You want to create a PostgreSQL function concat_names that takes two text parameters and returns their concatenation separated by a space. Which of the following is the correct function definition?

hard

A. CREATE FUNCTION concat_names(a text, b text) RETURNS text AS $$ RETURN a + ' ' + b; $$ LANGUAGE plpgsql;

B. CREATE FUNCTION concat_names(a text, b text) RETURNS text AS $$ BEGIN RETURN a || ' ' || b; END; $$ LANGUAGE plpgsql;

C. CREATE FUNCTION concat_names(a text, b text) RETURNS text AS $$ BEGIN RETURN concat(a, ' ', b); END $$ LANGUAGE plpgsql;

D. CREATE FUNCTION concat_names(a text, b text) RETURNS text AS $$ BEGIN RETURN a & ' ' & b; END; $$ LANGUAGE plpgsql;

Solution

1. Step 1: Understand string concatenation in PostgreSQL

   PostgreSQL uses the || operator to concatenate strings.

2. Step 2: Evaluate each option's concatenation method

   CREATE FUNCTION concat_names(a text, b text) RETURNS text AS $$ BEGIN RETURN a || ' ' || b; END; $$ LANGUAGE plpgsql; uses || correctly with BEGIN...END and semicolons. CREATE FUNCTION concat_names(a text, b text) RETURNS text AS $$ RETURN a + ' ' + b; $$ LANGUAGE plpgsql; uses + which is invalid for text. CREATE FUNCTION concat_names(a text, b text) RETURNS text AS $$ BEGIN RETURN concat(a, ' ', b); END $$ LANGUAGE plpgsql; misses semicolon after END. CREATE FUNCTION concat_names(a text, b text) RETURNS text AS $$ BEGIN RETURN a & ' ' & b; END; $$ LANGUAGE plpgsql; uses & which is invalid.

3. Final Answer:

   CREATE FUNCTION concat_names(a text, b text) RETURNS text AS $$ BEGIN RETURN a || ' ' || b; END; $$ LANGUAGE plpgsql; -> Option B

4. Quick Check:

   Use || for text concatenation in PL/pgSQL [OK]

Hint: Use || operator for text concatenation in PostgreSQL functions [OK]

Common Mistakes:

• Using + or & instead of || for strings
• Forgetting semicolon after END
• Missing BEGIN...END block for multiple statements