Postgres functions basics in Supabase - Mini Project: Build & Apply

Postgres functions basics

📖 Scenario: You are working with a Supabase database. You want to create a simple function in Postgres that calculates the total price of an order by multiplying quantity and unit price.

🎯 Goal: Create a Postgres function named calculate_total_price that takes two numeric inputs: quantity and unit_price. The function should return the total price as a numeric value.

📋 What You'll Learn

Create a function named calculate_total_price

The function must accept two parameters: quantity and unit_price of type numeric

The function must return a numeric value representing the total price

Use the RETURNS clause and RETURN statement correctly

Use the LANGUAGE plpgsql declaration

💡 Why This Matters

🌍 Real World

Postgres functions help automate calculations and logic inside the database, reducing the need for application-side code.

💼 Career

Knowing how to write database functions is essential for backend developers and cloud engineers working with managed databases like Supabase.

1

Create the function header

Write the first line to create a function named calculate_total_price that accepts two parameters: quantity and unit_price, both of type numeric. Use the syntax: CREATE FUNCTION calculate_total_price(quantity numeric, unit_price numeric)

Supabase

# Your code here

Hint

Start by writing the function name and parameters exactly as shown.

2

Add the return type and function body start

Add the line to specify the function returns a numeric value and start the function body with AS $$ and BEGIN. The line should be: RETURNS numeric AS $$ BEGIN

Supabase

CREATE FUNCTION calculate_total_price(quantity numeric, unit_price numeric)
# Your code here

Hint

Specify the return type and start the function body with AS $$ BEGIN.

3

Write the return statement

Inside the function body, write the line to return the product of quantity and unit_price using RETURN quantity * unit_price;

Supabase

CREATE FUNCTION calculate_total_price(quantity numeric, unit_price numeric)
RETURNS numeric AS $$ BEGIN
# Your code here

Hint

Use the RETURN keyword followed by the multiplication expression.

4

Close the function body and specify language

Add the lines to end the function body with END; $$ LANGUAGE plpgsql; to complete the function definition.

Supabase

CREATE FUNCTION calculate_total_price(quantity numeric, unit_price numeric)
RETURNS numeric AS $$ BEGIN
    RETURN quantity * unit_price;
# Your code here

Hint

Close the function with END;, the dollar quotes $$, and specify LANGUAGE plpgsql;.

Practice

1. What is the main purpose of a Postgres function in Supabase?

easy

A. To store reusable code inside the database

B. To create user interface elements

C. To manage server hardware

D. To backup the entire database automatically

Solution

1. Step 1: Understand what Postgres functions do

   Postgres functions hold code that can be run repeatedly inside the database.

2. Step 2: Compare options with this purpose

   Only To store reusable code inside the database matches this description; others relate to UI, hardware, or backups, which are unrelated.

3. Final Answer:

   To store reusable code inside the database -> Option A

4. Quick Check:

   Postgres functions = reusable database code [OK]

Hint: Functions store code to reuse inside the database [OK]

Common Mistakes:

• Confusing functions with UI components
• Thinking functions manage hardware
• Assuming functions handle backups automatically

2. Which of the following is the correct way to start defining a simple Postgres function?

easy

A. FUNCTION CREATE function_name() RETURNS type AS BEGIN ... END;

B. MAKE FUNCTION function_name RETURNS type { ... }

C. CREATE FUNCTION function_name() RETURNS type AS $$ BEGIN ... END; $$ LANGUAGE plpgsql;

D. DEFINE FUNCTION function_name() RETURNS type BEGIN ... END;

Solution

1. Step 1: Recall Postgres function syntax

   The correct syntax starts with CREATE FUNCTION, includes RETURNS, and uses $$ to wrap the body.

2. Step 2: Match options to syntax

   Only CREATE FUNCTION function_name() RETURNS type AS $$ BEGIN ... END; $$ LANGUAGE plpgsql; matches the exact syntax; others use incorrect keywords or order.

3. Final Answer:

   CREATE FUNCTION function_name() RETURNS type AS $$ BEGIN ... END; $$ LANGUAGE plpgsql; -> Option C

4. Quick Check:

   CREATE FUNCTION + RETURNS + $$ ... $$ = correct syntax [OK]

Hint: Postgres functions start with CREATE FUNCTION and use $$ for body [OK]

Common Mistakes:

• Using MAKE or DEFINE instead of CREATE
• Omitting LANGUAGE plpgsql
• Not wrapping function body with $$

3. Given this function, what will SELECT add_two_numbers(3, 4); return?

CREATE FUNCTION add_two_numbers(a integer, b integer) RETURNS integer AS $$ BEGIN RETURN a + b; END; $$ LANGUAGE plpgsql;

medium

A. NULL

B. 34

C. Error: function not found

D. 7

Solution

1. Step 1: Understand function behavior

   The function adds two integers a and b and returns their sum.

2. Step 2: Calculate the result for inputs 3 and 4

   3 + 4 equals 7, so the function returns 7.

3. Final Answer:

   7 -> Option D

4. Quick Check:

   3 + 4 = 7 [OK]

Hint: Function adds inputs; 3 + 4 = 7 [OK]

Common Mistakes:

• Concatenating numbers as strings (34)
• Assuming function is missing or returns NULL
• Confusing syntax errors with output

4. Identify the error in this Postgres function:

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

medium

A. Missing semicolon after RETURN statement

B. Incorrect function name

C. Wrong language specified

D. No error, function is correct

Solution

1. Step 1: Check function syntax carefully

   In PL/pgSQL, statements inside BEGIN...END must end with semicolons.

2. Step 2: Locate missing semicolon

   The RETURN statement lacks a semicolon at the end, causing a syntax error.

3. Final Answer:

   Missing semicolon after RETURN statement -> Option A

4. Quick Check:

   Statements inside BEGIN...END need semicolons [OK]

Hint: Check for missing semicolons inside BEGIN...END [OK]

Common Mistakes:

• Forgetting semicolon after RETURN
• Assuming function name or language is wrong
• Thinking function is correct without semicolon

5. You want to create a Postgres function that returns the length of a text input only if it is not empty; otherwise, it returns NULL. Which function definition correctly implements this?

hard

A. CREATE FUNCTION text_length_if_not_empty(input text) RETURNS integer AS $$ BEGIN RETURN length(input); END; $$ LANGUAGE plpgsql;

B. CREATE FUNCTION text_length_if_not_empty(input text) RETURNS integer AS $$ BEGIN IF input = '' THEN RETURN NULL; ELSE RETURN length(input); END IF; END; $$ LANGUAGE plpgsql;

C. CREATE FUNCTION text_length_if_not_empty(input text) RETURNS integer AS $$ BEGIN IF input IS NULL THEN RETURN 0; ELSE RETURN length(input); END IF; END; $$ LANGUAGE plpgsql;

D. CREATE FUNCTION text_length_if_not_empty(input text) RETURNS integer AS $$ BEGIN IF input = NULL THEN RETURN NULL; ELSE RETURN length(input); END IF; END; $$ LANGUAGE plpgsql;

Solution

1. Step 1: Understand the condition to check empty string

   The function must check if input equals empty string ('') and return NULL in that case.

2. Step 2: Evaluate each option's condition

   CREATE FUNCTION text_length_if_not_empty(input text) RETURNS integer AS $$ BEGIN IF input = '' THEN RETURN NULL; ELSE RETURN length(input); END IF; END; $$ LANGUAGE plpgsql; correctly checks input = '' and returns NULL; CREATE FUNCTION text_length_if_not_empty(input text) RETURNS integer AS $$ BEGIN RETURN length(input); END; $$ LANGUAGE plpgsql; always returns length; CREATE FUNCTION text_length_if_not_empty(input text) RETURNS integer AS $$ BEGIN IF input IS NULL THEN RETURN 0; ELSE RETURN length(input); END IF; END; $$ LANGUAGE plpgsql; checks NULL instead of empty; CREATE FUNCTION text_length_if_not_empty(input text) RETURNS integer AS $$ BEGIN IF input = NULL THEN RETURN NULL; ELSE RETURN length(input); END IF; END; $$ LANGUAGE plpgsql; uses incorrect NULL comparison.

3. Final Answer:

   CREATE FUNCTION text_length_if_not_empty(input text) RETURNS integer AS $$ BEGIN IF input = '' THEN RETURN NULL; ELSE RETURN length(input); END IF; END; $$ LANGUAGE plpgsql; -> Option B

4. Quick Check:

   Check empty string with = '' and return NULL [OK]

Hint: Use IF input = '' THEN RETURN NULL; ELSE RETURN length(input); [OK]

Common Mistakes:

• Using input = NULL instead of IS NULL
• Not checking for empty string
• Returning 0 instead of NULL for empty input