Postgres functions basics in Supabase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Postgres function?

A Postgres function is a set of SQL or procedural commands saved in the database. It can be called to perform tasks like calculations or data changes, making work easier and reusable.

beginner

How do you create a simple Postgres function that adds two numbers?

You use the CREATE FUNCTION statement with input parameters and a RETURN type. For example: CREATE FUNCTION add_numbers(a integer, b integer) RETURNS integer AS $$ BEGIN RETURN a + b; END; $$ LANGUAGE plpgsql;

beginner

What does the LANGUAGE plpgsql mean in a Postgres function?

It tells Postgres that the function uses the PL/pgSQL language, which allows procedural programming like variables and control flow inside the function.

beginner

Why use functions in Postgres instead of repeating SQL queries?

Functions save time by reusing code, reduce mistakes, and keep your database organized. They also can improve performance by running complex logic inside the database.

intermediate

What is the difference between a function and a stored procedure in Postgres?

Functions return a value and can be used in queries. Stored procedures do not return values and are called to perform actions like transactions. Postgres added procedures in version 11.

Which keyword starts the creation of a Postgres function?

A CREATE FUNCTION

B MAKE FUNCTION

C DEFINE FUNCTION

D START FUNCTION

The correct syntax to create a function in Postgres begins with CREATE FUNCTION.

What does a Postgres function always need to specify?

A Only return type

B Only input parameters

C Input parameters and return type

D No parameters or return type

Functions must define input parameters and what type of value they return.

Which language is commonly used inside Postgres functions for procedural logic?

A javascript

B plpgsql

C python

D sqlplus

PL/pgSQL is the procedural language designed for Postgres functions.

What is a key benefit of using Postgres functions?

A Replacing tables

B Making the database slower

C Avoiding all SQL queries

D Reusing code and improving performance

Functions help reuse code and can improve database performance.

When were stored procedures introduced in Postgres?

A Version 11

B Version 9

C Version 8

D Version 10

Stored procedures were added in Postgres version 11.

Explain what a Postgres function is and why it is useful.

Think about how a recipe helps you cook the same dish easily.

Describe the basic steps to create a Postgres function that returns the sum of two numbers.

Focus on the parts needed to add two numbers and return the result.

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