Postgres functions basics in Supabase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a simple Postgres function that returns a greeting message.

Supabase

CREATE FUNCTION greet() RETURNS text AS $$ BEGIN RETURN [1]; END; $$ LANGUAGE plpgsql;

Drag options to blanks, or click blank then click option'

A'Hello, world!'

BRETURN 'Hello, world!'

CHello, world!

DSELECT 'Hello, world!'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to use quotes around the string

Using SELECT instead of RETURN inside the function

The function must return a text string, so the greeting message should be enclosed in single quotes.

2fill in blank

medium

Complete the code to define a Postgres function that adds two integers and returns the result.

Supabase

CREATE FUNCTION add_numbers(a integer, b integer) RETURNS integer AS $$ BEGIN RETURN a [1] b; END; $$ LANGUAGE plpgsql;

Drag options to blanks, or click blank then click option'

A*

B-

C+

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication instead of addition

Forgetting to use the correct operator

The function should add the two input integers, so the plus sign '+' is used.

3fill in blank

hard

Fix the error in the function that returns the length of a text input.

Supabase

CREATE FUNCTION text_length(input_text text) RETURNS integer AS $$ BEGIN RETURN [1](input_text); END; $$ LANGUAGE plpgsql;

Drag options to blanks, or click blank then click option'

Alength

Blen

Ccount

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using len which is not a Postgres function

Using count which counts rows, not string length

The correct Postgres function to get the length of a string is length().

4fill in blank

hard

Fill both blanks to create a function that returns TRUE if a number is even, FALSE otherwise.

Supabase

CREATE FUNCTION is_even(num integer) RETURNS boolean AS $$ BEGIN RETURN (num [1] 2) [2] 0; END; $$ LANGUAGE plpgsql;

Drag options to blanks, or click blank then click option'

A%

B==

C=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '=' for comparison

Using '!=' which checks inequality

The modulo operator '%' checks the remainder. In Postgres, equality is tested with '='.

5fill in blank

hard

Fill all three blanks to create a function that returns the uppercase version of a text input if its length is greater than 5, else returns the original text.

Supabase

CREATE FUNCTION conditional_upper(text_input text) RETURNS text AS $$ BEGIN IF length(text_input) [1] 5 THEN RETURN [2](text_input); ELSE RETURN [3]; END IF; END; $$ LANGUAGE plpgsql;

Drag options to blanks, or click blank then click option'

A>

Bupper

Ctext_input

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' in the condition

Returning the function call 'upper' incorrectly

Not returning the original text in the else clause

The function checks if the length is greater than 5, then returns the uppercase text using upper(), otherwise returns the original text.

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