User-defined functions in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a simple function that returns the square of a number.

SQL

CREATE FUNCTION square(num INT) RETURNS INT AS $$ BEGIN RETURN num [1] num; END; $$ 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 '*' will add the number to itself, not square it.

Using '-' or '/' will cause incorrect results or errors.

The function multiplies the input number by itself to return its square.

2fill in blank

medium

Complete the code to create a function that returns the length of a given text.

SQL

CREATE FUNCTION text_length(txt TEXT) RETURNS INT AS $$ BEGIN RETURN [1](txt); END; $$ LANGUAGE plpgsql;

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

Acount

Bchar_length

Clength

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using count which counts rows, not string length.

Using char_length is valid but here only length is accepted.

The length function returns the number of characters in the text.

3fill in blank

hard

Fix the error in the function that calculates the factorial of a number.

SQL

CREATE FUNCTION factorial(n INT) RETURNS INT AS $$ BEGIN IF n = 0 THEN RETURN 1; ELSE RETURN n [1] factorial(n - 1); END IF; END; $$ LANGUAGE plpgsql;

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

A/

B+

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' will sum numbers incorrectly.

Using '-' or '/' will cause wrong results.

The factorial is calculated by multiplying the number by the factorial of one less than the number.

4fill in blank

hard

Fill both blanks to create a function that returns the maximum of two numbers.

SQL

CREATE FUNCTION max_two(a INT, b INT) RETURNS INT AS $$ BEGIN IF a [1] b THEN RETURN a; ELSE RETURN [2]; END IF; END; $$ LANGUAGE plpgsql;

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

A>

Ba

Cb

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' will reverse the logic.

Returning a in the else branch is incorrect.

The function compares a and b. If a is greater, it returns a, otherwise it returns b.

5fill in blank

hard

Fill all three blanks to create a function that returns a greeting message with the user's name.

SQL

CREATE FUNCTION greet_user(name TEXT) RETURNS TEXT AS $$ BEGIN RETURN 'Hello, ' || [1] || [2] || [3]; END; $$ LANGUAGE plpgsql;

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

Aname

B'!'

C' '

D'?'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to add space between name and punctuation.

Using wrong punctuation marks.

The function concatenates 'Hello, ' with the user's name, a space, and an exclamation mark to form a greeting.

Practice

1. What is the main purpose of a user-defined function in SQL?

easy

A. To create reusable code that returns a single value

B. To store large amounts of data

C. To create tables automatically

D. To delete records from a table

Solution

1. Step 1: Understand what user-defined functions do

   User-defined functions are designed to perform specific tasks and return a single value.

2. Step 2: Compare options with the function's purpose

   Only To create reusable code that returns a single value describes creating reusable code that returns a single value, which matches the function's role.

3. Final Answer:

   To create reusable code that returns a single value -> Option A

4. Quick Check:

   User-defined function = reusable single-value code [OK]

Hint: Functions return one value and reuse code easily [OK]

Common Mistakes:

• Confusing functions with tables or data storage
• Thinking functions delete or modify data directly
• Assuming functions return multiple rows or tables

2. Which of the following is the correct way to start creating a scalar user-defined function in SQL?

easy

A. DROP FUNCTION function_name();

B. CREATE FUNCTION function_name() RETURNS INT AS BEGIN RETURN 1; END;

C. SELECT FUNCTION function_name();

D. CREATE TABLE function_name();

Solution

1. Step 1: Identify the correct syntax for creating a function

   The correct syntax starts with CREATE FUNCTION, specifies the return type, and includes a BEGIN...END block.

2. Step 2: Check each option's validity

   CREATE FUNCTION function_name() RETURNS INT AS BEGIN RETURN 1; END; matches the correct syntax. Options A, B, and C are for other SQL commands (dropping, table creation, and selection).

3. Final Answer:

   CREATE FUNCTION function_name() RETURNS INT AS BEGIN RETURN 1; END; -> Option B

4. Quick Check:

   CREATE FUNCTION + RETURNS + BEGIN...END = correct syntax [OK]

Hint: Functions start with CREATE FUNCTION and specify RETURNS [OK]

Common Mistakes:

• Using CREATE TABLE instead of CREATE FUNCTION
• Trying to SELECT a function directly without calling it
• Confusing DROP FUNCTION with creation syntax

3. Given the function below, what will be the output of SELECT dbo.AddFive(10);?

CREATE FUNCTION dbo.AddFive(@num INT) RETURNS INT AS BEGIN RETURN @num + 5; END;

medium

A. 5

B. 10

C. 15

D. Syntax Error

Solution

1. Step 1: Understand the function logic

   The function takes an integer input and returns the input plus 5.

2. Step 2: Calculate the output for input 10

   10 + 5 = 15, so the function returns 15.

3. Final Answer:

   15 -> Option C

4. Quick Check:

   Input 10 + 5 = 15 [OK]

Hint: Add input to 5 as function returns input + 5 [OK]

Common Mistakes:

• Confusing input and output values
• Assuming function returns input unchanged
• Expecting syntax error due to missing parameters

4. Identify the error in the following user-defined function:

CREATE FUNCTION dbo.MultiplyByTwo(@val INT)
RETURNS INT
BEGIN
  SET @val = @val * 2;
  RETURN @val;
END;

medium

A. Missing AS keyword before BEGIN

B. Functions cannot modify input parameters

C. RETURN statement is not allowed in functions

D. Function must return VARCHAR, not INT

Solution

1. Step 1: Review function syntax rules

   In SQL, the AS keyword is required before the BEGIN block in function definitions.

2. Step 2: Check the given function

   The function is missing AS before BEGIN, causing a syntax error.

3. Final Answer:

   Missing AS keyword before BEGIN -> Option A

4. Quick Check:

   CREATE FUNCTION ... RETURNS ... AS BEGIN ... [OK]

Hint: Always include AS before BEGIN in function syntax [OK]

Common Mistakes:

• Trying to modify input parameters inside functions
• Omitting AS keyword before BEGIN
• Confusing function return types

5. You want to create a user-defined function that returns the length of a string only if the string is not empty; otherwise, it returns 0. Which of the following function definitions correctly implements this logic?

hard

A. CREATE FUNCTION dbo.StringLength(@input VARCHAR(100)) RETURNS INT AS BEGIN RETURN NULL; END;

B. CREATE FUNCTION dbo.StringLength(@input VARCHAR(100)) RETURNS INT AS BEGIN IF @input = '' RETURN 1; ELSE RETURN LEN(@input); END;

C. CREATE FUNCTION dbo.StringLength(@input VARCHAR(100)) RETURNS INT AS BEGIN RETURN CASE WHEN @input = '' THEN 1 ELSE LEN(@input) END; END;

D. CREATE FUNCTION dbo.StringLength(@input VARCHAR(100)) RETURNS INT AS BEGIN RETURN LEN(@input); END;

Solution

1. Step 1: Understand the requirement

   The function must return 0 if the string is empty, otherwise return its length.

2. Step 2: Analyze each option

   CREATE FUNCTION dbo.StringLength(@input VARCHAR(100)) RETURNS INT AS BEGIN RETURN LEN(@input); END; correctly returns LEN(@input), and since LEN('')=0 it satisfies the logic. CREATE FUNCTION dbo.StringLength(@input VARCHAR(100)) RETURNS INT AS BEGIN IF @input = '' RETURN 1; ELSE RETURN LEN(@input); END; returns 1 for empty strings, incorrect. CREATE FUNCTION dbo.StringLength(@input VARCHAR(100)) RETURNS INT AS BEGIN RETURN CASE WHEN @input = '' THEN 1 ELSE LEN(@input) END; END; returns 1 for empty strings using CASE, incorrect. CREATE FUNCTION dbo.StringLength(@input VARCHAR(100)) RETURNS INT AS BEGIN RETURN NULL; END; always returns NULL, which is incorrect.

3. Final Answer:

   CREATE FUNCTION dbo.StringLength(@input VARCHAR(100)) RETURNS INT AS BEGIN RETURN LEN(@input); END; -> Option D

4. Quick Check:

   LEN('') = 0 satisfies condition [OK]

Hint: LEN('') returns 0 for empty strings [OK]

Common Mistakes:

• Returning incorrect value (like 1) for empty strings
• Using overly complex conditional logic unnecessarily
• Returning NULL instead of 0 for empty strings