Why advanced PL/pgSQL matters in PostgreSQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable in PL/pgSQL.

PostgreSQL

DECLARE my_var [1]; 

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

AIS

BINTEGER

CAS

DTYPE

Attempts:

3 left

💡 Hint

Common Mistakes

Using keywords like 'IS', 'AS', or 'TYPE' before the data type.

Omitting the data type altogether.

In PL/pgSQL, variables are declared using the variable_name TYPE; syntax inside the DECLARE block. Here, use INTEGER.

2fill in blank

medium

Complete the code to start a PL/pgSQL function.

PostgreSQL

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

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 an operator.

The function adds two numbers, so the operator should be +.

3fill in blank

hard

Fix the error in the IF statement condition.

PostgreSQL

IF total [1] 100 THEN RAISE NOTICE 'Total is over 100'; END IF;

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

A=

B<

C==

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '>'.

Using '=' which checks for equality, not greater than.

In PL/pgSQL, to check if total is greater than 100, use the > operator. The == operator is not valid in SQL.

4fill in blank

hard

Fill both blanks to complete the loop that sums numbers from 1 to 5.

PostgreSQL

DECLARE total INTEGER := 0; BEGIN FOR i IN [1] LOOP total := total + i; END LOOP; RETURN [2]; END;

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

A1..5

B0..5

Ctotal

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0..5 which includes zero unnecessarily.

Returning the loop variable instead of the total.

The loop runs from 1 to 5 using 1..5. The function returns the total sum.

5fill in blank

hard

Fill all three blanks to create a conditional that raises a notice if count is zero.

PostgreSQL

IF [1] = [2] THEN RAISE NOTICE '[3] is zero'; END IF;

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

Acount

B0

Dtotal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'total' instead of 'count'.

Using a string '0' instead of numeric 0.

The condition checks if count equals zero, then raises a notice saying 'count is zero'.

Practice

1. What is one main benefit of using advanced PL/pgSQL in PostgreSQL?

easy

A. It replaces the need for any SQL queries.

B. It disables database transactions.

C. It automatically creates user interfaces.

D. It allows writing complex logic inside the database for better performance.

Solution

1. Step 1: Understand PL/pgSQL purpose

   PL/pgSQL is designed to write procedural code inside PostgreSQL to handle complex logic.

2. Step 2: Identify the benefit

   Writing logic inside the database improves performance by reducing data transfer and centralizing processing.

3. Final Answer:

   It allows writing complex logic inside the database for better performance. -> Option D

4. Quick Check:

   Advanced PL/pgSQL improves performance [OK]

Hint: Think about why logic inside DB helps speed [OK]

Common Mistakes:

• Thinking PL/pgSQL replaces all SQL queries
• Confusing PL/pgSQL with UI tools
• Assuming it disables transactions

2. Which of the following is the correct way to declare a variable in PL/pgSQL?

easy

A. DECLARE myvar INTEGER := 10;

B. myvar INTEGER := 10;

C. DECLARE myvar := 10 INTEGER;

D. VAR myvar INTEGER = 10;

Solution

1. Step 1: Recall PL/pgSQL variable declaration syntax

   Variables are declared inside a DECLARE block with type and optional initialization.

2. Step 2: Check each option

   DECLARE myvar INTEGER := 10; correctly uses DECLARE, variable name, type, and initialization. Others have syntax errors.

3. Final Answer:

   DECLARE myvar INTEGER := 10; -> Option A

4. Quick Check:

   Variable declaration needs DECLARE and type [OK]

Hint: Remember DECLARE block is mandatory for variables [OK]

Common Mistakes:

• Omitting DECLARE keyword
• Placing type after initialization
• Using VAR instead of DECLARE

3. What will be the output of this PL/pgSQL function?

CREATE OR REPLACE FUNCTION add_numbers(a INTEGER, b INTEGER) RETURNS INTEGER AS $$
BEGIN
  RETURN a + b;
END;
$$ LANGUAGE plpgsql;

SELECT add_numbers(3, 5);

medium

A. 8

B. 35

C. Syntax error

D. NULL

Solution

1. Step 1: Understand function logic

   The function takes two integers and returns their sum using RETURN a + b.

2. Step 2: Evaluate the SELECT call

   Calling add_numbers(3, 5) returns 3 + 5 = 8.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   3 + 5 = 8 [OK]

Hint: Add the two input numbers as the function returns sum [OK]

Common Mistakes:

• Concatenating numbers as strings
• Expecting syntax error due to missing semicolon
• Assuming NULL return without explicit return

4. Identify the error in this PL/pgSQL block:

DO $$
DECLARE
  counter INTEGER := 0
BEGIN
  counter := counter + 1;
  RAISE NOTICE 'Counter: %', counter;
END;
$$ LANGUAGE plpgsql;

medium

A. Variable counter cannot be initialized

B. RAISE NOTICE syntax is incorrect

C. Missing semicolon after variable declaration

D. LANGUAGE plpgsql is not allowed in DO blocks

Solution

1. Step 1: Check variable declaration syntax

   In PL/pgSQL, each statement must end with a semicolon. The declaration line lacks a semicolon.

2. Step 2: Verify other parts

   RAISE NOTICE syntax is correct, variable initialization is allowed, and LANGUAGE plpgsql is required.

3. Final Answer:

   Missing semicolon after variable declaration -> Option C

4. Quick Check:

   Statements must end with semicolon [OK]

Hint: Check semicolons after DECLARE lines [OK]

Common Mistakes:

• Ignoring missing semicolon errors
• Misreading RAISE NOTICE syntax
• Thinking variable initialization is disallowed

5. You want to create a PL/pgSQL function that returns the factorial of a number using recursion. Which of these function definitions correctly implements this?

hard

A. CREATE FUNCTION factorial(n INTEGER) RETURNS INTEGER AS $$ BEGIN RETURN n * factorial(n - 1); END; $$ LANGUAGE plpgsql;

B. CREATE FUNCTION factorial(n INTEGER) RETURNS INTEGER AS $$ BEGIN IF n <= 1 THEN RETURN 1; ELSE RETURN n * factorial(n - 1); END IF; END; $$ LANGUAGE plpgsql;

C. CREATE FUNCTION factorial(n INTEGER) RETURNS INTEGER AS $$ BEGIN IF n = 0 THEN RETURN 0; ELSE RETURN n * factorial(n - 1); END IF; END; $$ LANGUAGE plpgsql;

D. CREATE FUNCTION factorial(n INTEGER) RETURNS INTEGER AS $$ BEGIN WHILE n > 1 LOOP RETURN n * factorial(n - 1); END LOOP; END; $$ LANGUAGE plpgsql;

Solution

1. Step 1: Understand factorial base case

   Factorial of 0 or 1 is 1, so base case must return 1 when n <= 1.

2. Step 2: Check recursive call correctness

   CREATE FUNCTION factorial(n INTEGER) RETURNS INTEGER AS $$ BEGIN IF n <= 1 THEN RETURN 1; ELSE RETURN n * factorial(n - 1); END IF; END; $$ LANGUAGE plpgsql; correctly returns 1 for base case and multiplies n by factorial(n-1) otherwise.

3. Step 3: Identify errors in other options

   CREATE FUNCTION factorial(n INTEGER) RETURNS INTEGER AS $$ BEGIN RETURN n * factorial(n - 1); END; $$ LANGUAGE plpgsql; lacks base case, causing infinite recursion. CREATE FUNCTION factorial(n INTEGER) RETURNS INTEGER AS $$ BEGIN IF n = 0 THEN RETURN 0; ELSE RETURN n * factorial(n - 1); END IF; END; $$ LANGUAGE plpgsql; returns 0 for n=0, which is incorrect. CREATE FUNCTION factorial(n INTEGER) RETURNS INTEGER AS $$ BEGIN WHILE n > 1 LOOP RETURN n * factorial(n - 1); END LOOP; END; $$ LANGUAGE plpgsql; misuses WHILE loop and RETURN inside loop.

4. Final Answer:

   Correctly implements recursive factorial with base case and recursion. -> Option B

5. Quick Check:

   Base case + recursion needed for factorial [OK]

Hint: Always include base case in recursion [OK]

Common Mistakes:

• Missing base case causing infinite recursion
• Returning wrong value for factorial(0)
• Using loops incorrectly with RETURN inside