FOREACH for array iteration in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the FOREACH statement do in PostgreSQL?

FOREACH loops through each element in an array, allowing you to perform actions on each item one by one.

beginner

How do you declare a FOREACH loop to iterate over an integer array in PostgreSQL?

Use: FOREACH element_variable IN ARRAY array_variable LOOP ... END LOOP;

intermediate

Can FOREACH be used to iterate over arrays of types other than integers?

Yes, FOREACH works with arrays of any data type, like text[], integer[], or custom types.

beginner

What happens if the array used in FOREACH is empty?

The loop body does not execute at all because there are no elements to iterate over.

intermediate

Why is FOREACH useful compared to a traditional FOR loop in PostgreSQL?

FOREACH simplifies looping over arrays directly without needing to manage index counters manually.

What keyword starts the loop to iterate over an array in PostgreSQL?

A WHILE

B FOR

C FOREACH

D LOOP

FOREACH is the specific keyword used to loop over each element in an array.

Which of these is the correct syntax to iterate over an array named my_array?

A FOREACH item IN ARRAY my_array LOOP ... END LOOP;

B FOREACH item IN my_array LOOP ... END LOOP;

C FOR item IN ARRAY my_array LOOP ... END LOOP;

D FOR item IN my_array LOOP ... END LOOP;

FOREACH requires the IN ARRAY clause to specify the array to iterate over.

What type of variable should 'item' be in FOREACH item IN ARRAY my_array?

A Same type as elements in my_array

B Any type

C Always integer

D Always text

The loop variable must match the data type of the array elements.

If my_array is empty, what happens inside the FOREACH loop?

A Loop runs infinitely

B Loop runs once with NULL

C Error occurs

D Loop runs zero times

No elements means the loop body does not execute.

FOREACH is best used when you want to:

A Loop a fixed number of times

B Process each element of an array easily

C Loop through table rows

D Create arrays

FOREACH is designed to iterate over array elements simply.

Explain how to use FOREACH to loop through an array in PostgreSQL.

Think about the syntax and what each part means.

Describe a real-life scenario where FOREACH for array iteration would be helpful in a database function.

Imagine you have a list of things to handle one by one.

Practice

1. What is the main purpose of the FOREACH statement in PostgreSQL when working with arrays?

easy

A. To convert an array into a string.

B. To loop through each element of an array easily inside PL/pgSQL.

C. To sort the elements of an array in ascending order.

D. To create a new array from existing tables.

Solution

1. Step 1: Understand FOREACH usage context

   FOREACH is used inside PL/pgSQL to iterate over array elements one by one.

2. Step 2: Identify the main purpose

   It simplifies looping through arrays without manual index handling.

3. Final Answer:

   To loop through each element of an array easily inside PL/pgSQL. -> Option B

4. Quick Check:

   FOREACH loops over array elements [OK]

Hint: FOREACH loops over array elements inside PL/pgSQL [OK]

Common Mistakes:

• Confusing FOREACH with array creation or sorting
• Thinking FOREACH works outside PL/pgSQL
• Assuming FOREACH converts arrays to strings

2. Which of the following is the correct syntax to iterate over an integer array arr using FOREACH in PL/pgSQL?

easy

A. FOREACH element IN ARRAY arr LOOP ... END LOOP;

B. FOREACH element FROM arr LOOP ... END LOOP;

C. FOREACH element ON arr LOOP ... END LOOP;

D. FOREACH element OVER arr LOOP ... END LOOP;

Solution

1. Step 1: Recall FOREACH syntax

   The correct syntax uses 'IN ARRAY' to specify the array to loop over.

2. Step 2: Match syntax options

   Only FOREACH element IN ARRAY arr LOOP ... END LOOP; uses 'IN ARRAY' correctly; others use invalid keywords.

3. Final Answer:

   FOREACH element IN ARRAY arr LOOP ... END LOOP; -> Option A

4. Quick Check:

   FOREACH ... IN ARRAY ... is correct syntax [OK]

Hint: Use 'IN ARRAY' to loop over arrays with FOREACH [OK]

Common Mistakes:

• Using FROM, ON, or OVER instead of IN ARRAY
• Omitting LOOP or END LOOP keywords
• Trying to use FOREACH outside PL/pgSQL

3. Consider the following PL/pgSQL block:

DECLARE
  arr integer[] := ARRAY[2, 4, 6];
  sum integer := 0;
BEGIN
  FOREACH val IN ARRAY arr LOOP
    sum := sum + val;
  END LOOP;
  RAISE NOTICE '%', sum;
END;

What will be the output when this block runs?

medium

A. Syntax error

B. 24

C. 12

D. 0

Solution

1. Step 1: Understand the loop iteration

   The FOREACH loops over arr elements: 2, 4, and 6.

2. Step 2: Calculate the sum

   sum starts at 0, then adds 2 + 4 + 6 = 12.

3. Final Answer:

   12 -> Option C

4. Quick Check:

   2 + 4 + 6 = 12 [OK]

Hint: Sum array elements by adding each in FOREACH loop [OK]

Common Mistakes:

• Assuming FOREACH does not add elements
• Confusing sum initialization
• Expecting syntax error due to RAISE NOTICE

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

DECLARE
  arr text[] := ARRAY['a', 'b', 'c'];
  ch text;
BEGIN
  FOREACH ch IN arr LOOP
    RAISE NOTICE '%', ch;
  END LOOP;
END;

medium

A. Variable 'ch' must be declared as integer, not text.

B. Array declaration syntax is incorrect.

C. RAISE NOTICE cannot print variables inside loops.

D. FOREACH must use 'IN ARRAY' before the array variable.

Solution

1. Step 1: Check FOREACH syntax

   FOREACH requires 'IN ARRAY' before the array variable to iterate properly.

2. Step 2: Identify the error

   The code uses 'IN arr' instead of 'IN ARRAY arr', causing syntax error.

3. Final Answer:

   FOREACH must use 'IN ARRAY' before the array variable. -> Option D

4. Quick Check:

   FOREACH ... IN ARRAY ... is correct syntax [OK]

Hint: Always write 'FOREACH var IN ARRAY arr' [OK]

Common Mistakes:

• Omitting 'ARRAY' keyword in FOREACH
• Wrong variable type for array elements
• Misunderstanding RAISE NOTICE usage

5. You want to write a PL/pgSQL function that takes an integer array and returns the count of even numbers using FOREACH. Which of the following code snippets correctly implements this?

hard

A. CREATE FUNCTION count_evens(arr integer[]) RETURNS integer AS $$
DECLARE
val integer;
count integer := 0;
BEGIN
FOREACH val IN ARRAY arr LOOP
IF val % 2 = 0 THEN
count := count + 1;
END IF;
END LOOP;
RETURN count;
END;
$$ LANGUAGE plpgsql;

B. CREATE FUNCTION count_evens(arr integer[]) RETURNS integer AS $$
DECLARE
val integer;
count integer := 0;
BEGIN
FOREACH val FROM arr LOOP
IF val % 2 = 0 THEN
count := count + 1;
END IF;
END LOOP;
RETURN count;
END;
$$ LANGUAGE plpgsql;

C. CREATE FUNCTION count_evens(arr integer[]) RETURNS integer AS $$
DECLARE
val integer;
count integer := 0;
BEGIN
FOR val IN ARRAY arr LOOP
IF val % 2 = 0 THEN
count := count + 1;
END IF;
END LOOP;
RETURN count;
END;
$$ LANGUAGE plpgsql;

D. CREATE FUNCTION count_evens(arr integer[]) RETURNS integer AS $$
DECLARE
val integer;
count integer := 0;
BEGIN
FOREACH val ON ARRAY arr LOOP
IF val % 2 = 0 THEN
count := count + 1;
END IF;
END LOOP;
RETURN count;
END;
$$ LANGUAGE plpgsql;

Solution

1. Step 1: Check FOREACH syntax correctness

   CREATE FUNCTION count_evens(arr integer[]) RETURNS integer AS $$ DECLARE val integer; count integer := 0; BEGIN FOREACH val IN ARRAY arr LOOP IF val % 2 = 0 THEN count := count + 1; END IF; END LOOP; RETURN count; END; $$ LANGUAGE plpgsql; uses 'FOREACH val IN ARRAY arr LOOP', which is the correct syntax.

2. Step 2: Verify logic for counting even numbers

   Inside the loop, it checks if val is even and increments count correctly.

3. Final Answer:

   FOREACH val IN ARRAY arr LOOP with IF val % 2 = 0 logic -> Option A

4. Quick Check:

   FOREACH ... IN ARRAY ... and correct IF condition [OK]

Hint: Use 'FOREACH val IN ARRAY arr' and check val % 2 = 0 [OK]

Common Mistakes:

• Using 'FROM' or 'ON ARRAY' instead of 'IN ARRAY'
• Confusing FOREACH with FOR loops
• Missing RETURN statement or wrong logic