Why FOREACH for array iteration in PostgreSQL? - Purpose & Use Cases

The Big Idea

Discover how a simple loop can save you hours of tedious work!

The Scenario

Imagine you have a list of your favorite movies stored on paper. To find each movie's release year, you have to look up each one separately, writing down the year every time. This takes a lot of time and effort.

The Problem

Manually checking each movie one by one is slow and easy to mess up. You might skip a movie or write the wrong year. If the list grows, it becomes overwhelming and frustrating.

The Solution

Using FOREACH in PostgreSQL lets you automatically go through each item in an array one by one. It saves time and avoids mistakes by handling the repetition for you.

Before vs After

✗ Before

SELECT array[1,2,3,4]; -- then manually access each element by index

✓ After

FOREACH element IN ARRAY my_array LOOP
  -- process element
END LOOP;

What It Enables

With FOREACH, you can easily and safely process every item in a list without writing repetitive code.

Real Life Example

Suppose you store user IDs in an array and want to send a notification to each user. FOREACH lets you loop through all IDs and send messages one by one automatically.

Key Takeaways

Manually handling each array item is slow and error-prone.

FOREACH loops through array items automatically.

This makes your code simpler, faster, and less buggy.

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