FOREACH for array iteration in PostgreSQL - Step-by-Step Execution

Concept Flow - FOREACH for array iteration

Start FOREACH loop

↓

Pick next element from array

↓

Execute loop body with element

↓

More elements?

Yes→Pick next element

No↓

End loop

The FOREACH loop picks each element from the array one by one and runs the loop body with that element until all elements are processed.

Execution Sample

PostgreSQL

DO $$
DECLARE
  arr integer[] := ARRAY[10, 20, 30];
  val integer;
BEGIN
  FOREACH val IN ARRAY arr LOOP
    RAISE NOTICE 'Value: %', val;
  END LOOP;
END $$;

This code loops through each number in the array and prints it.

Execution Table

Step	Current Element (val)	Action	Output
1	10	Start loop, pick first element	Value: 10
2	20	Pick next element	Value: 20
3	30	Pick next element	Value: 30
4	N/A	No more elements, exit loop	Loop ends

💡 All elements in the array have been processed, so the loop ends.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
val	NULL	10	20	30	30

Key Moments - 3 Insights

What is the value of 'val' after the loop ends?

30. It retains the value of the last element assigned during the loop (see variable_tracker 'Final' column).

What happens if the array is empty?

The loop body never runs because there are no elements to pick, so no output is produced.

Can FOREACH be used with arrays of types other than integers?

Yes, FOREACH works with any array type, like text[], boolean[], etc., as long as the loop variable matches the element type.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'val' at step 2?

A 10

B 20

C 30

D NULL

💡 Hint

Check the 'Current Element (val)' column at step 2 in the execution_table.

At which step does the FOREACH loop end?

A Step 3

B Step 2

C Step 4

D Step 1

💡 Hint

Look for the row where the action says 'No more elements, exit loop' in the execution_table.

If the array was empty, what would the output be?

A No output

B Value: NULL

C Error

D Value: 0

💡 Hint

Refer to the key_moments section about empty arrays and loop execution.

Concept Snapshot

FOREACH loops through each element in an array.
Syntax: FOREACH var IN ARRAY array_var LOOP ... END LOOP;
Each iteration assigns the next element to var.
Loop ends when all elements are processed.
Works with any array type matching var's type.

Full Transcript

The FOREACH loop in PostgreSQL lets you run code for each element in an array. It picks elements one by one, assigns them to a variable, and runs the loop body. When no elements remain, the loop stops. If the array is empty, the loop body never runs. This works for arrays of any type, as long as the loop variable matches the element type. The example code loops through an integer array and prints each number. The variable holding the element changes each step and retains the last value after the loop ends.

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