FOREACH for array iteration in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to iterate over each element in the array using FOREACH.

PostgreSQL

DECLARE
  my_array integer[] := ARRAY[1, 2, 3];
  element integer;
BEGIN
  FOREACH element [1] ARRAY my_array LOOP
    RAISE NOTICE '%', element;
  END LOOP;
END;

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

AFROM

BOF

CON

DIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using OF instead of IN causes a syntax error.

Using FROM or ON are not valid keywords here.

The correct syntax to iterate over an array in PostgreSQL using FOREACH is FOREACH element IN ARRAY my_array.

2fill in blank

medium

Complete the code to declare an array of text values.

PostgreSQL

DECLARE
  fruits text[] := [1];
BEGIN
  NULL;
END;

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

AARRAY['apple', 'banana', 'cherry']

B['apple', 'banana', 'cherry']

C('apple', 'banana', 'cherry')

D{'apple', 'banana', 'cherry'}

Attempts:

3 left

💡 Hint

Common Mistakes

Using JSON-style [] notation causes errors.

Using parentheses () is for tuples, not arrays.

In PostgreSQL, arrays are declared using the ARRAY[...] syntax.

3fill in blank

hard

Fix the error in the FOREACH loop to correctly iterate over the array.

PostgreSQL

DECLARE
  nums integer[] := ARRAY[10, 20, 30];
  n integer;
BEGIN
  FOREACH n [1] nums LOOP
    RAISE NOTICE '%', n;
  END LOOP;
END;

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

AIN ARRAY

BIN

CFROM ARRAY

DON ARRAY

Attempts:

3 left

💡 Hint

Common Mistakes

Using IN without ARRAY causes syntax errors.

Using ON ARRAY or FROM ARRAY are invalid.

The FOREACH loop requires the keyword IN ARRAY before the array variable to iterate correctly.

4fill in blank

hard

Fill both blanks to create a FOREACH loop that iterates over a text array and prints each fruit.

PostgreSQL

DECLARE
  fruits text[] := ARRAY['apple', 'banana', 'cherry'];
  fruit text;
BEGIN
  FOREACH fruit [1] [2] fruits LOOP
    RAISE NOTICE '%', fruit;
  END LOOP;
END;

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

AOF

BIN

CARRAY

DTABLE

Attempts:

3 left

💡 Hint

Common Mistakes

Using OF instead of IN causes errors.

Using TABLE instead of ARRAY is invalid here.

The correct syntax is FOREACH fruit IN ARRAY fruits to iterate over the array.

5fill in blank

hard

Fill all three blanks to declare an integer array, iterate over it with FOREACH, and print each number.

PostgreSQL

DECLARE
  numbers [1] := [2];
  num integer;
BEGIN
  FOREACH num [3] numbers LOOP
    RAISE NOTICE '%', num;
  END LOOP;
END;

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

Ainteger[]

BARRAY[5, 10, 15]

CIN ARRAY

Dtext[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using text[] instead of integer[] for numbers.

Not using IN ARRAY in FOREACH loop.

Incorrect array literal syntax.

We declare numbers as an integer array with integer[], assign it using ARRAY[5, 10, 15], and iterate using FOREACH num IN ARRAY numbers.

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