Array aggregation with ARRAY_AGG in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all user IDs aggregated into an array.

PostgreSQL

SELECT [1](user_id) FROM users;

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AARRAY_AGG

BCOUNT

CMAX

DSUM

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT instead of ARRAY_AGG returns a number, not an array.

Using SUM or MAX returns a single value, not an array.

The ARRAY_AGG function collects values into an array. Here, it gathers all user_id values.

2fill in blank

medium

Complete the code to aggregate product names into an array grouped by category.

PostgreSQL

SELECT category, [1](product_name) FROM products GROUP BY category;

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AAVG

BCOUNT

CMIN

DARRAY_AGG

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT returns the number of products, not their names.

Using AVG or MIN does not work on text fields.

ARRAY_AGG collects all product_name values per category into arrays.

3fill in blank

hard

Fix the error in the code to aggregate emails into an array.

PostgreSQL

SELECT ARRAY_AGG[1] FROM contacts;

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A(DISTINCT email)

B[

C{

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Missing parentheses causes syntax errors.

Using brackets or braces is invalid syntax.

To aggregate unique emails, use ARRAY_AGG(DISTINCT email). The parentheses and DISTINCT keyword are required.

4fill in blank

hard

Complete the code to aggregate distinct tags into an array grouped by post ID.

PostgreSQL

SELECT post_id, ARRAY_AGG[1]tags FROM post_tags GROUP BY post_id;

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A(DISTINCT

B)

C;

D]

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to close parentheses causes errors.

Using semicolon or brackets instead of closing parenthesis is invalid.

Use ARRAY_AGG(DISTINCT tags) with parentheses and DISTINCT to get unique tags per post.

5fill in blank

hard

Fill all three blanks to aggregate user names in uppercase into an array grouped by city.

PostgreSQL

SELECT city, ARRAY_AGG([1]([2])) AS users FROM users GROUP BY [3];

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AUPPER

Buser_name

Ccity

DLOWER

Attempts:

3 left

💡 Hint

Common Mistakes

Using LOWER instead of UPPER changes case incorrectly.

Grouping by user_name instead of city changes the grouping.

This query groups users by city and aggregates their names in uppercase using ARRAY_AGG(UPPER(user_name)).