String aggregation with STRING_AGG in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the STRING_AGG function do in PostgreSQL?

STRING_AGG combines multiple string values from rows into a single string, separated by a specified delimiter.

beginner

How do you specify the separator in STRING_AGG?

You provide the separator as the second argument, for example: STRING_AGG(column_name, ', ') combines values separated by a comma and space.

intermediate

Can STRING_AGG be used with an ORDER BY clause? If yes, why?

Yes, STRING_AGG supports ORDER BY inside its syntax to control the order of concatenated strings, ensuring predictable output.

beginner

Write a simple example of STRING_AGG usage to combine names separated by a semicolon.

SELECT STRING_AGG(name, '; ') FROM users;

intermediate

What happens if STRING_AGG is used on an empty set of rows?

STRING_AGG returns NULL if there are no rows to aggregate.

What is the purpose of STRING_AGG in PostgreSQL?

A To combine multiple string values into one string with a separator

B To count the number of rows in a table

C To find the maximum value in a column

D To split a string into multiple rows

STRING_AGG combines multiple strings into one, separated by a specified delimiter.

How do you specify the separator in STRING_AGG?

A As the second argument

B As the first argument

C Using a separate function

D It uses a default separator and cannot be changed

The separator is the second argument in STRING_AGG, for example: STRING_AGG(column, ', ').

Which clause can be used inside STRING_AGG to order the concatenated strings?

A GROUP BY

B HAVING

C ORDER BY

D WHERE

ORDER BY inside STRING_AGG controls the order of concatenated strings.

What does STRING_AGG return if no rows are found?

A An error

B An empty string

C Zero

D NULL

STRING_AGG returns NULL when there are no rows to aggregate.

Which of the following is a valid STRING_AGG usage?

A STRING_AGG(name)

B STRING_AGG(name, ', ' ORDER BY name)

C STRING_AGG(name, ', ', GROUP BY name)

D STRING_AGG(name, ', ', WHERE age > 20)

STRING_AGG can include ORDER BY inside parentheses to order the concatenated strings.

Explain how STRING_AGG works and how you can control the order of concatenated strings.

Think about how to join many names into one string with a chosen separator and order.

Write a query using STRING_AGG to list all product names separated by a dash (-), ordered alphabetically.

Use STRING_AGG(product_name, ' - ' ORDER BY product_name) from products.