In PostgreSQL, the string_agg() function is an aggregate function which returns a string containing values of what is between the brackets ().

Structure

SELECT string_agg(expression)
FROM TABLE
WHERE [CONSTRAINTS]
GROUP BY [EXPRESSION]

Example

Suppose we want to output the sporting equipment that each individual has borrowed. Here is how we would do that:

SELECT b.name, string_agg(e.name, ', ') as equipment from borrowers b
JOIN equipment_borrowed e ON e.id = b.id
WHERE b.active = 1
GROUP BY b.name;

Output

name	equipment
Tom	Basketball, Soccer ball
Jenn	Volleyball, Tennis equipment
Tony	Hockey stick, Hockey puck