Você pode juntar as tabelas e usar
array_agg para combinar os valores separados por uma vírgula SELECT a.id, array_agg(b.name) assignments
FROM Project a
INNER JOIN assignment b
ON a.id = b.project_ID
GROUP BY a.id
Demonstração do SQLFiddle
ou usando
STRING_AGG SELECT a.id, STRING_AGG(b.name, ', ' ORDER BY b.name) assignments
FROM Project a
INNER JOIN assignment b
ON a.id = b.project_ID
GROUP BY a.id
