如何在postgres中将多个值作为单个字符串收集？

Question

我有桌子：

Project table

id name
-------
1  A
2  B

Assignment table

id name project_id
-------------------
1  A1   1
2  A2   1
3  A3   2

我希望编写一个返回每个项目的查询，其中包含从中创建的赋值的名称，如：

project_id  assignments
-----------------------
1            A1,A2
2            A3

有没有办法实现这个目标？

Answer 1

您可以加入表并使用array_agg组合以逗号分隔的值

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

SQLFiddle Demo

或使用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

SQLFiddle Demo