PostgreSQL 检查值是否存在于另一个表中

Question

I'm trying to find a solution in PostgreSQL of how I can add to the output of the query extra column with value if id exists in another table or not:

I need several things:

1. Do a join between two tables
2. Add a new column into the result output where I check if exists in the third table or not

My tables:

• announcement
• author
• chapter

announcement table

| id | author_id | date | group_id |... |

author table

| id | name | email |... |

chapter table

| id | announcement_id |... |

This is what I have now. I did a left outer join and it works as I expected:

select announcement.id, announcement.date, author.id as publisher_id, author.name as publisher_name
from announcement 
left outer join author 
on announcement.author_id = author.id
where announcement.group_id = 123 and announcement.date >= '2022-06-01'::date;

with output:

| id | date       | publisher_id | publisher_name |
| 1  | 2020-07-01 | 12           | John           |
| 2  | 2020-07-04 | 123          | Arthur         |

Now I can't find a solution of how to add an extra column with_chapters to the query response, where I will check if announcement.id exists in chapter table under announcement_id column .

For example, chapter table can have such data:

| id | announcement_id |
| 1  | 1               |
| 2  | 1               |
| 3  | 1               |

So we see that some announcements can appear in chapters several times (so i'm looking for at least 1 match). And some announcements doesn't have chapters at all.

Output finally should be like that:

| id | date       | publisher_id | publisher_name | with_chapters |
| 1  | 2020-07-01 | 12           | John           | true          |
| 2  | 2020-07-04 | 123          | Arthur         | false         |

Thanks a lot for any help:)

Answer 1

While EXISTS (subquery) is usually used in the WHERE clause, it returns an ordinary Boolean and so can be used in the select list.

SELECT blah1, blah2, 
       EXISTS (select 1 from chapter where chapter.announcement_id=announcement.id) as with_chapter
FROM ...