Query database for distinct values and aggregate data based on condition - sql

Question

I am trying to extract distinct items from a Postgres database pairing a column from a table with a column from another table based on a condition. Simplified version looks like this:
CREATE TABLE users
(
id SERIAL PRIMARY KEY,
name VARCHAR(255)
);
CREATE TABLE photos
(
id INT PRIMARY KEY,
user_id INTEGER REFERENCES users(id),
flag VARCHAR(255)
);
INSERT INTO users VALUES (1, 'Bob');
INSERT INTO users VALUES (2, 'Alice');
INSERT INTO users VALUES (3, 'John');
INSERT INTO photos VALUES (1001, 1, 'a');
INSERT INTO photos VALUES (1002, 1, 'b');
INSERT INTO photos VALUES (1003, 1, 'c');
INSERT INTO photos VALUES (1004, 2, 'a');
INSERT INTO photos VALUES (1004, 2, 'x');
What I need is to extract each user name, only once, and a flag value for each of them. The flag value should prioritize a specific one, let's say b. So, the result should look like:
Bob b
Alice a
Where Bob owns a photo having the b flag, while Alice does not and John has no photos. For Alice the output for the flag value is not important (a or x would be just as good) as long as she owns no photo flagged b.
The closest thing I found were some self-join queries where the flag value would have been aggregated using min() or max(), but I am looking for a particular value, which is not first, nor last. Moreover, I found out that you can define your own aggregate functions, but I wonder if there is an easier way of conditioning the query in order to obtain the required data.
Thank you!

Answer 1

Here is a method with aggregation:
select u.name,
coalesce(max(flag) filter (where flag = 'b'),
min(flag)
) as flag
from users u left join
photos p
on u.id = p.user_id
group by u.id, u.name;
That said, a more typical method would be a prioritization query. Perhaps:
select distinct on (u.id) u.name, p.flag
from users u left join
photos p
on u.id = p.user_id
order by u.id, (p.flag = 'b') desc;