Add conditional statement in PostgreSQL query - postgresql

Question

I have this query:
WITH grouped_messages AS
(SELECT
um.conversation_id,
array_agg(json_build_object('message', um.message, 'sent_at', um.created_at))
FROM user_messages um
INNER JOIN users_conversations c ON c.conversation_id = um.conversation_id
WHERE c.user_id = '#{user_id}' AND um.user_id != '#{user_id}' #{ "and um.created_at >= '#{timestamp}'" IF TIMESTAMP}
GROUP BY um.conversation_id
),
senders AS
(SELECT
gm.conversation_id,
json_build_object('id', u.id, 'user_name', u.user_name, 'avatar',
('https://my-staging.s3.amazonaws.com/public/uploads/user/' || u.id :: TEXT ||
'/avatar.jpg')) AS sender
FROM grouped_messages AS gm
INNER JOIN users_conversations c ON c.conversation_id = gm.conversation_id
INNER JOIN users u ON u.id = c.user_id
WHERE u.id != '#{user_id}'
)
SELECT json_object_agg(grouped_messages.conversation_id,
json_build_object('new_messages', grouped_messages.array_agg, 'sender', senders.sender))
FROM grouped_messages
INNER JOIN senders ON senders.conversation_id = grouped_messages.conversation_id
Which outputs, for example:
{
"62": {
"new_messages": [
{
"message": "some",
"sent_at": "2016-05-30T20:19:53.786024"
},
{
"message": null,
"sent_at": "2016-05-30T20:19:26.408814"
}
],
"sender": {
"id": "e4ba308b-a5cf-47ad-b8d6-d774eb325411",
"user_name": null,
"avatar": "https://my-staging.s3.amazonaws.com/public/uploads/user/e4ba308b-a5cf-47ad-b8d6-d774eb325411/avatar.jpg"
}
}
}
Now, what I need is to have a conditional statement in this query.
If u (user) avatar is not NULL then apply this 'https://my-staging.s3.amazonaws.com/public/uploads/user/' || u.id::text || '/avatar.jpg', otherwise just insert ':null' string (so that any JSON parser recognizes it as a null object).
Not sure how to put this if-else here in select statement and also how to ensure that part about JSON parser.

Answer 1

worked for me:
(SELECT gm.conversation_id, json_build_object('id', u.id, 'user_name', u.user_name,
'avatar', (
CASE WHEN u.avatar IS NULL THEN ':null'
ELSE ('https://my-staging.s3.amazonaws.com/public/uploads/user/' || u.id::text || '/avatar.jpg')
END))
You can provide case-statement in SELECT block as if it were value (with optional AS).

Answer 2

This is a good use of the CASE expression. In this scenario, you want to return either a hard URL to their avatar if the avatar field is not null, and NULL otherwise.
This is accomplished thus (omitting the other portions of the query for readability):
json_build_object('id', u.id, 'user_name', u.user_name,
'avatar',
CASE WHEN u.avatar IS NULL
THEN NULL
ELSE
(
'https://my-staging.s3.amazonaws.com/public/uploads/user/'
|| u.id :: TEXT ||
'/avatar.jpg') END) AS sender