[英]PostgreSQL select distinct by two columns
I have a table我有一张桌子
senderId![]() |
receiverId![]() |
date![]() |
---|---|---|
1 ![]() |
2 ![]() |
"2022-08-10T07:21:12.881Z" ![]() |
2 ![]() |
1 ![]() |
"2022-08-10T07:28:12.881Z" ![]() |
2 ![]() |
1 ![]() |
"2022-08-10T07:22:12.881Z" ![]() |
1 ![]() |
2 ![]() |
"2022-08-10T07:25:12.881Z" ![]() |
Current query当前查询
SELECT DISTINCT ON ("senderId", "receiverId") "sender"."id" AS "senderId", "receiver"."id" AS "receiverId", cm."createdAt" AS "createdAt" FROM "chat_message" "cm" LEFT JOIN "user" "sender" ON "sender"."id"="cm"."senderId" LEFT JOIN "user" "receiver" ON "receiver"."id"="cm"."receiverId" WHERE "senderId" = 2 OR "receiverId" = 2 ORDER BY "senderId", "receiverId" ASC, cm."createdAt" DESC
Current result当前结果
senderId![]() |
receiverId![]() |
date![]() |
---|---|---|
1 ![]() |
2 ![]() |
"2022-08-10T07:25:12.881Z" ![]() |
2 ![]() |
1 ![]() |
"2022-08-10T07:28:12.881Z" ![]() |
Expected result预期结果
senderId![]() |
receiverId![]() |
date![]() |
---|---|---|
2 ![]() |
1 ![]() |
"2022-08-10T07:28:12.881Z" ![]() |
So, I need to select only one row in this pair where the date is bigger所以,我只需要 select 这对中日期较大的一行
We can use a LEAST
/ GREATEST
trick here along with DISTINCT ON
:我们可以在这里使用
LEAST
/ GREATEST
技巧和DISTINCT ON
:
SELECT DISTINCT ON (LEAST(senderId, receiverId),
GREATEST(senderId, receiverId))
LEAST(senderId, receiverId) AS senderId,
GREATEST(senderId, receiverId) AS receiverId,
createdAt
FROM chat_message
ORDER BY 1, 2, createdAt DESC;
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