PostgreSQL select 由兩列區分

Question

我有一張桌子

發件人ID	接收者 ID	日期
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”

當前查詢

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

當前結果

發件人ID	接收者 ID	日期
1	2	“2022-08-10T07:25:12.881Z”
2	1	“2022-08-10T07:28:12.881Z”

預期結果

發件人ID	接收者 ID	日期
2	1	“2022-08-10T07:28:12.881Z”

所以，我只需要 select 這對中日期較大的一行

Answer 1

我們可以在這里使用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;