sql - PostgreSQL聚合函数选择2个对应字段

Question

我有以下子查询:

LEFT OUTER JOIN
(
    SELECT
        MAX(testresult."testdate") AS beforeresult,
        testschedule."test_id",
        testschedule."asset_id",
        testschedule."testdate" AS testdate,
        testschedule."testresult_id" as scheduleresult_id                                        
    FROM
        "public"."testresult" testresult 
        INNER JOIN "public"."testschedule" testschedule 
            ON testschedule."asset_id" = testresult."asset_id" 
            AND testschedule."test_id" = testresult."test_id" 
    WHERE
        "testresult"."client_id" = 25368272
        AND testresult."testdate" < testschedule."mintolerancedate"
        AND testschedule."testdate" > '2016-10-01' 
        AND testschedule."testdate" < '2016-10-20'
    GROUP BY
        testschedule."asset_id", 
        testschedule."test_id", 
        testschedule."testdate", 
        testschedule."testresult_id" 
    ORDER BY MAX(testresult."testdate")
) lasttr
    ON "testschedule"."asset_id" = lasttr."asset_id"
    AND "testschedule"."test_id" = lasttr."test_id"
    AND testschedule."testdate" = lasttr.testdate

这给了我正确的测试日期。但是，我还需要与日期对应的 testschedule."testresult_id"。有没有办法从上面的查询中选择这个？

Answer 1

您可以摆弄窗口函数，或者在最新版本中，进行左侧连接:

LEFT JOIN LATERAL
(SELECT ts."test_id", ts."asset_id", ts."testdate" AS testdate, ts."testresult_id" as scheduleresult_id,
        tr.*
 FROM "public"."testresult" tr INNER JOIN
      "public"."testschedule" ts
      ON ts."asset_id" = tr."asset_id" and ts."test_id" = tr."test_id" 
 WHERE tr."client_id" = 25368272 AND
       tr."testdate" < ts."mintolerancedate" AND
       ts."testdate" > '2016-10-01' and ts."testdate" < '2016-10-20' AND
       "testschedule"."asset_id" = ts."asset_id" AND
       "testschedule"."test_id" = ts."test_id"
 ORDER BY tr."testdate" DESC
 FETCH FIRST 1 ROW ONLY
) lasttr

横向连接类似于 FROM 子句中的相关子查询。您可以返回任意数量的列。没有 ON 子句，因为连接条件在子查询的 WHERE 子句中。