從MySQL移動后查詢中的PostgreSQL分組錯誤

Question

我已經從MySQL遷移到PostgreSQL，但是遇到了奇怪的問題。 我的查詢在MySQL中運行完美。

SELECT MIN("events"."schedule") AS schedule 
FROM "events"  
WHERE ("events"."state" IN (1)) AND ("events"."schedule" >= '2014-07-01') 
GROUP BY EXTRACT(YEAR FROM "events"."schedule"), 
EXTRACT(MONTH FROM "events"."schedule")  
ORDER BY "events"."schedule" DESC

PostgreSQL給我這個錯誤：

ERROR:  column "events.schedule" must appear in the GROUP BY clause or be used in an aggregate function
LINE 6: ORDER BY "events"."schedule" DESC
             ^

********** Error **********

ERROR: column "events.schedule" must appear in the GROUP BY clause or be used in an aggregate function
SQL state: 42803
Character: 238

當我同時按兩列對數據進行分組時，應如何重寫此查詢以在GROUP BY子句中包含“ events.schedule”？

Answer 1

如果要按結果列（分鍾（時間表））排序，則可以使用以下命令：

SELECT MIN("events"."schedule") AS schedule
  FROM "events"
 WHERE "events"."state" IN (1)
   AND "events"."schedule" >= '2014-07-01'
 GROUP BY EXTRACT(YEAR FROM "events"."schedule"),
          EXTRACT(MONTH FROM "events"."schedule")
 ORDER BY 1 desc

出現錯誤的原因是因為您嘗試按Schedule列本身進行排序，而不是匯總的min（schedule）

Answer 2

您還可以簡化很多：

SELECT MIN(schedule) AS schedule 
FROM   events  
WHERE  state = 1
AND    schedule >= '2014-07-01'
GROUP  BY date_trunc('month', schedule)
ORDER  BY 1 DESC;

特別是，將兩個EXTRACT調用替換為單個date_trunc() 。 便宜點

Answer 3

您最初的查詢實際上沒有任何意義，因此PostgreSQL拒絕它是正確的。 我不確定MySQL為什么接受它或它實際上在執行哪種排序。

如果您的目標是根據輸出schedule列進行訂購，則可以直接引用SELECT別名（而不是events.schedule ：

SELECT MIN("events"."schedule") AS schedule 
FROM "events"  
WHERE ("events"."state" IN (1)) AND ("events"."schedule" >= '2014-07-01') 
GROUP BY EXTRACT(YEAR FROM "events"."schedule"), 
EXTRACT(MONTH FROM "events"."schedule")
ORDER BY schedule DESC

否則，您可能需要通過提供一些示例數據來闡明您的問題，以表明您希望如何對結果進行排序。