Question

是否可以写

select min(next) from participant;

作为esqueleto查询？

更新

我发现了min_功能。但是，代码

nextMessageTime =
  from $ \p -> min_ (p ^. ParticipantNext)

显然有一些模棱两可的类型：

Could not deduce (Database.Esqueleto.Internal.Language.From
                    query expr backend (query (Entity (ParticipantGeneric backend0))))
  arising from the ambiguity check for ‘nextMessageTime’
from the context (Database.Esqueleto.Internal.Language.From
                    query expr backend (query (Entity (ParticipantGeneric backend2))),
                  Esqueleto query1 query backend1)
  bound by the inferred type for ‘nextMessageTime’:
             (Database.Esqueleto.Internal.Language.From
                query expr backend (query (Entity (ParticipantGeneric backend2))),
              Esqueleto query1 query backend1) =>
             query (Value (Maybe Int64))
  at Presta/DB/Queries.hs:(15,1)-(16,42)
The type variable ‘backend0’ is ambiguous
When checking that ‘nextMessageTime’
  has the inferred type ‘forall (query :: * -> *)
                                (expr :: * -> *)
                                backend
                                (query1 :: * -> *)
                                backend1
                                backend2.
                         (Database.Esqueleto.Internal.Language.From
                            query expr backend (query (Entity (ParticipantGeneric backend2))),
                          Esqueleto query1 query backend1) =>
                         query (Value (Maybe Int64))’
Probable cause: the inferred type is ambiguous

Answer 1

我设法使用持久性rawSql：

来实现这一点

nextMessageTime = do
  [Single t] <-
    rawSql
      "SELECT min(next) FROM participant"
      []
  return (t :: Int64)

（但是如果您正在复制此代码，请注意这种无穷无尽的模式。）

聚合函数作为选择结果

1 个答案: