如何使用光滑进行聚合-Java 学习之路

我想强制光滑创建像

select max(price) from coffees where ...

但slick's documentation没有帮助

val q = Coffees.map(_.price) //this is query Query[Coffees.type, ...]
val q1 = q.min // this is Column[Option[Double]]
val q2 = q.max
val q3 = q.sum
val q4 = q.avg

因为那些q1-q4不是查询，我无法获得结果，但可以在其他查询中使用它们 .

这个说法

for {
  coffee <- Coffees
} yield coffee.price.max

生成正确的查询但不推荐使用（生成警告：" method max in class ColumnExtensionMethods is deprecated: Use Query.max instead"） . How to generate such query without warnings?

另一个问题是通过以下方式聚合：

"select name, max(price) from coffees group by name"

试图解决它

for {
  coffee <- Coffees
} yield (coffee.name, coffee.price.max)).groupBy(x => x._1)

产生

select x2.x3, x2.x3, x2.x4 from (select x5."COF_NAME" as x3, max(x5."PRICE") as x4 from "coffees" x5) x2 group by x2.x3

这会导致明显的db错误

column "x5.COF_NAME" must appear in the GROUP BY clause or be used in an aggregate function

How to generate such query?

1 回答

据我所知，第一个就是简单

Query(Coffees.map(_.price).max).first

第二个

val maxQuery = Coffees
  .groupBy { _.name }
  .map { case (name, c) =>
    name -> c.map(_.price).max
  }

maxQuery.list

要么

val maxQuery = for {
  (name, c) <- Coffees groupBy (_.name)
} yield name -> c.map(_.price).max

maxQuery.list

回复于 2024-04-28T19:50:28+08:00

如何使用光滑进行聚合

1 回答

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