如何使用 slick 进行聚合 [英] How to make aggregations with slick

问题描述

我想强制 slick 创建像

I want to force slick to create queries like

select max(price) from coffees where ... 

但是 slick 的文档没有帮助

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.

Because those q1-q4 aren't queries, I can't get the results but can use them inside other queries.

本声明

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

生成正确的查询但已被弃用(生成警告:不推荐使用类 ColumnExtensionMethods 中的方法 max:改为使用 Query.max").如何在没有警告的情况下生成这样的查询?

generates right query but is deprecated (generates warning: " method max in class ColumnExtensionMethods is deprecated: Use Query.max instead"). How to generate such query without warnings?

另一个问题是与 group by 聚合:

Another issue is to aggregate with group by:

"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

导致明显的数据库错误

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

如何生成这样的查询?

推荐答案

据我所知是第一个简单

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

或

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

maxQuery.list

这篇关于如何使用 slick 进行聚合的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！