F＃查询连接

Question

I am using SqlDataConnection data provider in F# to migrate some rows, part of that migration is to make a join between 3 tables like this, think of it as an inheritance of tables A , B , C where B and C inherit from A so the thing is I need to get is (in Linq-like):

        Bs.Join(As, b.PK, a.FK).Select(new {...})
.Concat(Cs.Join(As, c.PK, a.FK).Select(new {...})

In F# , the closest I got to this was:

 let result = seq {
     yield! query { ... }
     yield! query { ... }
 }

but I've been told this will produce 2 SQL queries and the overall result will be on-memory. The question being: is there a way to make this "concatenation" as a query computation expression without using seq so that all happens in a single SQL query?

Answer 1

Here's some example code that combines two queries using SQL UNION or UNION ALL .

First, the setup. Note that I've added logging to dbContext so you can see what happens behind the scenes.

#r "System.Data.dll"
#r "System.Data.Linq.dll"
#r "FSharp.Data.TypeProviders.dll"

open System
open System.Linq
open Microsoft.FSharp.Data.TypeProviders

type sql = SqlDataConnection<connStr>

let createDbContext() = 
    let dbContext = sql.GetDataContext()
    // add logging to console
    dbContext.DataContext.Log <- System.Console.Out 
    dbContext 

let db = createDbContext()
let products = db.Product

let q1 = query { for x in products do select x }
let q2 = query { for y in products do select y }

The Union extension method combines queries as one query using UNION

let qUnion = q1.Union(q2)
qUnion.ToList() |> Seq.toList

Here's the logged output:

SELECT [t2].[Id], [t2].[Name]
FROM (
    SELECT [t0].[Id], [t0].[Name]
    FROM [dbo].[Product] AS [t0]
    UNION
    SELECT [t1].[Id], [t1].[Name]
    FROM [dbo].[Product] AS [t1]
    ) AS [t2]

The Concat extension method combines queries as one query using UNION ALL

let qConcat = q1.Concat(q2)
qConcat.ToList() |> Seq.toList

Here's the logged output:

SELECT [t2].[Id], [t2].[Name]
FROM (
    SELECT [t0].[Id], [t0].[Name]
    FROM [dbo].[Product] AS [t0]
    UNION ALL
    SELECT [t1].[Id], [t1].[Name]
    FROM [dbo].[Product] AS [t1]
    ) AS [t2]

There's no special syntax for unions in query expressions, AFAIK.