CodeGo.net> CLR用户定义的聚合函数,以获取序列的第一个元素

我需要一个FIRST聚合函数,该函数返回序列的第一个元素,并将其与HAVING子句一起使用.

这个问题可以看作是我先前的问题：FIRST aggregate function which I can use with HAVING clause的延续.

事实证明,只有聚合函数才能解决我的问题,因此我尝试创建一个函数：

[Serializable]
[SqlUserDefinedAggregate(
    Format.UserDefined, //use clr serialization to serialize the intermediate result
    IsInvariantToNulls = true, //optimizer property
    IsInvariantToDuplicates = false, //optimizer property
    IsInvariantToOrder = false, //optimizer property
    MaxByteSize = 8000) //maximum size in bytes of persisted value
]
public struct GetFirst : IBinarySerialize {

    private string allValues;

    public void Init() {
        allValues = string.Empty;
    }

    private void incrementAndAdd(SqlInt32 value) {

        allValues += (value.Value.ToString() + ",");
    }

    public void Accumulate(SqlInt32 Value) {

        incrementAndAdd(Value);
    }

    public void Merge(GetFirst Group) {

    }

    public SqlInt32 Terminate() {
        // Put your code here
        return new SqlInt32(int.Parse(allValues.Split(',')[0]));
    }

    // This is a place-holder member field
    private SqlInt32 var1;


    public void Read(System.IO.BinaryReader r) {

        allValues = r.ReadString();
    }

    public void Write(System.IO.BinaryWriter w) {

        w.Write(this.allValues);
    }
}

这就是我的用法：

DECLARE @fooTable AS TABLE(
    ID INT,
    CategoryName NVARCHAR(100),
    Name NVARCHAR(100),
    MinAllow INT,
    Price DECIMAL(18,2)
);

INSERT INTO @fooTable  VALUES(1, 'Cat1', 'Product1', 3, 112.2);
INSERT INTO @fooTable  VALUES(2, 'Cat2', 'Product2', 4, 12.34);
INSERT INTO @fooTable  VALUES(3, 'Cat1', 'Product3', 5, 233.32);
INSERT INTO @fooTable  VALUES(4, 'Cat3', 'Product4', 4, 12.43);
INSERT INTO @fooTable  VALUES(5, 'Cat3', 'Product5', 1, 13.00);
INSERT INTO @fooTable  VALUES(7, 'Cat4', 'Product7', 1, 15.00);
INSERT INTO @fooTable  VALUES(6, 'Cat4', 'Product6', 3, 13.00);

DECLARE @minAllowParam AS INT = 3;

SELECT ft.CategoryName, SUM(ft.Price), dbo.GetFirst(ft.MinAllow) FROM @fooTable ft
GROUP BY ft.CategoryName
HAVING dbo.GetFirst(ft.MinAllow) >= @minAllowParam;

它有时会返回正确的结果,有时却不会,并且我不确定我是否正确实施了该方法.如果我有鉴于此的要求,对此有何想法？

解决方法:

我认为您的问题是IsInvariantToOrder尚未实施.

Reserved for future use. This property is not currently used by the
query processor: order is currently not guaranteed.

因此,不能保证在运行之间将以正确的顺序甚至是相同的顺序调用您的函数.

我对C#不太了解,但是我认为可以在函数中使用两个参数,其中第二个是要排序的字段.在返回第一个值之前,将对保存在列表中并在第二个字段上排序.

更新：

我不使用C#,所以这里可能有很多错误的错误做法,我没有看到.但是看起来在我进行的测试中效果很好.

using System;
using System.Data;
using Microsoft.SqlServer.Server;
using System.Data.SqlTypes;
using System.IO;
using System.Text;

[Serializable]
[SqlUserDefinedAggregate(
    Format.UserDefined, //use clr serialization to serialize the intermediate result
    IsInvariantToNulls = true, //optimizer property
    IsInvariantToDuplicates = false, //optimizer property
    IsInvariantToOrder = false, //optimizer property
    MaxByteSize = 8000) //maximum size in bytes of persisted value
]

public class GetFirst : IBinarySerialize
{
    private int Value;
    private int OrderBy;

[SqlFunctionAttribute(IsDeterministic = true)]
    public void Init()
    {
      Value = 0;
      OrderBy = int.MaxValue;
    }

[SqlFunctionAttribute(IsDeterministic = true)]
    public void Accumulate(SqlInt32 SValue, SqlInt32 SOrderBy)
    {
        if (SValue.IsNull)
        {
            return;
        }

        if (SOrderBy.IsNull)
        {
            return;
        }

        if (SOrderBy.Value < OrderBy)
        {
            Value = SValue.Value;
            OrderBy = SOrderBy.Value;
        }
    }

[SqlFunctionAttribute(IsDeterministic = true)]
    public void Merge(GetFirst other)
    {
        if (other.OrderBy < OrderBy)
        {
            Value = other.Value;
            OrderBy = other.OrderBy;
        }
    }

[SqlFunctionAttribute(IsDeterministic = true)]
    public SqlInt32 Terminate()
    {
      return new SqlInt32(Value);
    }

[SqlFunctionAttribute(IsDeterministic = true)]
    public void Read(BinaryReader r)
    {
        Value = r.ReadInt32();
        OrderBy = r.ReadInt32();
    }

[SqlFunctionAttribute(IsDeterministic = true)]
    public void Write(BinaryWriter w)
    {
        w.Write(Value);
        w.Write(OrderBy);
    }
}

安装：

CREATE ASSEMBLY GetFirstAsm FROM 'First.dll'
GO
CREATE AGGREGATE GetFirst (@Value int, @OrderBy int) RETURNS int
EXTERNAL NAME GetFirstAsm.GetFirst

像这样使用：

declare @T table
(
  ID1 int,
  ID2 int,
  Val int,
  Grp int
)

insert into @T values
(1, 5, '1', 1),
(2, 4, '2', 1),
(3, 3, '3', 1),
(4, 2, '4', 2),
(5, 1, '5', 2)

select Grp, dbo.GetFirst(Val, ID1) as Val
from @T
group by Grp

select Grp, dbo.GetFirst(Val, ID2) as Val
from @T
group by Grp

结果：

(5 row(s) affected)
Grp         Val
----------- -----------
1           1
2           4

(2 row(s) affected)

Grp         Val
----------- -----------
1           3
2           5

(2 row(s) affected)

标签：aggregate-functions,sql-server-2008-r2,sqlclr,c,sql-server
来源： https://codeday.me/bug/20191201/2081143.html