Linq2SQL分組和總和優化

Question

我已經在這上面敲了一段時間了。 我想實現這一目標：

SELECT [t2].[nArtKey], [t2].[value] AS [nQty]
FROM (
    SELECT SUM([t0].[nQty]) AS [value], [t1].[nArtKey]
        FROM [vdatStockTransactions] AS [t0]
        INNER JOIN [regArtSKU] AS [t1] ON [t0].[nSKU] = [t1].[nSKU]
        GROUP BY [t1].[nArtKey]
        ) AS [t2]
    INNER JOIN [regArticles] AS [t3] ON [t2].[nArtKey] = [t3].[nArtKey]
    INNER JOIN [regGroupConnector] AS [t4] ON [t2].[nArtKey] = [t4].[nArtKey]
WHERE [t2].[value] > @p0

到目前為止，我對LINQ所擁有的東西幾乎完全滿足了我的需求，數量方面...

from trans in context.vdatStockTransactions
join sku in context.regArtSKUs on trans.nSKU equals sku.nSKU
group trans by new { sku.nArtKey } into grp
where grp.Sum(g => g.nQty) > 0
join art in context.regArticles on grp.Key.nArtKey equals art.nArtKey       
join ca in context.regGroupConnectors on grp.Key.nArtKey equals ca.nArtKey
select new
{
    nArtKey = grp.Key.nArtKey,
    //nQty = grp.Sum(g => g.nQty)
};

但是，如果我取消注釋nQty，則會得到以下信息：

SELECT [t7].[nArtKey], [t7].[value] AS [nQty]
FROM (
    SELECT [t3].[nArtKey], (
        SELECT SUM([t5].[nQty])
        FROM [vdatStockTransactions] AS [t5]
        INNER JOIN [regArtSKU] AS [t6] ON [t5].[nSKU] = [t6].[nSKU]
        WHERE [t2].[nArtKey] = [t6].[nArtKey]
        ) AS [value], [t2].[value] AS [value2]
    FROM (
        SELECT SUM([t0].[nQty]) AS [value], [t1].[nArtKey]
        FROM [vdatStockTransactions] AS [t0]
        INNER JOIN [regArtSKU] AS [t1] ON [t0].[nSKU] = [t1].[nSKU]
        GROUP BY [t1].[nArtKey]
        ) AS [t2]
    INNER JOIN [regArticles] AS [t3] ON [t2].[nArtKey] = [t3].[nArtKey]
    INNER JOIN [regGroupConnector] AS [t4] ON [t2].[nArtKey] = [t4].[nArtKey]
    ) AS [t7]
WHERE [t7].[value2] > @p0

為什么要創建額外的子查詢，有什么辦法可以避免呢？ 它在性能上有很大的不同，所以我真的很想弄清楚這一點。 我想要的只是使用數量（[t2]。[value]）並將其放入SELECT語句。

只有在group by之后有多個連接時，才存在該額外的子查詢，因此，如果我刪除其中一個，則查詢將生成預期的SQL。

注意：我精簡了原始查詢，只保留了生成此行為的部分，因此更容易理解。

Answer 1

我還沒有測試過，但這可以解決問題：

var quantities =
    from trans in context.vdatStockTransactions
    group trans by trans.SKU.nArtKey into grp
    select new
    {
        nQty = grp.Sum(g => g.nQty), 
        nArtKey = grp.Key 
    };

var results =
    from quantity in quantities
    join art in context.regArticles
        on quantity.nArtKey equals art.nArtKey       
    join ca in context.regGroupConnectors 
        on quantity.nArtKey equals ca.nArtKey
    where quantity.nQty > 0
    select quantity;