如何實現這個SQL塊包含（有，加入和GROUP BY）使用LINQ

我的SQL語句是如何實現這個SQL塊包含（有，加入和GROUP BY）使用LINQ

SELECT c.type,c.title,c.datereg, d.ranknum 
FROM T_News AS c 
    INNER JOIN (
    SELECT a.id, COUNT(*) AS ranknum 
    FROM T_News AS a 
      INNER JOIN T_News AS b 
      ON (a.type = b.type) 
      AND (a.datereg >= b.datereg) 
    GROUP BY a.id 
    HAVING COUNT(*) <= 3 
) AS d ON (c.id = d.id) 
ORDER BY c.type, d.ranknum

，我得到http://rickosborne.org/blog/2008/01/sql-getting-top-n-rows-for-a-grouped-query/

爲獲得TOP N行的分組查詢

EFUnitOfWork EF = new EFUnitOfWork(); 
T_NewsRepository News = new T_NewsRepository(); 
News.UnitOfWork = EF; 
var query = 
    from news1 in News.All() 
    join news2 in News.All() 
     on news1.type equals news2.type into resjoin 
    group news1 by news1.id into idgroup 
    where idgroup.Count() <= 3 
    select new { idgroup }; 

var x = query.ToList();

我沒有得到任何錯誤，但「哪裏idgroup.Count（）< = 3」，沒有工作，我得到的所有行以dB爲結果

來源

2012-02-03 amir

你得到了什麼錯誤？ – doogle 2012-02-03 12:14:22

我沒有得到任何錯誤，但「其中idgroup.Count（）<= 3」沒有工作，我得到所有行數據庫作爲結果 – amir 2012-02-03 13:04:38

將其分解爲最小的組件，然後再編寫較大的查詢。讓我們先從最裏面的查詢是有道理的：

SELECT 
    a.id, COUNT(*) AS ranknum 
FROM 
    T_News AS a 
     INNER JOIN T_News AS b ON 
      (a.type = b.type) AND 
      (a.datereg >= b.datereg) 
GROUP BY 
    a.id 
HAVING 
    COUNT(*) <= 3

我想這個轉換爲：

// Items with counts/ranknum 
var ranknum = 
    from a in News.All() 
    join b in News.All() on 
     a.type equals b.type 
    where 
     a.datereg > b.datereg 
    group by a.id into g 
    select new { g.Key as id, g.Count() as ranknum }; 

// Filter the ranknum. 
ranknum = ranknum.Where(rn => rn.ranknum <= 3);

然後加入與外部查詢：

SELECT 
    c.type,c.title,c.datereg, d.ranknum 
FROM 
    T_News AS c 
     INNER JOIN (<sub-query from above>) as d ON 
      c.id = d.id 
ORDER BY 
    c.type, d.ranknum

這部分變得簡單，因爲它只是兩個現有查詢之間的連接。

var query = 
    from c in News.All() 
    join rn in ranknum on c.id = rn.id 
    orderby c.type, rn.ranknum 
    select new { c.type, c.title, c.datereg, rn.ranknum };

機會是，LINQ到實體產生這種情況的SQL會看起來真的醜，而且很可能是低效的，在這種情況下，你可能要考慮將這種邏輯在存儲過程然後通過LINQ-to-Entities調用即（對於更復雜的查詢通常是這樣）。

來源

2012-02-03 13:40:12 casperOne

如何實現這個SQL塊包含（有，加入和GROUP BY）使用LINQ

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