整理一下Entity Framework的查询

Entity Framework是个好东西，虽然没有Hibernate功能强大，但使用更简便。今天整理一下常见SQL如何用EF来表达，Func形式和Linq形式都会列出来（本人更喜欢Func形式）。

1、简单查询：

SQL：

SELECT * FROM [Clients] WHERE Type=1 AND Deleted=0 ORDER BY ID

EF：

//Func形式 
    var clients = ctx.Clients.Where(c => c.Type == 1 && c.Deleted == 0) 
                .OrderBy(c => c.ID) 
                .ToList(); 
     
    //Linq形式 
    var clients = from c in ctx.Clients 
                  where c.Type == 1 && c.Deleted==0 
                  orderby c.ID 
                  select c;

2、查询部分字段：

SQL：

SELECT ID,Name FROM [Clients] WHERE Status=1

EF：

//Func形式 
    var clients = ctx.Clients.Where(c => c.Status == 1) 
                .Select(c => new { c.ID, Name = c.ComputerName }) 
                .ToList(); 
     
    //Linq形式 
    var clients = from c in ctx.Clients 
                  where c.Status == 1 
                  select new { c.ID, Name = c.ComputerName }; ：

3、查询单一记录：

SQL:

SELECT * FROM [Clients] WHERE ID=100

EF：

//Func形式 
    var client = ctx.Clients.FirstOrDefault(c => c.ID == 100); 
     
    //Linq形式 
    var client = (from c in ctx.Clients 
                where c.ID = 100 
                select c).FirstOrDefault();

4、LEFT JOIN 连接查询

SQL：

SELECT  c.ID ,
        c.ComputerName ,
        g.Name GroupName
FROM    [Clients] c
        LEFT JOIN [Groups] g ON c.GroupID = g.ID
WHERE   c.Status = 1

EF：

//Func形式 
    var clients = ctx.Clients.Where(c => c.Status == 1) 
                .Select(c => new  
                { 
                    c.ID, 
                    c.ComputerName, 
                    GroupName = ctx.Groups.FirstOrDefault(g => g.ID == c.GroupID).Name 
                }) 
                .ToList(); 
     
    //Linq形式 
    var clients = from c in ctx.Clients 
                where c.Status == 1 
                select new 
                { 
                    c.ID, 
                    c.ComputerName, 
                    GroupName = (from g in ctx.Groups 
                                where g.ID == c.GroupID 
                                select g.Name).FirstOrDefault() 
                };

5、INNER JOIN 连接查询：

SQL：

SELECT  c.ID ,
        c.ComputerName ,
        g.Name GroupName
FROM    [Clients] c
        INNER JOIN [Groups] g ON c.GroupID = g.ID
WHERE   c.Status = 1
ORDER BY g.Name

EF：

//Func形式 
    var clients = ctx.Clients.Where(c => c.Status == 1) 
                .Join(ctx.Group, c => c.GroupID, g => g.ID, (c,g) =>  
                { 
                    c.ID, 
                    c.ComputerName, 
                    GroupName = g.Name 
                }) 
                .OrderBy(item => item.GroupName) 
                .ToList(); 
     
     
    //Linq形式1 
    var clients = from c in ctx.Clients 
                from g in ctx.Groups 
                where c.GroupID == g.ID 
                orderby g.Name 
                select new 
                { 
                    c.ID, 
                    c.ComputerName, 
                    GroupName = g.Name 
                }; 
     
    //Linq形式2 
    var clients = from c in ctx.Clients 
                where c.Status == 1 
                join g in ctx.Group 
                on c.GroupID equals g.ID into result 
                from r in result 
                order by r.Name 
                select new 
                { 
                    c.ID, 
                    c.ComputerName, 
                    GroupName = r.Name 
                };

6、分页

SQL：

-- 方案1 
SELECT TOP 10
        *
FROM    [Clients]
WHERE   Status = 1
        AND ID NOT IN ( SELECT TOP 20
                                ID
                        FROM    [Clients]
                        WHERE   Status = 1
                        ORDER BY ComputerName )
ORDER BY ComputerName 
     
--方案2 
SELECT  *
FROM    ( SELECT    * ,
                    ROW_NUMBER() OVER ( ORDER BY ComputerName ) AS RowNo
          FROM      [Clients]
          WHERE     Status = 1
        ) t
WHERE   RowNo >= 20
        AND RowNo < 30

EF：

//Func形式 
    var clients = ctx.Clients.Where(c => c.Status=1) 
                .OrderBy(c => c.ComputerName) 
                .Skip(20) 
                .Take(10) 
                .ToList(); 
     
    //Linq形式 
    var clients = (from c in ctx.Clients 
                orderby c.ComputerName 
                select c).Skip(20).Take(10);

7、分组统计：

SQL：

SELECT  Status ,
        COUNT(*) AS Cnt
FROM    [Clients]
GROUP BY Status
ORDER BY COUNT(*) DESC

EF：

//Func形式 
    var result = ctx.Clients.GroupBy(c => c.Status) 
                .Select(s => new 
                { 
                    Status = s.Key, 
                    Cnt = s.Count() 
                }) 
                .OrderByDescending(r => r.Cnt); 
     
    //Linq形式 
    var result = from c in ctx.Clients 
                group c by c.Status into r 
                orderby r.Count() descending 
                select new 
                { 
                    Status = r.Key, 
                    Cnt = r.Count() 
                };