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：

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

EF：

1. //Func形式
2. var clients = ctx.Clients.Where(c => c.Status == 1)
3. .Join(ctx.Group, c => c.GroupID, g => g.ID, (c,g) =>
4. {
5. c.ID,
6. c.ComputerName,
7. GroupName = g.Name
8. })
9. .OrderBy(item => item.GroupName)
10. .ToList();
11. //Linq形式1
12. var clients = from c in ctx.Clients
13. from g in ctx.Groups
14. where c.GroupID == g.ID
15. orderby g.Name
16. select new
17. {
18. c.ID,
19. c.ComputerName,
20. GroupName = g.Name
21. };
22. //Linq形式2
23. var clients = from c in ctx.Clients
24. where c.Status == 1
25. join g in ctx.Group
26. on c.GroupID equals g.ID into result
27. from r in result
28. order by r.Name
29. select new
30. {
31. c.ID,
32. c.ComputerName,
33. GroupName = r.Name
34. };

6、分页

SQL：

1. -- 方案1
2. SELECT TOP 10 * FROM [Clients] WHERE Status=1
3. AND ID NOT IN
4. (
5. SELECT TOP 20 ID FROM [Clients] WHERE Status=1
6. ORDER BY ComputerName
7. )
8. ORDER BY ComputerName
9. --方案2
10. SELECT * FROM
11. (
12. SELECT *, ROW_NUMBER() OVER(ORDER BY ComputerName) AS RowNo
13. FROM [Clients]
14. WHERE Status=1
15. )t
16. WHERE RowNo >= 20 AND RowNo < 30

EF：

1. //Func形式
2. var clients = ctx.Clients.Where(c => c.Status=1)
3. .OrderBy(c => c.ComputerName)
4. .Skip(20)
5. .Take(10)
6. .ToList();
7. //Linq形式
8. var clients = (from c in ctx.Clients
9. orderby c.ComputerName
10. select c).Skip(20).Take(10);

7、分组统计：

SQL：

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

EF：

1. //Func形式
2. var result = ctx.Clients.GroupBy(c => c.Status)
3. .Select(s => new
4. {
5. Status = s.Key,
6. Cnt = s.Count()
7. })
8. .OrderByDescending(r => r.Cnt);
9. //Linq形式
10. var result = from c in ctx.Clients
11. group c by c.Status into r
12. orderby r.Count() descending
13. select new
14. {
15. Status = r.Key,
16. Cnt = r.Count()
17. };

未完待续……