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：