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ADO.NET Entity Framework(5)esql (一)

目录

　　1　　esql的查询结果集 ObjectQuery　　1

　　1.1　　ObjectQuery<实体>　　1

　　1.2　　ObjectQuery<DbDataRecord>　　1

　　1.3　　ObjectQuery<简单类型>　　1

　　2　　esql的使用　　2

　　2.1　　it关键字　　2

　　2.2　　value 关键字　　2

　　2.3　　查询参数的使用　　3

　　2.4　　中文字段　　3

　　3　　得到esql与sql字串　　3

　　3.1　　CommandText属性　　4

　　3.2　　ToTraceString方法　　4

　　4　　ObjectQuery的Linq方法　　4

　　4.1　　Where　　4

　　4.2　　OrderBy　　4

　　4.3　　Select　　4

　　4.4　　SelectValue(projection)　　4

　　4.5　　Top(count)　　5

　　4.6　　Skip(keys,count)　　5

　　4.7　　分页 Skip Top　　5

　　4.8　　GroupBy(keys,projection)　　6

　　4.9　　Include(path)　　6

　　5　　esql注释,成员访问,分行　　6

　　6　　esql运算符　　7

　　6.1　　算术运算符　　7

　　6.2　　比效运算符　　7

　　6.3　　逻辑运算符　　7

　　6.4　　区间　　7

　　6.5　　集合运算　　8

　　7　　esql函数　　10

　　7.1　　统计类　　10

　　7.1.1　　联合使用　　11

　　7.1.2　　与group by一起使用　　11

　　7.2　　数学类　　11

　　7.3　　日期　　11

　　7.4　　字符　　12

　　8　　esql语句　　12

　　8.1　　查询语句　　12

　　8.2　　CASE语句　　14

　　9　　esql 类型　　14

　　9.1　　简单类型　　14

　　9.2　　REF　　16

　　9.3　　ROW　　16

　　9.4　　集合　　16

　　9.5　　Object 返回对像　　17

　　9.6　　CAST 类型转换　　17

　　9.7　　OFTYPE　　18

　　9.8　　TREAT　　18

　　9.9　　IS 类型判断　　19

　　10　　esql Namespace　　19

　　10.1　　使用SqlServer函数　　19

　　10.2　　使用NET的数据类型　　20

　　11　　esql关系,导航　　20

　　11.1　　KEY　　20

　　比起 LINQ to SQL，EF 除了提供 LINQ 查询方式, 还提供了 Entity SQL language

　　ESQL 类似 Hibernate 的 HSQL，ESQL 与SQL 语言的语法相似，以字符串的方式执行

　　esql的查询结果集 ObjectQuery ObjectQuery<实体>

myContext context = new myContext();
　　
string esql = "SELECT VALUE DBItemList FROM myContext.DBItemList";
　　
// ObjectQuery<DBItemList> query = new ObjectQuery<DBItemList>(esql, context);
　　
ObjectQuery<DBItemList> query = context.CreateQuery<DBItemList>(esql);
　　
foreach (DBItemList r in query)
{
Console.WriteLine(r.NameID);
}
myContext context = new myContext();
　　
string esql = "SELECT VALUE it FROM myContext.DBItemList as it";
　　
// ObjectQuery<DBItemList> query = new ObjectQuery<DBItemList>(esql, context);
　　
ObjectQuery<DBItemList> query = context.CreateQuery<DBItemList>(esql);
　　
foreach (DBItemList r in query)
{
Console.WriteLine(r.NameID);
}

　　ObjectQuery<DbDataRecord>

myContext context = new myContext();
　　
string esql = "SELECT it.NameID FROM myContext.DBItemList as it";
　　
//ObjectQuery<DbDataRecord> query = new ObjectQuery<DbDataRecord>(esql, context);
ObjectQuery<DbDataRecord> query = context.CreateQuery<DbDataRecord>(esql);
　　
foreach (DbDataRecord r in query)
{
Console.WriteLine(r["NameID"].ToString());
}

　　ObjectQuery<简单类型>

myContext context = new myContext();
　　
string esql = "SELECT value count(it.NameID) FROM myContext.DBItemList as it";
　　
// ObjectQuery<int> query = new ObjectQuery<int>(esql, context);
　　
ObjectQuery<int> query = context.CreateQuery<int>(esql);
foreach (int n in query)
{
Console.WriteLine(n);
}
myContext context = new myContext();
　　
string esql = "SELECT value it.NameID FROM myContext.DBItemList as it";
　　
// ObjectQuery<int> query = new ObjectQuery<int>(esql, context);
　　
ObjectQuery<string> query = context.CreateQuery<string>(esql);
foreach (string n in query)
{
Console.WriteLine(n);
}

　　esql的使用

　　可以在

　　ObjectQuery的Linq方法,

　　构造ObjectQuery,

　　context.CreateQuery返方法,

　　中使用esql,并得到返回的榄查询结果ObjectQuery

　　it关键字

　　[it] 出现在 ESQL 中, 由 ObjectQuery<T>.Name 属性设定，用于标示源查询对象(ObjectQuery)的名称，

　　类似于 "SELECT * FROM Tab as it WHERE it.ItemValue =14" 。

　　可以将这个默认值 "it" 改成其他字符串。

myContext context = new myContext();
context.DBItemList.Name = "wxd";
　　
ObjectQuery<DBItemList> list = context.DBItemList.Where("wxd.ItemValue=5");
myContext context = new myContext();
　　
var sql = "SELECT VALUE DBItemList FROM myContext.DBItemList";
var query = new ObjectQuery<DBItemList>(sql, context);
query.Name = "wxd";
　　
ObjectQuery<DBItemList> list = query.Where("wxd.ItemValue=@v", new ObjectParameter("v", 5));

　　value 关键字

　　value 后只能返回一个成员

myContext context = new myContext();
string esql = "SELECT value AVG(it.ItemValue) FROM myContext.DBItemList as it";
ObjectQuery<int> query = context.CreateQuery<int>(esql);
foreach (int n in query)
{
Console.WriteLine(n);
}
　　
/* print:
3
*/
string esql = "select value it.ItemID from myContext.DBItemList as it";
　　
ObjectQuery<string> query = context.CreateQuery<string>(esql);
　　
foreach (string r in query)
{
Console.WriteLine(r);
}
myContext context = new myContext();
　　
string esql = "select value row( it.ItemValue ,it.NameID,'wxd' as wxwinter) from myContext.DBItemList as it";
　　
ObjectQuery<DbDataRecord> query = context.CreateQuery<DbDataRecord>(esql);
　　
foreach (DbDataRecord r in query)
{
Console.WriteLine("{0},{1},{2}", r["ItemValue"], r["NameID"], r["wxwinter"]);
}

　　查询参数的使用

myContext context = new myContext();
　　
string esql = "SELECT VALUE it FROM myContext.DBItemList as it where it.ItemValue=@v1 or it.NameID=@v2";
　　
ObjectParameter v1 = new ObjectParameter("v1", 3);
ObjectParameter v2 = new ObjectParameter("v2", "n01");
ObjectQuery<DBItemList> query = context.CreateQuery<DBItemList>(esql,v1,v2);
　　
foreach (DBItemList r in query)
{
Console.WriteLine("{0},{1}",r.NameID,r.ItemValue);
}

　　中文字段

　　使用[]将字段括起来

myContext context = new myContext();
　　
ObjectQuery<typeTest> query = context.typeTest.Where("it.值 ==22.22");
System.Console.WriteLine(query.CommandText);
foreach (typeTest r in query)
{
Console.WriteLine("{0},{1},{2},{3},{4},{5}", r.a, r.b, r.c, r.d, r.e, r.值);
}

myContext context = new myContext();
　　
ObjectQuery<typeTest> query = context.typeTest.Where("it.[值] ==22.22");
System.Console.WriteLine(query.CommandText);
foreach (typeTest r in query)
{
Console.WriteLine("{0},{1},{2},{3},{4},{5}", r.a, r.b, r.c, r.d, r.e, r.值);
}

　　得到esql与sql字串

myContext context = new myContext();
　　
string esql = "SELECT VALUE it FROM myContext.DBItemList as it";
　　
ObjectQuery<DBItemList> query = context.CreateQuery<DBItemList>(esql);
　　
Console.WriteLine(query.CommandText);
　　
Console.WriteLine(query.ToTraceString())
SELECT VALUE it FROM myContext.DBItemList as it
SELECT
[Extent1].[AutoId] AS [AutoId],
[Extent1].[NameID] AS [NameID],
[Extent1].[ItemID] AS [ItemID],
[Extent1].[ItemValue] AS [ItemValue]
FROM [dbo].[DBItemList] AS [Extent1]

　　CommandText属性

　　得到esql字串

　　ToTraceString方法

　　得到sql字串

　　ObjectQuery的Linq方法 Where

　　用字符串为条件进行查询

ObjectQuery<T> Where(string predicate, params ObjectParameter[] parameters);
myContext context = new myContext();
ObjectQuery<DBItemList> list = context.DBItemList.Where("(it.ItemValue=5 or it .ItemValue=5) and it.NameID='n01'");
SELECT
[Extent1].[AutoId] AS [AutoId],
[Extent1].[NameID] AS [NameID],
[Extent1].[ItemID] AS [ItemID],
[Extent1].[ItemValue] AS [ItemValue]
FROM [dbo].[DBItemList] AS [Extent1]
WHERE (([Extent1].[ItemValue] = 5) OR ([Extent1].[ItemValue] = 5)) AND ([Extent1].[NameID] = 'n01')

　　OrderBy

　　排序

ObjectQuery<T> OrderBy(string keys, params ObjectParameter[] parameters);
myContext context = new myContext();
ObjectQuery<DBItemList> query = context.DBItemList.OrderBy("it.ItemValue,it.ItemID desc");
foreach (var r in query)
{
Console.WriteLine("{0},{1},{2},{3}", r.AutoId, r.ItemID, r.NameID, r.ItemValue);
}
SELECT
[Extent1].[AutoId] AS [AutoId],
[Extent1].[NameID] AS [NameID],
[Extent1].[ItemID] AS [ItemID],
[Extent1].[ItemValue] AS [ItemValue]
FROM [dbo].[DBItemList] AS [Extent1]
ORDER BY [Extent1].[ItemValue] ASC, [Extent1].[ItemID] DESC

　　Select

　　射影

ObjectQuery<DbDataRecord> Select(string projection, params ObjectParameter[] parameters);
myContext context = new myContext();
　　
ObjectQuery<DbDataRecord> list = context.DBItemList.Select(" it.ItemValue as a,it.NameID ");
SELECT
1 AS [C1],
[Extent1].[ItemValue] AS [ItemValue],
[Extent1].[NameID] AS [NameID]
FROM [dbo].[DBItemList] AS [Extent1]

　　SelectValue(projection)

　　返回只有一组字段的数组

ObjectQuery<TResultType> SelectValue<TResultType>(string projection, params ObjectParameter[] parameters);
myContext context = new myContext();
ObjectQuery<int> query = context.DBItemList.SelectValue<int>("it.ItemValue + it.AutoID");
foreach (var r in query)
{
Console.WriteLine(r);
}
SELECT
[Extent1].[ItemValue] + [Extent1].[AutoId] AS [C1]
FROM [dbo].[DBItemList] AS [Extent1]

　　Top(count)

　　集合的前n个元素

　　count : 前n个元素

ObjectQuery<T> Top(string count, params ObjectParameter[] parameters);
myContext context = new myContext();
ObjectQuery<DBItemList> query = context.DBItemList.Top("3"); ;
foreach (var r in query)
{
Console.WriteLine("{0},{1},{2},{3}", r.AutoId, r.ItemID, r.NameID, r.ItemValue);
　　
}
SELECT TOP (3)
[c].[AutoId] AS [AutoId],
[c].[NameID] AS [NameID],
[c].[ItemID] AS [ItemID],
[c].[ItemValue] AS [ItemValue]
FROM [dbo].[DBItemList] AS [c]

　　Skip(keys,count)

　　跳过集合的前n个元素,

　　keys : 用于排序的字段

　　count : 要跳过的记录个数

ObjectQuery<T> Skip(string keys, string count, params ObjectParameter[] parameters);
myContext context = new myContext();
ObjectQuery<DBItemList> query = context.DBItemList.Skip("it.ItemValue", "5");
foreach (var r in query)
{
Console.WriteLine("{0},{1},{2},{3}", r.AutoId, r.ItemID, r.NameID, r.ItemValue);
　　
}
SELECT
[Extent1].[AutoId] AS [AutoId],
[Extent1].[NameID] AS [NameID],
[Extent1].[ItemID] AS [ItemID],
[Extent1].[ItemValue] AS [ItemValue]
FROM ( SELECT [Extent1].[AutoId] AS [AutoId], [Extent1].[NameID] AS [NameID], [Extent1].[ItemID] AS [ItemID], [Extent1].[ItemValue] AS [ItemValue], row_number() OVER (ORDER BY [Extent1].[ItemValue] ASC) AS [row_number]
　　FROM [dbo].[DBItemList] AS [Extent1]
) AS [Extent1]
WHERE [Extent1].[row_number] > 5
ORDER BY [Extent1].[ItemValue] ASC

　　分页 Skip Top

　　Skip与Top一起使用

myContext context = new myContext();
ObjectQuery<DBItemList> query = context.DBItemList.Skip("it.ItemValue", "5").Top("3"); ;
foreach (var r in query)
{
Console.WriteLine("{0},{1},{2},{3}", r.AutoId, r.ItemID, r.NameID, r.ItemValue);
　　
}
SELECT TOP (3)
[Extent1].[AutoId] AS [AutoId],
[Extent1].[NameID] AS [NameID],
[Extent1].[ItemID] AS [ItemID],
[Extent1].[ItemValue] AS [ItemValue]
FROM ( SELECT [Extent1].[AutoId] AS [AutoId], [Extent1].[NameID] AS [NameID], [Extent1].[ItemID] AS [ItemID], [Extent1].[ItemValue] AS [ItemValue], row_number() OVER (ORDER BY [Extent1].[ItemValue] ASC) AS [row_number]
　　FROM [dbo].[DBItemList] AS [Extent1]
) AS [Extent1]
WHERE [Extent1].[row_number] > 5
ORDER BY [Extent1].[ItemValue] ASC

　　GroupBy(keys,projection)

　　分组

　　keys: GROUP BY的字段

　　projection : Select 的内容

ObjectQuery<DbDataRecord> GroupBy(string keys, string projection, params ObjectParameter[] parameters);
myContext context = new myContext();
ObjectQuery<DbDataRecord> query = context.DBItemList.GroupBy("it.ItemID", "it.ItemID,Sum(it.ItemValue) as ValueSum");
foreach (var r in query)
{
Console.WriteLine("{0},{1}", r["ItemID"], r["ValueSum"]);
}
/*
a,23
b,8
c,23
*/
SELECT
1 AS [C1],
[GroupBy1].[K1] AS [ItemID],
[GroupBy1].[A1] AS [C2]
FROM ( SELECT
　　[Extent1].[ItemID] AS [K1],
　　SUM([Extent1].[ItemValue]) AS [A1]
　　FROM [dbo].[DBItemList] AS [Extent1]
　　GROUP BY [Extent1].[ItemID]
) AS [GroupBy1]
SELECT it.ItemID,Sum(it.ItemValue) as ValueSum
FROM ( [DBItemList] ) AS it
GROUP BY it.ItemID

　　Include(path)

　　加载关联数据,参数为实体的[导航属性]的字串,调用Include("导航属性")后,关联数据会加载,这样就不用在[实体.导航属性]上调用Load()方法

ObjectQuery<T> Include(string path);
myContext context = new myContext();
　　
var r = context.DBItem.Include("DBItemList");
foreach (var dbitem in r)
{
foreach (var dbitemlist in dbitem.DBItemList)
{
Console.WriteLine("{0},{1}", dbitemlist.NameID, dbitemlist.ItemValue);
}
}

　　效果与下例相同

myContext context = new myContext();
　　
var r = context.DBItem;
foreach (var dbitem in r)
{
dbitem.DBItemList.Load();
foreach (var dbitemlist in dbitem.DBItemList)
{
Console.WriteLine("{0},{1}", dbitemlist.NameID, dbitemlist.ItemValue);
}
}

　　esql注释,成员访问,分行

注释 --
成员访问 .
分行 ;

　　esql运算符 算术运算符

加 +
减 -
乘 *
除 /
模 %
负 -

　　比效运算符

等于 =
大于 >
大于等于 >=
空判断 IS NOT NULL

　　IS NULL

小于 <
小天等于 <=
不等于 !=

　　<>

　　字符比效

 LIKE ''

　　NOT LIKE ''

% :

　　_ :

　　[ ] :

　　[^] :

　　逻辑运算符

与 AND

　　&&

非 NOT

　　!

或 OR

　　||

　　区间

　　

　　BETWEEN

　　之间

BETWEEN n AND m
NOT BETWEEN n AND m

myContext context = new myContext();
　　
string esql = "SELECT VALUE it FROM [DBItemList] AS it WHERE it.ItemValue BETWEEN 2 and 4";
　　
ObjectQuery<DBItemList> query = context.CreateQuery<DBItemList>(esql);
　　
foreach (DBItemList r in query)
{
Console.WriteLine("{0},{1}", r.ItemID, r.ItemValue);
}

myContext context = new myContext();
　　
ObjectQuery<DBItemList> query = context.DBItemList.Where(" it.ItemValue not BETWEEN 2 and 4");
　　
foreach (DBItemList r in query)
{
Console.WriteLine("{0},{1}", r.ItemID, r.ItemValue);
}

　　IN

　　在集合中

IN {v,v}
NOT IN{v,v}

myContext context = new myContext();
　　
string esql = "SELECT VALUE it FROM [DBItemList] AS it WHERE it.ItemValue IN {1,2,3}";
　　
ObjectQuery<DBItemList> query = context.CreateQuery<DBItemList>(esql);
　　
foreach (DBItemList r in query)
{
Console.WriteLine("{0},{1}", r.ItemID, r.ItemValue);
}

myContext context = new myContext();
　　
ObjectQuery<DBItemList> query = context.DBItemList.Where("it.ItemValue not IN {1,2,3}");
　　
foreach (DBItemList r in query)
{
Console.WriteLine("{0},{1}", r.ItemID, r.ItemValue);
}

　　EXISTS

　　存在

EXISTS(select from)
NOT EXISTS(select from)

myContext context = new myContext();
　　
string esql = "SELECT VALUE it FROM [DBItemList] AS it WHERE exists(Select VALUE it2 From DBItem as it2 Where it2.ItemID=it.ItemID )";
　　
ObjectQuery<DBItemList> query = context.CreateQuery<DBItemList>(esql);
　　
foreach (DBItemList r in query)
{
Console.WriteLine("{0},{1}", r.ItemID, r.ItemValue);
}

myContext context = new myContext();
　　
ObjectQuery<DBItemList> query = context.DBItemList.Where("exists(Select VALUE it2 From DBItem as it2 Where it2.ItemID=it.ItemID )");
　　
foreach (DBItemList r in query)
{
Console.WriteLine("{0},{1}", r.ItemID, r.ItemValue);
}

分页

SELECT VALUE it FROM ( [DBItemList] ) AS it
ORDER BY it.ItemValue
SKIP 5
LIMIT 3

　　集合运算

　　

　　Union

　　(合集)

　　连接不同集合

UNION --自动过滤相同项
UNION ALL --两个集合的相同项都会返回

myContext context = new myContext();
　　
string esql = "(select value it from myContext.DBItemList as it where it.ItemID == 'c' || it.ItemID == 'a' ) UNION (select value it from myContext.DBItemList as it where it.ItemID == 'c' || it.ItemID == 'b' )";
　　
ObjectQuery<DBItemList> query = context.CreateQuery<DBItemList>(esql);
　　
foreach (DBItemList r in query)
{
Console.WriteLine("{0},{1}", r.ItemID, r.ItemValue);
}

EXCEPT

　　(左并集)

　　从集合中删除其与另一个集合中相同的项

myContext context = new myContext();
　　
string esql = "(select value it from myContext.DBItemList as it where it.ItemID == 'c' || it.ItemID == 'a' ) EXCEPT (select value it from myContext.DBItemList as it where it.ItemID == 'c' || it.ItemID == 'b' )";
ObjectQuery<DBItemList> query = context.CreateQuery<DBItemList>(esql);
　　
foreach (DBItemList r in query)
{
Console.WriteLine("{0},{1}", r.ItemID, r.ItemValue);
}

INTERSECT

　　（交集）

　　获取不同集合的相同项

myContext context = new myContext();
　　
string esql = "(select value it from myContext.DBItemList as it where it.ItemID == 'c' || it.ItemID == 'a' ) INTERSECT (select value it from myContext.DBItemList as it where it.ItemID == 'c' || it.ItemID == 'b' )";
ObjectQuery<DBItemList> query = context.CreateQuery<DBItemList>(esql);
　　
foreach (DBItemList r in query)
{
Console.WriteLine("{0},{1}", r.ItemID, r.ItemValue);
}

　　ANYELEMENT

　　集合中的第一个

myContext context = new myContext();
　　
string esql = "ANYELEMENT(select value it from myContext.DBItemList as it where it.ItemID == 'a') ";
ObjectQuery<DBItemList> query = context.CreateQuery<DBItemList>(esql);
　　
//如果集合中有两个以上,只返回第一个到集合中
foreach (DBItemList r in query)
{
Console.WriteLine("{0},{1}", r.ItemID, r.ItemValue);
}

SELECT
[Element1].[AutoId] AS [AutoId],
[Element1].[NameID] AS [NameID],
[Element1].[ItemID] AS [ItemID],
[Element1].[ItemValue] AS [ItemValue]
FROM ( SELECT cast(1 as bit) AS X ) AS [SingleRowTable1]
LEFT OUTER JOIN (SELECT TOP (1)
　　[Extent1].[AutoId] AS [AutoId],
　　[Extent1].[NameID] AS [NameID],
　　[Extent1].[ItemID] AS [ItemID],
　　[Extent1].[ItemValue] AS [ItemValue]
　　FROM [dbo].[DBItemList] AS [Extent1]
　　WHERE [Extent1].[ItemID] = 'a' ) AS [Element1] ON 1 = 1

　　OVERLAPS

　　两个集合是否有相交部份

myContext context = new myContext();
　　
string esql = "(select value it from myContext.DBItemList as it where it.ItemID == 'c' || it.ItemID=='b' ) OVERLAPS (select value it from myContext.DBItemList as it where it.ItemID == 'a' || it.ItemID=='b')";
ObjectQuery<bool> query = context.CreateQuery<bool>(esql);
　　
foreach (bool r in query)
{
Console.WriteLine(r);
}
//print: True

SELECT
CASE WHEN ( EXISTS (SELECT
　　cast(1 as bit) AS [C1]
　　FROM (SELECT
　　　　[Extent1].[AutoId] AS [AutoId],
　　　　[Extent1].[NameID] AS [NameID],
　　　　[Extent1].[ItemID] AS [ItemID],
　　　　[Extent1].[ItemValue] AS [ItemValue]
　　　　FROM [dbo].[DBItemList] AS [Extent1]
　　　　WHERE ([Extent1].[ItemID] = 'c') OR ([Extent1].[ItemID] = 'b')
　　INTERSECT
　　　　SELECT
　　　　[Extent2].[AutoId] AS [AutoId],
　　　　[Extent2].[NameID] AS [NameID],
　　　　[Extent2].[ItemID] AS [ItemID],
　　　　[Extent2].[ItemValue] AS [ItemValue]
　　　　FROM [dbo].[DBItemList] AS [Extent2]
　　　　WHERE ([Extent2].[ItemID] = 'a') OR ([Extent2].[ItemID] = 'b')) AS [Intersect1]
)) THEN cast(1 as bit) WHEN ( NOT EXISTS (SELECT
　　cast(1 as bit) AS [C1]
　　FROM (SELECT
　　　　[Extent3].[AutoId] AS [AutoId],
　　　　[Extent3].[NameID] AS [NameID],
　　　　[Extent3].[ItemID] AS [ItemID],
　　　　[Extent3].[ItemValue] AS [ItemValue]
　　　　FROM [dbo].[DBItemList] AS [Extent3]
　　　　WHERE ([Extent3].[ItemID] = 'c') OR ([Extent3].[ItemID] = 'b')
　　INTERSECT
　　　　SELECT
　　　　[Extent4].[AutoId] AS [AutoId],
　　　　[Extent4].[NameID] AS [NameID],
　　　　[Extent4].[ItemID] AS [ItemID],
　　　　[Extent4].[ItemValue] AS [ItemValue]
　　　　FROM [dbo].[DBItemList] AS [Extent4]
　　　　WHERE ([Extent4].[ItemID] = 'a') OR ([Extent4].[ItemID] = 'b')) AS [Intersect2]
)) THEN cast(0 as bit) END AS [C1]
FROM ( SELECT cast(1 as bit) AS X ) AS [SingleRowTable1]

　　Set

　　去掉重复项

myContext context = new myContext();
　　
string esql = "set(select value it.ItemID from myContext.DBItemList as it)";
ObjectQuery<string> query = context.CreateQuery<string>(esql);
　　
foreach (string r in query)
{
Console.WriteLine(r);
}
//去掉了重复项

SELECT
[Distinct1].[ItemID] AS [ItemID]
FROM ( SELECT DISTINCT
　　[Extent1].[ItemID] AS [ItemID]
　　FROM [dbo].[DBItemList] AS [Extent1]
) AS [Distinct1]

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