? 前言
LINQ to SQL 中需要对两个或多个数据集进行操作，比如：合并、取交集等，主要使用下面四个方法，这四个方法都是 System.Linq.IQueryable<out T> 接口的扩展方法，并且都是延迟加载方法，下面是使用的简单示例。
? 示例数据，数据表：Subject（科目表）、Score（成绩表）
1. Concat() 方法
1) 方法声明
public static IQueryable<TSource> Concat<TSource>(this IQueryable<TSource> source1, IEnumerable<TSource> source2);
2) 描述
合并两个数据集，不去重复，对应的 SQL 语句是 UNION ALL。
3) 示例1：获取所有科目Id 和 考试过的科目Id 列表（科目Id 可以重复）
1. C# 代码
var query = (from t1 in DbContext.Subject
select t1.SubjectId
).Concat(
from t1 in DbContext.Score
select t1.SubjectId
);
var result = query.ToList();
2. 生成SQL
SELECT
[UnionAll1].[SubjectId] AS [C1]
FROM (SELECT
[Extent1].[SubjectId] AS [SubjectId]
FROM [dbo].[Subject] AS [Extent1]
UNION ALL
SELECT
[Extent2].[SubjectId] AS [SubjectId]
FROM [dbo].[Score] AS [Extent2]) AS [UnionAll1]
3. 执行结果
2. Union() 方法
1) 方法声明
public static IQueryable<TSource> Union<TSource>(this IQueryable<TSource> source1, IEnumerable<TSource> source2);
2) 描述
合并两个数据集，去重复，对应的 SQL 语句是 UNION ALL + DISTINCT。与想象中似乎不太一样，实际写 SQL 时，一般都会用 UNION 就好了哇。
3) 示例1：获取所有科目Id 和 考试过的科目Id 列表（科目Id 不能重复）
1. C# 代码
var query = (from t1 in DbContext.Subject
select t1.SubjectId
).Union(
from t1 in DbContext.Score
select t1.SubjectId
);
var result = query.ToList();
2. 生成SQL
SELECT
[Distinct1].[C1] AS [C1]
FROM ( SELECT DISTINCT
[UnionAll1].[SubjectId] AS [C1]
FROM (SELECT
[Extent1].[SubjectId] AS [SubjectId]
FROM [dbo].[Subject] AS [Extent1]
UNION ALL
SELECT
[Extent2].[SubjectId] AS [SubjectId]
FROM [dbo].[Score] AS [Extent2]) AS [UnionAll1]
) AS [Distinct1]
3. 执行结果
3. Intersect() 方法
1) 方法声明（2个重载）
public static IQueryable<TSource> Intersect<TSource>(this IQueryable<TSource> source1, IEnumerable<TSource> source2);
public static IQueryable<TSource> Intersect<TSource>(this IQueryable<TSource> source1, IEnumerable<TSource> source2, IEqualityComparer<TSource> comparer);
2) 描述
取两个数据集的交集（在数据集A出现的记录，并且在数据集B也出现的记录），对应的 SQL 语句是 INTERSECT。
3) 示例1：获取考试过的科目Id
1. C# 代码
var query = (from t1 in DbContext.Subject
select t1.SubjectId
).Intersect(
from t1 in DbContext.Score
select t1.SubjectId
);
var result = query.ToList();
2. 生成SQL
SELECT
[Intersect1].[SubjectId] AS [C1]
FROM (SELECT
[Extent1].[SubjectId] AS [SubjectId]
FROM [dbo].[Subject] AS [Extent1]
INTERSECT
SELECT
[Extent2].[SubjectId] AS [SubjectId]
FROM [dbo].[Score] AS [Extent2]) AS [Intersect1]
3. 执行结果
4. Concat() 方法
1) 方法声明（2个重载）
public static IQueryable<TSource> Except<TSource>(this IQueryable<TSource> source1, IEnumerable<TSource> source2);
public static IQueryable<TSource> Except<TSource>(this IQueryable<TSource> source1, IEnumerable<TSource> source2, IEqualityComparer<TSource> comparer);
2) 描述
取两个数据集的差集（在数据集A出现的记录，在数据集B没出现的记录），对应的 SQL 语句是 EXCEPT。
3) 示例1：获取没有考试过的科目Id
1. C# 代码
var query = (from t1 in DbContext.Subject
select t1.SubjectId
).Except(
from t1 in DbContext.Score
select t1.SubjectId
);
var result = query.ToList();
2. 生成SQL
SELECT
[Except1].[SubjectId] AS [C1]
FROM (SELECT
[Extent1].[SubjectId] AS [SubjectId]
FROM [dbo].[Subject] AS [Extent1]
EXCEPT
SELECT
[Extent2].[SubjectId] AS [SubjectId]
FROM [dbo].[Score] AS [Extent2]) AS [Except1]
3. 执行结果
? 总结
简单测试，以备不时之需。