Created Linq to Sql (markdown)

Linq-to-Sql.md

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+[中文](LinqToSql) | **English**
+
+原本不支持 IQueryable 主要出于使用习惯的考虑，编写代码的智能总会提示出现一堆你不想使用的方法（对不起，我有强迫症），IQueryable 自身提供了一堆没法实现的方法，还有外部入侵的扩展方法，严重影响编码体验。如下图：
+
+![image](https://user-images.githubusercontent.com/16286519/57295126-5dd7bd00-70fc-11e9-99c0-d1c46423afa2.png)
+
+v1.4.0+ 版本请使用以下命令安装（老版本不需要安装）：
+
+> dotnet add package FreeSql.Extensions.Linq
+
+## 特别说明
+
+* 请尽量不要在 ISelect 模式下的使用 Linq 方法：GroupJoin、Select、SelectMany、Join、DefaultIfEmpty；
+
+* 如果一定要在 ISelect 中使用 .Select() 方法，请务必在 .ToList() 之前调用它；
+
+## IQueryable
+
+FreeSql 提供强大的数据查询对象 ISelect。
+
+FreeSql.Extensions.Linq v1.4.0+ 实现了 IQueryable 查询对象常用功能，以便在各框架中交互使用。
+
+```csharp
+//将 ISelect 转为 IQueryable
+IQueryable<Student> queryable = fsql.Select<Student>().AsQueryable();
+
+//Linq 方法查询
+var t1 = queryable.Where(a => a.id == 1).FirstOrDefault();
+
+//将 IQueryable 还原为 ISelect
+ISelect<Studeng> select = queryable.RestoreToSelect();
+```
+
+注意：IQueryable 的实现目前不支持 GroupBy，可以考虑使用 RestoreSelect 方法转回 ISelect 进行查询
+
+## Where
+```csharp
+var t1 = (
+    from a in fsql.Select<Student>()
+    where a.id == item.id
+    select a
+).ToList();
+```
+
+## Specify Fields: Select
+```csharp
+var t1 = (
+    from a in fsql.Select<Student>()
+    where a.id == item.id
+    select new { a.id }
+).ToList();
+```
+
+## CaseWhen
+```csharp
+var t1 = (
+    from a in fsql.Select<Student>()
+    where a.id == item.id
+    select new {
+        a.id,
+        a.name,
+        testsub = new {
+            time = a.age > 10 ? "大于" : "小于或等于"
+        }
+    }
+).ToList();
+```
+
+## Join
+```csharp
+var t1 = (
+    from a in fsql.Select<Student>()
+    join b in fsql.Select<School>() on a.id equals b.StudentId
+    select a
+).ToList();
+
+var t2 = (
+    from a in fsql.Select<Student>()
+    join b in fsql.Select<School>() on a.id equals b.StudentId
+    select new { a.id, bid = b.id }
+).ToList();
+
+var t3 = (
+    from a in fsql.Select<Student>()
+    join b in fsql.Select<School>() on a.id equals b.StudentId
+    where a.id == item.id
+    select new { a.id, bid = b.id }
+).ToList();
+```
+
+## LeftJoin
+```csharp
+var t1 = (
+    from a in fsql.Select<Student>()
+    join b in fsql.Select<School>() on a.id equals b.StudentId into temp
+    from tc in temp.DefaultIfEmpty()
+    select a
+).ToList();
+
+var t2 = (
+    from a in fsql.Select<Student>()
+    join b in fsql.Select<School>() on a.id equals b.StudentId into temp
+    from tc in temp.DefaultIfEmpty()
+    select new { a.id, bid = tc.id }
+).ToList();
+
+var t3 = (
+    from a in fsql.Select<Student>()
+    join b in fsql.Select<School>() on a.id equals b.StudentId into temp
+    from tc in temp.DefaultIfEmpty()
+    where a.id == item.id
+    select new { a.id, bid = tc.id }
+).ToList();
+```
+
+## Multi-table Query: From
+```csharp
+var t1 = (
+    from a in fsql.Select<Student>()
+    from b in fsql.Select<School>()
+    where a.id == b.StudentId
+    select a
+).ToList();
+
+var t2 = (
+    from a in fsql.Select<Student>()
+    from b in fsql.Select<School>()
+    where a.id == b.StudentId
+    select new { a.id, bid = b.id }
+).ToList();
+
+var t3 = (
+    from a in fsql.Select<Student>()
+    from b in fsql.Select<School>()
+    where a.id == b.StudentId
+    where a.id == item.id
+    select new { a.id, bid = b.id }
+).ToList();
+```
+
+## Grouping: GroupBy
+
+```csharp
+var t1 = (
+    from a in fsql.Select<Student>()
+    where a.id == item.id
+    group a by new {a.id, a.name } into g
+    select new {
+        g.Key.id, g.Key.name,
+        cou = g.Count(),
+        avg = g.Avg(g.Value.age),
+        sum = g.Sum(g.Value.age),
+        max = g.Max(g.Value.age),
+        min = g.Min(g.Value.age)
+    }
+).ToList();
+```
+
+## Reference
+
+- [《Query from Multi Tables》](Query-from-Multi-Tablea)
+- [《Return Data》](Return-Data)
+- [《Repository Layer》](Repository-Layer)
+- [《FreeSql Optimization: Lazy Loading》](Lazy-Loading)
+- [《FreeSql Optimization: Greed Loading》](Greed-Loading)
+- [《Expression Function》](Expression-Function)