FreeSql （二十四）Linq To Sql 语法使用介绍

原本不支持 IQueryable 主要出于使用习惯的考虑，如果继承 IQueryable，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：

原以为必须实现 IQueryable 才可以实现，结果一次惊喜，原来只要有对应的方法就成。

虽然支持了，但是还是推荐使用【链式 + lambda】！！！

特别说明

这次功能更新，ISelect 增加了 5个方法，对【链式 + lambda】的用户可能会造成少许影响，我在注释上标明了，如下图：

特别是 .Select()，原先没有支持，该功能与 ToList(a => new Dto{}) 合并实现的。

需要避免一下坑：

如果一定要使用 .Select() 方法，请务必在 .ToList() 之前调用它；
请减少图中方法在【链式 + labmda】模式下的使用；

所有 ISelect 都可以使用 linq to sql，包括 Repository、DbContext；

Where

var t1 = (from a in fsql.Select<Student>()where a.id == item.idselect a
).ToList();

Select(指定字段)

var t1 = (from a in fsql.Select<Student>()where a.id == item.idselect new { a.id }
).ToList();

CaseWhen

var t1 = (from a in fsql.Select<Student>()where a.id == item.idselect new {a.id,a.name,testsub = new {time = a.age > 10 ? "大于" : "小于或等于"}}
).ToList();

Join

var t1 = (from a in fsql.Select<Student>()join b in fsql.Select<School>() on a.id equals b.StudentIdselect a
).ToList();var t2 = (from a in fsql.Select<Student>()join b in fsql.Select<School>() on a.id equals b.StudentIdselect 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.StudentIdwhere a.id == item.idselect new { a.id, bid = b.id }
).ToList();

LeftJoin

var t1 = (from a in fsql.Select<Student>()join b in fsql.Select<School>() on a.id equals b.StudentId into tempfrom 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 tempfrom 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 tempfrom tc in temp.DefaultIfEmpty()where a.id == item.idselect new { a.id, bid = tc.id }
).ToList();

From(多表查询)

var t1 = (from a in fsql.Select<Student>()from b in fsql.Select<School>()where a.id == b.StudentIdselect a
).ToList();var t2 = (from a in fsql.Select<Student>()from b in fsql.Select<School>()where a.id == b.StudentIdselect 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.StudentIdwhere a.id == item.idselect new { a.id, bid = b.id }
).ToList();

GroupBy(分组)

var t1 = (from a in fsql.Select<Student>()where a.id == item.idgroup a by new {a.id, a.name } into gselect 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();