linq2db.EntityFrameworkCore 介绍

linq2db.EntityFrameworkCore 是一个ef core的插件，对linq语法的扩展

对复杂的sql都有很好的支持，他是基于linq2db （provided by LINQ To DB）

如果你使用了linq2db的语法扩展那么你必须使用下面的方法进行查询

 // ToLinqToDB是必须的
 var temp = qry.ToLinqToDB().ToList();

---

下面是 linq2db 的冰山一角

JOIN

1. InnerJoin

 var qry = from t1 in db.T
           from t2 in db.T2.InnerJoin(m => m.T1Id == t1.Id)

2.LeftJoin

 var qry = from t1 in db.T
           from t2 in db.T2.LeftJoin(m => m.T1Id == t1.Id)

3.RightJoin

 var qry = from t1 in db.T
           from t2 in db.T2.RightJoin(m => m.T1Id == t1.Id)

---

SUM

// 相比于原来linq，简洁了很多。
var qry = from t1 in db.T
          from t2 in db.T2.LeftJoin(m => m.T1Id == t1.Id)
          select Sql.Ext.Sum(t1.Type == "A" ? t1.Number * (t1.SalePrice-t2.OriginalPrice) : 0).ToValue();

CountExt

//我要查t2中不重复的 t1的Id有多少个
var qry = from t1 in db.T
          from t2 in db.T2.LeftJoin(m => m.T1Id == t1.Id)
          where t1.some==''
          group new {t1,t2} by t2.some into g
          select new 
          {
              //相当于sql Count(distinct t2.T1Id)
              Number = g.CountExt(m => m.t2.T1Id, Sql.AggregateModifier.Distinct)
          }

对于一些sql函数的支持

DatePart

var qry = from t1 in db.T
                    where t1.SaleDate > beginTime 
                    group t1 by Sql.DatePart(Sql.DateParts.Month, t1 .SaleDate) into g
                    select new
                    {
                        Month = g.Key,
                        FlowAmount = g.Sum(m => m.SaleWay == "A" ? Sql.Abs(m.Number * m.SalePrice) : 0) -
                                     g.Sum(m => m.SaleWay == "B" ? Sql.Abs(m.Number * m.SalePrice) : 0)
                    };

当然还有更多的扩展方法，分别位于
包含于 Sql ， Sql.Ext，AnalyticFunctions 中
linq2db文档 : https://linq2db.github.io/index.html

当然还有批量更新的操作

如果是需要使用，那么最好再程序开始时运行以下代码

//因为他是幂等的 ，所以可以多次运行
LinqToDBForEFTools.Initialize();

以下代码都是从github上抄下来的。

// fast insert big recordsets
ctx.BulkCopy(new BulkCopyOptions {...}, items);

// query for retrieving products that do not have duplicates by Name
var query =
	from p in ctx.Products
	from op in ctx.Products.LeftJoin(op => op.ProductID != p.ProductID && op.Name == p.Name)
	where Sql.ToNullable(op.ProductID) == null
	select p;

// insert these records into the same or another table
query.Insert(ctx.Products.ToLinqToDBTable(), s => new Product { Name = s.Name ... });

// update these records by changing name based on previous value
query.Update(prev => new Product { Name = "U_" + prev.Name ... });

// delete records that matched by query
query.Delete();