.NET 中 Entity Framework 的数据库查询

Entity Framework 支持三种类型的数据库查询。

• LINQ to Entities
• Entity SQL
• Native SQL

其中，LINQ（音：link）是较为推荐的方式。它又分为 LINQ Method syntax 与 LINQ Query syntax，如下所示。

// LINQ Method syntax
using (var context = new CompanyDBEntities()) 
{ 
    var L2EQuery = context.Employees.where(e => e.EmployeeName== "Ben"); 
    var employee= L2EQuery.FirstOrDefault(); 
} 

// LINQ Query syntax
using (var context = new CompanyDBEntities()) 
{ 
    var L2EQuery = from ep in context.Employees    
                   where ep.EmployeeName == "Bill" 
                   select ep; 
     var employee = L2EQuery.FirstOrDefault(); 
} 

Entity SQL 与 Native SQL 都是基于拼接 SQL 语句的方式，如下所示。

// Entity SQL
string sqlString = "SELECT VALUE ep FROM CompanyDBEntities.Employees " + "AS ep WHERE ep.EmployeeName == 'Ben'"; 
var objctx = (ctx as IObjectContextAdapter).ObjectContext; 
ObjectQuery employee = objctx.CreateQuery(sqlString); 
Employee newEmployee = employee.First(); 

// Native SQL
using (var ctx = new CompanyDBEntities()) 
{ 
    var employeeName = ctx.Employees.SqlQuery("Select EmployeeId, EmployeeName from Employee where EmployeeName='Ben'").FirstOrDefault(); 
} 

此外，Entity Framework 提供了丰富的函数，应了解并掌握用法。以下列举一些。

• Aggregate*
• Average
• All
• Any
• ElementAt/ElementAtOrDefault*
• Single/SingleOrDefault*
• Contains
• First/FirstOrDefault/Last/LastOrDefault*
• Count
• Max/Min*
• OrderBy/OrderByDescending
• Reverse
• SelectMany
• SequenceEqual
• Skip/SkipWhile
• Take/TakeWhile

其中以星号标记的将返回 Entity 实体。