我的项目模型是数据库优先,并使用远程访问另一台服务器上的数据库。我需要使用原始SQL查询,因为我的查询非常复杂,我觉得SQL比LINQ更舒服。
我是这样做的:
string query = "select * from Inquiry_TBL where ...";
using (educationEntities db = new educationEntities())
{
var list = db.Database.SqlQuery<Inquiry_TBL>(query);
ViewData["total"] = list.Count();
}
问题是有时我得到查询结果在一秒钟内,有时它只是保持加载很长一段时间,并给我一个错误,'调用'读取'当数据阅读器关闭不是一个有效的操作。'
为什么?是我的代码有问题,还是因为我使用远程访问另一台服务器?切换到本地服务器能解决问题吗?
实体框架代码优先API包括使您能够将SQL命令直接传递给数据库的方法。您有以下选项:
•使用DbSet。返回实体类型查询的SqlQuery方法。返回的对象必须是DbSet对象所期望的类型,除非关闭跟踪,否则它们将由数据库上下文自动跟踪。(参见下面关于AsNoTracking方法的部分)
•使用数据库。SqlQuery方法,用于返回非实体类型的查询。返回的数据不被数据库上下文跟踪,即使使用此方法检索实体类型也是如此。
•使用数据库。ExecuteSqlCommand用于非查询命令。
调用查询返回实体:
public async Task<ActionResult> Details(int? id)
{
if (id == null)
{
return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
}
// Commenting out original code to show how to use a raw SQL query.
//Department department = await db.Departments.FindAsync(id);
// Create and execute raw SQL query.
string query = "SELECT * FROM Department WHERE DepartmentID = @p0";
Department department = await db.Departments.SqlQuery(query, id).SingleOrDefaultAsync();
if (department == null)
{
return HttpNotFound();
}
return View(department);
}
调用返回其他类型对象的查询:
public ActionResult About()
{
//Commenting out LINQ to show how to do the same thing in SQL.
//IQueryable<EnrollmentDateGroup> = from student in db.Students
// group student by student.EnrollmentDate into dateGroup
// select new EnrollmentDateGroup()
// {
// EnrollmentDate = dateGroup.Key,
// StudentCount = dateGroup.Count()
// };
// SQL version of the above LINQ code.
string query = "SELECT EnrollmentDate, COUNT(*) AS StudentCount "
+ "FROM Person "
+ "WHERE Discriminator = 'Student' "
+ "GROUP BY EnrollmentDate";
IEnumerable<EnrollmentDateGroup> data = db.Database.SqlQuery<EnrollmentDateGroup>(query);
return View(data.ToList());
}
调用Update查询:
[HttpPost]
public ActionResult UpdateCourseCredits(int? credit)
{
if (credit != null)
{
ViewBag.RowsAffected = db.Database.ExecuteSqlCommand(
"UPDATE Course SET Credits = Credits * {0}", credit);
}
return View();
}
有关更多信息,请查看MVC 5 Web应用程序的高级实体框架6场景(12/12)。