Here are some rules to remember for Entity SQL queries:
1.Use SELECT VALUE when projecting more than one type;
2.When querying with SELECT,the ObjectQuery type must be a DbDataRecord ;
3.You can use SELECT VALUE when projecting a single value or entity;
4.When querying with SELECT VALUE,the ObjectQuery type must be the same type as the value being returned.
用 Entity SQL 查询“包装”和“未包装” 结果的规则：
下面是几条用Entity SQL 查询需要记住的规则：
1、当投射的类型超过一个时，用 select value ;
2、当用 select 查询时，ObjectQuery 结果集类型必须为DbDataRecord类型；
3、当查询单个值或单个实体的时候，推荐使用 select value 查询；
4、当用 select value 进行查询时，ObjectQuery 类型必须和返回值类型保持一致；
Examples:
1、Wrapped Contact needs to be cast:
private static void WrappedContactInEntitySql() { using (var context = new PEF()) { var queryString = "select c from PEF.Contacts as c where c.FirstName =‘Robert‘ "; var contacts = context.CreateQuery<DbDataRecord>(queryString); foreach (DbDataRecord c in contacts) { var contact = (Contact)c[0]; Console.WriteLine("{0} {1} {2}", contact.Title.Trim(), contact.FirstName.Trim(), contact.LastName.Trim()); } } Console.WriteLine("Press Enter..."); Console.ReadLine(); }
2、Unwrapped Contact does not need to be cast:
private static void UnwrappedContactInEntitySql() { using (var context = new PEF()) { var queryString = "select value c from PEF.Contacts as c where c.FirstName = ‘Robert‘ "; var contacts = new ObjectQuery<Contact>(queryString, context); foreach (var contact in contacts) { Console.WriteLine("{0} {1} {2}", contact.Title.Trim(), contact.FirstName.Trim(), contact.LastName.Trim()); } } Console.WriteLine("Press Enter..."); Console.ReadLine(); }
Entity SQL rules for Wrapped and Unwrapped Results
原文：http://www.cnblogs.com/ARMdong/p/3516001.html