Linq和EF 做 单一条件查询 和 复合条件 查询 以及 多表 联合查询 示例

var table2Object = (from t1 in db.table1
             join t2 in db.table2 on t1.id equals t2.id         
             select t2).FirstOrDefault();

var table2Object = (from t1 in db.table1
             join t2 in db.table2
             on new { t1.id, t1.another_id } equals new { t2.id, t2.another_id }
             select t2).FirstOrDefault();

var table2Object = (from t1 in db.table1
             join t2 in db.table2
             on new { t1.id, t1.another_id } equals new { t2.id, t2.another_id }
             join t3 in db.table3
             on new { t2.id, t2.another_id } equals new { t3.id, t3.another_id }
             select t2).FirstOrDefault();