数据库多表查询

　　string strSql = "select e.MaterialEnterID,e.MaterialID,e.Num,e.EnterDate,p.ParentComName " +
             　　      "from t_MaterialEnter e left join t_ParentCompany p on e.ParentComID = p.ParentComID";

　　string strSql = "select ml.MaterialLibID,p.ParentComName,m.MaterialName,m.MaterialType,ml.TotalNum " +
                            "from (t_MaterialLib ml left join t_ParentCompany p on ml.ParentComID = p.ParentComID) " +
                            "left join t_Material m on ml.MaterialID = m.MaterialID";

　　string strSql = "select e.MaterialEnterID,p.ParentComName,m.MaterialName,m.MaterialType,prod.ProductName,prod.ProductType,e.Num,e.EnterDate " +
                            "from ((t_MaterialEnter e left join t_ParentCompany p on e.ParentComID = p.ParentComID) " +
                            "left join t_Material m on e.MaterialID = m.MaterialID) " +
                            "left join t_Product prod on e.ProductID = prod.ProductID";

注意：

（1）、left join on 关键字

（2）、空格

（3）、有的数据库产品需要括号，有的没必要