DataTable Linq 左连接

（一）Linq左连接案例分析

有如下两个DataTable：localDt和schemaDt

要实现这两个DataTable的左连接，得到如下效果：

分析一：

var temp1 = (from schema in schemaDt.AsEnumerable()
             join local in localDt.AsEnumerable() 
             on schema.Field<string>("name") equals local.Field<string>("sql_name")
             into tempTable
             from item in tempTable.DefaultIfEmpty<DataRow>()
             select item).ToList();

分析二

var temp2 = (from local in localDt.AsEnumerable()
             join schema in schemaDt.AsEnumerable() 
             on local.Field<string>("sql_name") equals schema.Field<string>("name")
             into tempTable
             from item in tempTable.DefaultIfEmpty()
             select item).ToList();

分析三：

var temp3 = (from schema in schemaDt.AsEnumerable()
             join local in localDt.AsEnumerable() on schema.Field<string>("name") equals local.Field<string>("sql_name")
             into tempTable
             from item in tempTable.DefaultIfEmpty<DataRow>(localDt.NewRow())
             select item).ToList();

分析四：

var temp4 = (from schema in schemaDt.AsEnumerable()
            join local in localDt.AsEnumerable() on schema.Field<string>("name") equals local.Field<string>("sql_name")
            into tempTable
            from item in tempTable.DefaultIfEmpty<DataRow>(schemaDt.NewRow())
            select item).ToList();