C# linq left join 操作表和datatable

https://blog.csdn.net/The_flying_pig/article/details/79568072

操作表：

现有sfcusn表，sfcmo表，为保证sfcusn表数据的完整使用left join 查询。结构如下

sql语句

---oracle写法
SELECT A.usn,A.mo,A.upn,A.stage,B.line FROM sfcusn A,sfcmo B WHERE A.mo = B.mo(+)

linq语句

var query= from A in db.sfcusn
join B in db.sfcmo on A.mo equals B.mo into temp
from t in temp.DefaultIfEmpty()
select new
{
    usn = A.usn,
    mo = A.mo,
    upn = A.upn,
    stage = A.stage,
    line = t==null?"":t.line //判断第二集合中可能为空
};

操作datatable：

现需对两个datatable进行left join查询，结构如下

这里需要用到A，B中三个栏位。

如果是查询表用sql写当然是很简单了，可以仿看上面的sql写出来。

linq在操作datatable，注意字段的类型。

var query = from aa in A.AsEnumerable()
         join bb in B.AsEnumerable()
         on new { USERID = aa.Field<string>("USERID"), TRNDATE = aa.Field<string>("TRNDATE"), MODEC = aa.Field<string>("MODEC") }
         equals new { USERID = bb.Field<string>("USERID"), TRNDATE = bb.Field<string>("TRNDATE"), MODEC = bb.Field<string>("MODEC") }
         into temp
         from tt in temp.DefaultIfEmpty()
         select new
         {
            USERID = aa.Field<string>("USERID"),
            TOTAL = tt == null ? 0 : tt.Field<decimal>("TOTAL"),
            ECOUNT = aa.Field<decimal>("ECOUNT") == null ? 0 : aa.Field<decimal>("ECOUNT"),
            RATE = (100 * (tt == null ? 1 : (tt.Field<decimal>("TOTAL") == 0 ? 1 : aa.Field<decimal>("ECOUNT") / tt.Field<decimal>("TOTAL")))).ToString("#0.00") + "%",
            TRNDATE = a.Field<string>("TRNDATE") == null ? "" : a.Field<string>("TRNDATE"),
            MODEC = aa.Field<string>("MODEC") == null ? "" : aa.Field<string>("MODEC")
        };