数据表与 linq 表达式的左外连接
本文关键字:连接 表达式 linq 数据表 | 更新日期: 2023-09-27 18:31:40
我有两个DataTable
:
DataTable dtFields = new DataTable("tmpFieldTable");
dtFields.Columns.Add("FieldID");
dtFields.Columns.Add("CDGroupID");
dtFields.Columns.Add("CDCaption");
dtFields.Columns.Add("fldIndex");
和
DataTable dtCDGroup = new DataTable("tmpCDGroup");
dtCDGroup.Columns.Add("CDGroupID");
dtCDGroup.Columns.Add("Name");
dtCDGroup.Columns.Add("Priority");
我使用以下 LINQ 表达式来连接这些表:
var resultList = dtFields.AsEnumerable()
.Join(dtCDGroup.AsEnumerable(),
fieldList => fieldList.Field<string>("CDGroupID"),
cd => cd.Field<string>("CDGroupID"),
(fieldList, cd) => new
{
FieldID = fieldList.Field<string>("FieldID"),
CdGroup = cd.Field<string>("Name"),
CDCaption = fieldList.Field<string>("CDCaption"),
Priority = (cd.Field<string>("Priority") ?? "99"),
fldIndex = fieldList.Field<string>("fldIndex").ToString()
})
.OrderBy(result => result.Priority)
.ThenBy(result => result.fldIndex);
如何使用这些DataTable
执行左外连接?
左连接本身不是使用 lambda 表达式提供的。 但您可以使用 GroupJoin 和 SelectMany 方法实现此目的。
var result = new string[] { "A", "B" }.GroupJoin(new string[] { "B" }, l => l, r => r, (l, r) => new { l, r }).ToList();
Console.WriteLine(result.Count());
这将导致结果 result[0].r.Count() = 0 和 result[1].r.Count() = 1。
希望这会有所帮助。
问候。
只需将DefaultIfEmpty()
添加到表中:
resultList = dtFields.AsEnumerable()
.Join(dtCDGroup.AsEnumerable().DefaultIfEmpty(),
fieldList => fieldList.Field<string>("CDGroupID"),
cd => cd.Field<string>("CDGroupID"),
(fieldList, cd) => new
{
FieldID = fieldList.Field<string>("FieldID"),
CdGroup = cd.Field<string>("Name"),
CDCaption = fieldList.Field<string>("CDCaption"),
Priority = (cd.Field<string>("Priority") ?? "99"),
fldIndex = fieldList.Field<string>("fldIndex").ToString()
})
.OrderBy(result => result.Priority)
.ThenBy(result => result.fldIndex);