C# 如何在c中左外连接两个数据表？

我认为这是左外连接，但我不能100%确定两个数据表都有以下表和条件，同时保留两个表中的所有列

dtblLeft：

 id   col1   anotherColumn2
 1    1      any2
 2    1      any2
 3    2      any2
 4    3      any2
 5    3      any2
 6    3      any2
 7           any2

dtblRight：

 col1   col2      anotherColumn1
 1      Hi        any1
 2      Bye       any1
 3      Later     any1
 4      Never     any1

DTBL已加入：

 id   col1  col2     anotherColumn1     anotherColumn2
 1    1     Hi       any1               any2
 2    1     Hi       any1               any2
 3    2     Bye      any1               any2
 4    3     Later    any1               any2
 5    3     Later    any1               any2
 6    3     Later    any1               any2
 7                                      any2

条件：

在dtblLeft中，col1不需要具有唯一值。 在dtblRight中，col1具有唯一的值。 如果dtblLeft在col1中缺少外键，或者它有一个在dtblRight中不存在的外键，则将插入空字段或null字段。 加入col1。 我可以使用常规的DataTable操作、LINQ或任何东西

我尝试了这个，但它删除了重复项：

dtblA.PrimaryKey = new DataColumn[] {dtblA.Columns["col1"]}

DataTable dtblJoined = new DataTable();
dtblJoined.Merge(dtblA, false, MissingSchemaAction.AddWithKey);
dtblJoined.Merge(dtblB, false, MissingSchemaAction.AddWithKey);

编辑1：

这与我想要的很接近，但它只包含位于以下位置的其中一个表中的列：

编辑2：

这个问题的答案似乎对我有用，但我不得不改变一下，如下所示：

DataTable targetTable = dtblA.Clone();
var dt2Columns = dtblB.Columns.OfType<DataColumn>().Select(dc =>
new DataColumn(dc.ColumnName, dc.DataType, dc.Expression, dc.ColumnMapping));
var dt2FinalColumns = from dc in dt2Columns.AsEnumerable()
                   where targetTable.Columns.Contains(dc.ColumnName) == false
                   select dc;

targetTable.Columns.AddRange(dt2FinalColumns.ToArray());

var rowData = from row1 in dtblA.AsEnumerable()
                          join row2 in dtblB.AsEnumerable()
                          on row1["col1"] equals row2["col1"]
                          select row1.ItemArray.Concat(row2.ItemArray.Where(r2 => row1.ItemArray.Contains(r2) == false)).ToArray();

 foreach (object[] values in rowData)
      targetTable.Rows.Add(values);

我还发现了这个，我可能会尝试一下，因为它看起来更简洁

编辑3 2013年11月18日：

---

更新表以反映更多情况。

您可能可以使用LINQ并执行以下操作：

var dtblJoined = from dB in dtblB.AsEnumerable()
                 join dA in dtblA.AsEnumerable() on dA.col1 equals dB.col1 into dAB
                 from d in dAB.DefaultIfEmpty()
                 select new (col1 = dB.col1, ; col2 = (dB.col1 == dA.col1) ? dA.col2 : null);

---

这将返回一个IEnumerable作为结果，而不是一个DataTable，但我认为它应该让您更接近您所寻找的内容。不过可能需要稍微调整一下

这只是两个表之间的内部联接：

var query = (from x in a.AsEnumerable()
              join y in b.AsEnumerable() on x.Field<int>("col1") equals y.Field<int>("col1")
              select new { col1= y.Field<int>("col1"), col2=x.Field<int>("col2") }).ToList();

---

谢谢大家的帮助。以下是我根据多种资源得出的结论：

public static class DataTableHelper
{
    public enum JoinType
    {
        /// <summary>
        /// Same as regular join. Inner join produces only the set of records that match in both Table A and Table B.
        /// </summary>
        Inner = 0,
        /// <summary>
        /// Same as Left Outer join. Left outer join produces a complete set of records from Table A, with the matching records (where available) in Table B. If there is no match, the right side will contain null.
        /// </summary>
        Left = 1
    }

    /// <summary>
    /// Joins the passed in DataTables on the colToJoinOn.
    /// <para>Returns an appropriate DataTable with zero rows if the colToJoinOn does not exist in both tables.</para>
    /// </summary>
    /// <param name="dtblLeft"></param>
    /// <param name="dtblRight"></param>
    /// <param name="colToJoinOn"></param>
    /// <param name="joinType"></param>
    /// <returns></returns>
    /// <remarks>
    /// <para>http://stackoverflow.com/questions/2379747/create-combined-datatable-from-two-datatables-joined-with-linq-c-sharp?rq=1</para>
    /// <para>http://msdn.microsoft.com/en-us/library/vstudio/bb397895.aspx</para>
    /// <para>http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html</para>
    /// <para>http://stackoverflow.com/questions/406294/left-join-and-left-outer-join-in-sql-server</para>
    /// </remarks>
    public static DataTable JoinTwoDataTablesOnOneColumn(DataTable dtblLeft, DataTable dtblRight, string colToJoinOn, JoinType joinType)
    {
        //Change column name to a temp name so the LINQ for getting row data will work properly.
        string strTempColName = colToJoinOn + "_2";
        if (dtblRight.Columns.Contains(colToJoinOn))
            dtblRight.Columns[colToJoinOn].ColumnName = strTempColName;

        //Get columns from dtblLeft
        DataTable dtblResult = dtblLeft.Clone();

        //Get columns from dtblRight
        var dt2Columns = dtblRight.Columns.OfType<DataColumn>().Select(dc => new DataColumn(dc.ColumnName, dc.DataType, dc.Expression, dc.ColumnMapping));

        //Get columns from dtblRight that are not in dtblLeft
        var dt2FinalColumns = from dc in dt2Columns.AsEnumerable()
                              where !dtblResult.Columns.Contains(dc.ColumnName)
                              select dc;

        //Add the rest of the columns to dtblResult
        dtblResult.Columns.AddRange(dt2FinalColumns.ToArray());

        //No reason to continue if the colToJoinOn does not exist in both DataTables.
        if (!dtblLeft.Columns.Contains(colToJoinOn) || (!dtblRight.Columns.Contains(colToJoinOn) && !dtblRight.Columns.Contains(strTempColName)))
        {
            if (!dtblResult.Columns.Contains(colToJoinOn))
                dtblResult.Columns.Add(colToJoinOn);
            return dtblResult;
        }

        switch (joinType)
        {

            default:
            case JoinType.Inner:
                #region Inner
                //get row data
                //To use the DataTable.AsEnumerable() extension method you need to add a reference to the System.Data.DataSetExtension assembly in your project. 
                var rowDataLeftInner = from rowLeft in dtblLeft.AsEnumerable()
                                       join rowRight in dtblRight.AsEnumerable() on rowLeft[colToJoinOn] equals rowRight[strTempColName]
                                       select rowLeft.ItemArray.Concat(rowRight.ItemArray).ToArray();


                //Add row data to dtblResult
                foreach (object[] values in rowDataLeftInner)
                    dtblResult.Rows.Add(values);

                #endregion
                break;
            case JoinType.Left:
                #region Left
                var rowDataLeftOuter = from rowLeft in dtblLeft.AsEnumerable()
                                       join rowRight in dtblRight.AsEnumerable() on rowLeft[colToJoinOn] equals rowRight[strTempColName] into gj
                                       from subRight in gj.DefaultIfEmpty()
                                       select rowLeft.ItemArray.Concat((subRight== null) ? (dtblRight.NewRow().ItemArray) :subRight.ItemArray).ToArray();


                //Add row data to dtblResult
                foreach (object[] values in rowDataLeftOuter)
                    dtblResult.Rows.Add(values);

                #endregion
                break;
        }

        //Change column name back to original
        dtblRight.Columns[strTempColName].ColumnName = colToJoinOn;

        //Remove extra column from result
        dtblResult.Columns.Remove(strTempColName);

        return dtblResult;
    }
}

编辑3：

该方法现在可以正常工作，并且在表有2000多行时仍然很快。如有任何建议/建议/改进，将不胜感激

编辑4：

---

我有一个特定的场景，使我意识到前一个版本实际上是在进行内部连接。已修改该函数以修复该问题。我在这里使用了信息来解决这个问题。

这也是自然加入。请告诉我们当dtblA有“4”而dtblB没有时应该发生什么。以防您错过了SO上关于同一问题的其他帖子，已更新以显示dtblA4@adam-c此代码已从两个表中获取唯一的列。

col1 col2
1    Hi 
1    Hi 
2    Bye 
3    Later 
3    Later 
3    Later 

public static class DataTableHelper
{
    public enum JoinType
    {
        /// <summary>
        /// Same as regular join. Inner join produces only the set of records that match in both Table A and Table B.
        /// </summary>
        Inner = 0,
        /// <summary>
        /// Same as Left Outer join. Left outer join produces a complete set of records from Table A, with the matching records (where available) in Table B. If there is no match, the right side will contain null.
        /// </summary>
        Left = 1
    }

    /// <summary>
    /// Joins the passed in DataTables on the colToJoinOn.
    /// <para>Returns an appropriate DataTable with zero rows if the colToJoinOn does not exist in both tables.</para>
    /// </summary>
    /// <param name="dtblLeft"></param>
    /// <param name="dtblRight"></param>
    /// <param name="colToJoinOn"></param>
    /// <param name="joinType"></param>
    /// <returns></returns>
    /// <remarks>
    /// <para>http://stackoverflow.com/questions/2379747/create-combined-datatable-from-two-datatables-joined-with-linq-c-sharp?rq=1</para>
    /// <para>http://msdn.microsoft.com/en-us/library/vstudio/bb397895.aspx</para>
    /// <para>http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html</para>
    /// <para>http://stackoverflow.com/questions/406294/left-join-and-left-outer-join-in-sql-server</para>
    /// </remarks>
    public static DataTable JoinTwoDataTablesOnOneColumn(DataTable dtblLeft, DataTable dtblRight, string colToJoinOn, JoinType joinType)
    {
        //Change column name to a temp name so the LINQ for getting row data will work properly.
        string strTempColName = colToJoinOn + "_2";
        if (dtblRight.Columns.Contains(colToJoinOn))
            dtblRight.Columns[colToJoinOn].ColumnName = strTempColName;

        //Get columns from dtblLeft
        DataTable dtblResult = dtblLeft.Clone();

        //Get columns from dtblRight
        var dt2Columns = dtblRight.Columns.OfType<DataColumn>().Select(dc => new DataColumn(dc.ColumnName, dc.DataType, dc.Expression, dc.ColumnMapping));

        //Get columns from dtblRight that are not in dtblLeft
        var dt2FinalColumns = from dc in dt2Columns.AsEnumerable()
                              where !dtblResult.Columns.Contains(dc.ColumnName)
                              select dc;

        //Add the rest of the columns to dtblResult
        dtblResult.Columns.AddRange(dt2FinalColumns.ToArray());

        //No reason to continue if the colToJoinOn does not exist in both DataTables.
        if (!dtblLeft.Columns.Contains(colToJoinOn) || (!dtblRight.Columns.Contains(colToJoinOn) && !dtblRight.Columns.Contains(strTempColName)))
        {
            if (!dtblResult.Columns.Contains(colToJoinOn))
                dtblResult.Columns.Add(colToJoinOn);
            return dtblResult;
        }

        switch (joinType)
        {

            default:
            case JoinType.Inner:
                #region Inner
                //get row data
                //To use the DataTable.AsEnumerable() extension method you need to add a reference to the System.Data.DataSetExtension assembly in your project. 
                var rowDataLeftInner = from rowLeft in dtblLeft.AsEnumerable()
                                       join rowRight in dtblRight.AsEnumerable() on rowLeft[colToJoinOn] equals rowRight[strTempColName]
                                       select rowLeft.ItemArray.Concat(rowRight.ItemArray).ToArray();


                //Add row data to dtblResult
                foreach (object[] values in rowDataLeftInner)
                    dtblResult.Rows.Add(values);

                #endregion
                break;
            case JoinType.Left:
                #region Left
                var rowDataLeftOuter = from rowLeft in dtblLeft.AsEnumerable()
                                       join rowRight in dtblRight.AsEnumerable() on rowLeft[colToJoinOn] equals rowRight[strTempColName] into gj
                                       from subRight in gj.DefaultIfEmpty()
                                       select rowLeft.ItemArray.Concat((subRight== null) ? (dtblRight.NewRow().ItemArray) :subRight.ItemArray).ToArray();


                //Add row data to dtblResult
                foreach (object[] values in rowDataLeftOuter)
                    dtblResult.Rows.Add(values);

                #endregion
                break;
        }

        //Change column name back to original
        dtblRight.Columns[strTempColName].ColumnName = colToJoinOn;

        //Remove extra column from result
        dtblResult.Columns.Remove(strTempColName);

        return dtblResult;
    }
}