How to Efficiently Merge Multiple DataTables with Varying Column Structures in SQL Server?

Combining Multiple DataTables into a Single DataTable

When working with data in SQL Server, it's sometimes necessary to combine data from multiple tables into a single, unified dataset. This can be achieved using Merge operations, which allow you to append rows from different tables into a target table. However, if the tables have different column structures, the resulting merged table may have padding or misaligned data.

Existing Solution and Limitations

The provided solution attempts to solve this issue by using a Merge loop to combine DataTables from multiple unknown tables with varying column structures. While this method works, it can be tedious and may result in data misalignment.

Alternative Approach Using LINQ

To overcome these limitations, an alternative approach using LINQ (Language Integrated Query) is available:

public static DataTable MergeAll(this IList<DataTable> tables, String primaryKeyColumn)
{
    if (!tables.Any())
        throw new ArgumentException("Tables must not be empty", "tables");
    if(primaryKeyColumn != null)
        foreach(DataTable t in tables)
            if(!t.Columns.Contains(primaryKeyColumn))
                throw new ArgumentException("All tables must have the specified primarykey column " + primaryKeyColumn, "primaryKeyColumn");

    if(tables.Count == 1)
        return tables[0];

    DataTable table = new DataTable("TblUnion");
    table.BeginLoadData(); // Turns off notifications, index maintenance, and constraints while loading data
    foreach (DataTable t in tables)
    {
        table.Merge(t); // same as table.Merge(t, false, MissingSchemaAction.Add);
    }
    table.EndLoadData();

    if (primaryKeyColumn != null)
    {
        // since we might have no real primary keys defined, the rows now might have repeating fields
        // so now we're going to "join" these rows ...
        var pkGroups = table.AsEnumerable()
            .GroupBy(r => r[primaryKeyColumn]);
        var dupGroups = pkGroups.Where(g => g.Count() > 1);
        foreach (var grpDup in dupGroups)
        { 
            // use first row and modify it
            DataRow firstRow = grpDup.First();
            foreach (DataColumn c in table.Columns)
            {
                if (firstRow.IsNull(c))
                {
                    DataRow firstNotNullRow = grpDup.Skip(1).FirstOrDefault(r => !r.IsNull(c));
                    if (firstNotNullRow != null)
                        firstRow[c] = firstNotNullRow[c];
                }
            }
            // remove all but first row
            var rowsToRemove = grpDup.Skip(1);
            foreach(DataRow rowToRemove in rowsToRemove)
                table.Rows.Remove(rowToRemove);
        }
    }

    return table;
}

Usage

To use the MergeAll method, pass a list of DataTables and optionally specify a common primary key column name:

var tables = new[] { tblA, tblB, tblC };
DataTable TblUnion = tables.MergeAll("c1");

Advantages of LINQ Approach

• Simplifies the merging process by eliminating the need for manual loops.
• Allows for optional specification of a primary key to merge rows when columns differ.
• Automatically combines and adjusts data values for repeating primary key rows.

Note: This solution requires that all tables have a unique column name for the primary key, if specified.

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