/// <summary>
/// Determine the sum of a column for records that match the supplied filter.
/// </summary>
/// <param name="tableName">The name of the table.</param>
/// <param name="fieldName">The name of the field.</param>
/// <param name="filter">Expression.</param>
/// <returns>The sum of the specified column from the matching rows.</returns>
public decimal Sum(string tableName, string fieldName, Expression filter)
{
if (String.IsNullOrEmpty(tableName))
{
throw new ArgumentNullException(nameof(tableName));
}
if (String.IsNullOrEmpty(fieldName))
{
throw new ArgumentNullException(nameof(fieldName));
}
DataTable result = Query(PostgresqlHelper.SumQuery(tableName, fieldName, _SumColumnName, filter));
if (result != null &&
result.Rows.Count > 0 &&
result.Rows[0].Table.Columns.Contains(_SumColumnName) &&
result.Rows[0][_SumColumnName] != null &&
result.Rows[0][_SumColumnName] != DBNull.Value)
{
return(Convert.ToDecimal(result.Rows[0][_SumColumnName]));
}
return(0m);
}
