ADO.NET 1.x对于DataTable虽然公开了Select方法,但是未能提供如SELECT DISTINCT的方法,而这样的功能往往在实际项目中是需要的。
下面是常用的几种方法:
方法一:(参见http://support.microsoft.com/kb/326176/zh-cn)
方法二:(参考http://weblogs.asp.net/eporter/archive/2005/02/10/370548.aspx)
ADO.NET 2.0 虽然仍然未提供对DISTINCT的支持,但DataView提供了几个重载版本的ToDataTable,可以简单对DISTINCT支持:
在微软.NET 3.0中,LINQ提供了对复杂查询的支持,有待学习、体验。
下面是常用的几种方法:
方法一:(参见http://support.microsoft.com/kb/326176/zh-cn)
Code
1. Add the following Private method to the class definition. This method is the same as the method that is used in other DataSetHelper articles. It is used to compare field values (including NULL).private bool ColumnEqual(object A, object B)
{
// Compares two values to see if they are equal. Also compares DBNULL.Value.
// Note: If your DataTable contains object fields, then you must extend this
// function to handle them in a meaningful way if you intend to group on them.
if ( A == DBNull.Value && B == DBNull.Value ) // both are DBNull.Value
return true;
if ( A == DBNull.Value || B == DBNull.Value ) // only one is DBNull.Value
return false;
return ( A.Equals(B) ); // value type standard comparison
}
2. Add the following Public method to the class definition. This method copies unique values of the field that you select into a new DataTable. If the field contains NULL values, a record in the destination table will also contain NULL values.public DataTable SelectDistinct(string TableName, DataTable SourceTable, string FieldName)
{
DataTable dt = new DataTable(TableName);
dt.Columns.Add(FieldName, SourceTable.Columns[FieldName].DataType);
object LastValue = null;
foreach (DataRow dr in SourceTable.Select("", FieldName))
{
if ( LastValue == null || !(ColumnEqual(LastValue, dr[FieldName])) )
{
LastValue = dr[FieldName];
dt.Rows.Add(new object[]{LastValue});
}
}
if (ds != null)
ds.Tables.Add(dt);
return dt;
}
1. Add the following Private method to the class definition. This method is the same as the method that is used in other DataSetHelper articles. It is used to compare field values (including NULL).private bool ColumnEqual(object A, object B)
{
// Compares two values to see if they are equal. Also compares DBNULL.Value.
// Note: If your DataTable contains object fields, then you must extend this
// function to handle them in a meaningful way if you intend to group on them.
if ( A == DBNull.Value && B == DBNull.Value ) // both are DBNull.Value
return true;
if ( A == DBNull.Value || B == DBNull.Value ) // only one is DBNull.Value
return false;
return ( A.Equals(B) ); // value type standard comparison
}
2. Add the following Public method to the class definition. This method copies unique values of the field that you select into a new DataTable. If the field contains NULL values, a record in the destination table will also contain NULL values.public DataTable SelectDistinct(string TableName, DataTable SourceTable, string FieldName)
{
DataTable dt = new DataTable(TableName);
dt.Columns.Add(FieldName, SourceTable.Columns[FieldName].DataType);
object LastValue = null;
foreach (DataRow dr in SourceTable.Select("", FieldName))
{
if ( LastValue == null || !(ColumnEqual(LastValue, dr[FieldName])) )
{
LastValue = dr[FieldName];
dt.Rows.Add(new object[]{LastValue});
}
}
if (ds != null)
ds.Tables.Add(dt);
return dt;
}
方法二:(参考http://weblogs.asp.net/eporter/archive/2005/02/10/370548.aspx)
Code
private static DataTable SelectDistinct(DataTable SourceTable, params string[] FieldNames)
{
object[] lastValues;
DataTable newTable;
DataRow[] orderedRows;
if (FieldNames == null || FieldNames.Length == 0)
throw new ArgumentNullException("FieldNames");
lastValues = new object[FieldNames.Length];
newTable = new DataTable();
foreach (string fieldName in FieldNames)
newTable.Columns.Add(fieldName, SourceTable.Columns[fieldName].DataType);
orderedRows = SourceTable.Select("", string.Join(", ", FieldNames));
foreach (DataRow row in orderedRows)
{
if (!fieldValuesAreEqual(lastValues, row, FieldNames))
{
newTable.Rows.Add(createRowClone(row, newTable.NewRow(), FieldNames));
setLastValues(lastValues, row, FieldNames);
}
}
return newTable;
}
private static bool fieldValuesAreEqual(object[] lastValues, DataRow currentRow, string[] fieldNames)
{
bool areEqual = true;
for (int i = 0; i < fieldNames.Length; i++)
{
if (lastValues[i] == null || !lastValues[i].Equals(currentRow[fieldNames[i]]))
{
areEqual = false;
break;
}
}
return areEqual;
}
private static DataRow createRowClone(DataRow sourceRow, DataRow newRow, string[] fieldNames)
{
foreach (string field in fieldNames)
newRow[field] = sourceRow[field];
return newRow;
}
private static void setLastValues(object[] lastValues, DataRow sourceRow, string[] fieldNames)
{
for (int i = 0; i < fieldNames.Length; i++)
lastValues[i] = sourceRow[fieldNames[i]];
}
private static DataTable SelectDistinct(DataTable SourceTable, params string[] FieldNames)
{
object[] lastValues;
DataTable newTable;
DataRow[] orderedRows;
if (FieldNames == null || FieldNames.Length == 0)
throw new ArgumentNullException("FieldNames");
lastValues = new object[FieldNames.Length];
newTable = new DataTable();
foreach (string fieldName in FieldNames)
newTable.Columns.Add(fieldName, SourceTable.Columns[fieldName].DataType);
orderedRows = SourceTable.Select("", string.Join(", ", FieldNames));
foreach (DataRow row in orderedRows)
{
if (!fieldValuesAreEqual(lastValues, row, FieldNames))
{
newTable.Rows.Add(createRowClone(row, newTable.NewRow(), FieldNames));
setLastValues(lastValues, row, FieldNames);
}
}
return newTable;
}
private static bool fieldValuesAreEqual(object[] lastValues, DataRow currentRow, string[] fieldNames)
{
bool areEqual = true;
for (int i = 0; i < fieldNames.Length; i++)
{
if (lastValues[i] == null || !lastValues[i].Equals(currentRow[fieldNames[i]]))
{
areEqual = false;
break;
}
}
return areEqual;
}
private static DataRow createRowClone(DataRow sourceRow, DataRow newRow, string[] fieldNames)
{
foreach (string field in fieldNames)
newRow[field] = sourceRow[field];
return newRow;
}
private static void setLastValues(object[] lastValues, DataRow sourceRow, string[] fieldNames)
{
for (int i = 0; i < fieldNames.Length; i++)
lastValues[i] = sourceRow[fieldNames[i]];
}
ADO.NET 2.0 虽然仍然未提供对DISTINCT的支持,但DataView提供了几个重载版本的ToDataTable,可以简单对DISTINCT支持:
Code
C#
public DataTable ToTable (
bool distinct,
params string[] columnNames
)
public DataTable ToTable (
string tableName,
bool distinct,
params string[] columnNames
)
参数
tableName
返回的 DataTable 的名称。
distinct
如果为 true,则返回的 DataTable 将包含所有列都具有不同值的行。默认值为 false。
columnNames
一个字符串数组,包含要包括在返回的 DataTable 中的列名的列表。DataTable 包含指定的列,其顺序与这些列在该数组中的顺序相同。
返回值
一个新的 DataTable 实例,其中包含所请求的行和列。
C#
public DataTable ToTable (
bool distinct,
params string[] columnNames
)
public DataTable ToTable (
string tableName,
bool distinct,
params string[] columnNames
)
参数
tableName
返回的 DataTable 的名称。
distinct
如果为 true,则返回的 DataTable 将包含所有列都具有不同值的行。默认值为 false。
columnNames
一个字符串数组,包含要包括在返回的 DataTable 中的列名的列表。DataTable 包含指定的列,其顺序与这些列在该数组中的顺序相同。
返回值
一个新的 DataTable 实例,其中包含所请求的行和列。
在微软.NET 3.0中,LINQ提供了对复杂查询的支持,有待学习、体验。