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GetSchemaTable Method

OdbcDataReader.GetSchemaTable Method

Note: This namespace, class, or member is supported only in version 1.1 of the .NET Framework.

Returns a DataTable that describes the column metadata of the OdbcDataReader.

[Visual Basic]
Public Overridable Function GetSchemaTable() As DataTable Implements _
   IDataReader.GetSchemaTable
[C#]
public virtual DataTable GetSchemaTable();
[C++]
public: virtual DataTable* GetSchemaTable();
[JScript]
public function GetSchemaTable() : DataTable;

Return Value

A DataTable that describes the column metadata.

Implements

IDataReader.GetSchemaTable

Exceptions

Exception Type Condition
InvalidOperationException The OdbcDataReader is closed.

Remarks

The GetSchemaTable method returns metadata about each column in the following order:

DataReader Column Description
ColumnName The name of the column; this might not be unique. If the column name cannot be determined, a null value is returned. This name always reflects the most recent naming of the column in the current view or command text.
ColumnOrdinal The ordinal of the column. This is zero for the bookmark column of the row, if any. Other columns are numbered starting with 1. This column cannot contain a null value.
ColumnSize The maximum possible length of a value in the column. For columns that use a fixed-length data type, this is the size of the data type.
NumericPrecision If DbType is a numeric data type, this is the maximum precision of the column. The precision depends on the definition of the column. If DbType is not a numeric data type, this is a null value. If the underlying ODBC driver returns a precision value for a non-numeric data type, this value is used in the schema table.
NumericScale If DbType is is Decimal, the number of digits to the right of the decimal point. Otherwise, this is a null value. If the underlying ODBC driver returns a precision value for a non-numeric data type, this value is used in the schema table.
DataType Maps to the common language runtime type of DbType.
ProviderType The underlying driver type.
IsLong true if the column contains a Binary Long Object (BLOB) that contains very long data. The definition of very long data is driver-specific.
AllowDBNull true if the consumer can set the column to a null value or if the driver cannot determine whether the consumer can set the column to a null value. Otherwise, false. A column may contain null values, even if it cannot be set to a null value.
IsReadOnly true if the column cannot be modified; otherwise false.
IsRowVersion Set if the column contains a persistent row identifier that cannot be written to, and has no meaningful value except to identity the row.
IsUnique true: No two rows in the base table-the table returned in BaseTableName-can have the same value in this column. IsUnique is guaranteed to be true if the column constitutes a key by itself or if there is a constraint of type UNIQUE that applies only to this column. false: The column can contain duplicate values in the base table. The default for this column is false.
IsKey true: The column is one of a set of columns in the rowset that, taken together, uniquely identify the row. The set of columns with IsKey set to true must uniquely identify a row in the rowset. There is no requirement that this set of columns is a minimal set of columns. This set of columns may be generated from a base table primary key, a unique constraint or a unique index. false: The column is not required to uniquely identify the row.
IsAutoIncrement true if the column assigns values to new rows in fixed increments; otherwise false. The default for this column is false.
BaseSchemaName The name of the schema in the data source that contains the column. NULL if the base catalog name cannot be determined. The default for this column is a null value.
BaseCatalogName The name of the catalog in the data store that contains the column. NULL if the base catalog name cannot be determined. The default for this column is a null value.
BaseTableName The name of the table or view in the data store that contains the column. A null value if the base table name cannot be determined. The default of this column is a null value.
BaseColumnName The name of the column in the data store. This might be different than the column name returned in the ColumnName column if an alias was used. A null value if the base column name cannot be determined or if the rowset column is derived, but not identical to, a column in the data store. The default for this column is a null value.

A row will be returned for every column in the results set.

The .NET Framework Data Provider for ODBC assumes that metadata information is available from an ODBC driver after one of SQLPrepare, SQLExecute, or SQLExecuteDirect functions are called. For "SchemaOnly" command behavior to work correctly, SQLPrepare must return the necessary metadata information. Not all ODBC Drivers support this function or return metadata information. In such cases, part or all of the information in the SchemaTable will be missing. After calling SQLPrepare, The data provider calls the ODBC SQLColAttribute function to find the metadata information related to each column in the query results (for example, IsLong, IsUnique, AllowDBNull, BaseTableName, BaseColumnName). If the underlying driver does not return some of this information, the corresponding values in the SchemaTable will not be set correctly.

The .NET Framework Data Provider for ODBC also calls SQLPrimaryKeys to retrieve the key information for every table. If the underlying ODBC driver does not support this function, the data provider calls SQLStatistics and chooses one of the unique indexes as the primary key for the table. This may not always give the desired results.

OdbcCommandBuilder needs the correct identification of the primary keys of the table in order to work properly. If the BaseTableName is not returned for every column in the query results, the .NET Framework Data Provider for ODBC attempts to parse the SQL statement to find the table names involved in the query. This works with UPDATE, INSERT, DELETE and simple SELECT statements, but not with stored procedures or SELECT statements based on joins. In cases where some or all of the schema information is missing from this table, the OdbcCommandBuilder will not work correctly, since it does not have enough schema information to automatically generate the correct INSERT, UPDATE, or DELETE statements.

Note[note] To ensure that metadata columns return the correct information, you must call ExecuteReader with the behavior parameter set to KeyInfo. Otherwise, some of the columns in the schema table may return default, null, or incorrect data.

Note   The Microsoft Jet ODBC driver always returns unique index and primary key columns as nullable regardless of whether they are nullable or not. The driver also does not return primary key information; it only returns a list of unique indexes and their columns, including primary key columns, without differentiating among them.

Requirements

Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family

See Also

OdbcDataReader Class | OdbcDataReader Members | System.Data.Odbc Namespace

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