Bewerken

Share via

SQLDescribeCol Function

  • Artikel

Conformance
Version Introduced: ODBC 1.0 Standards Compliance: ISO 92

Summary
SQLDescribeCol returns the result descriptor - column name,type, column size, decimal digits, and nullability - for one column in the result set. This information also is available in the fields of the IRD.

Syntax

  
SQLRETURN SQLDescribeCol(  
      SQLHSTMT       StatementHandle,  
      SQLUSMALLINT   ColumnNumber,  
      SQLCHAR *      ColumnName,  
      SQLSMALLINT    BufferLength,  
      SQLSMALLINT *  NameLengthPtr,  
      SQLSMALLINT *  DataTypePtr,  
      SQLULEN *      ColumnSizePtr,  
      SQLSMALLINT *  DecimalDigitsPtr,  
      SQLSMALLINT *  NullablePtr);

Arguments

StatementHandle
[Input] Statement handle.

ColumnNumber
[Input] Column number of result data, ordered sequentially in increasing column order, starting at 1. The ColumnNumber argument can also be set to 0 to describe the bookmark column.

ColumnName
[Output] Pointer to a null-terminated buffer in which to return the column name. This value is read from the SQL_DESC_NAME field of the IRD. If the column is unnamed or the column name cannot be determined, the driver returns an empty string.

If ColumnName is NULL, NameLengthPtr will still return the total number of characters (excluding the null-termination character for character data) available to return in the buffer pointed to by ColumnName.

BufferLength
[Input] Length of the *ColumnName buffer, in characters.

NameLengthPtr
[Output] Pointer to a buffer in which to return the total number of characters (excluding the null termination) available to return in *ColumnName. If the number of characters available to return is greater than or equal to BufferLength, the column name in *ColumnName is truncated to BufferLength minus the length of a null-termination character.

DataTypePtr
[Output] Pointer to a buffer in which to return the SQL data type of the column. This value is read from the SQL_DESC_CONCISE_TYPE field of the IRD. This will be one of the values in SQL Data Types, or a driver-specific SQL data type. If the data type cannot be determined, the driver returns SQL_UNKNOWN_TYPE.

In ODBC 3.x, SQL_TYPE_DATE, SQL_TYPE_TIME, or SQL_TYPE_TIMESTAMP is returned in *DataTypePtr for date, time, or timestamp data, respectively; in ODBC 2.x, SQL_DATE, SQL_TIME, or SQL_TIMESTAMP is returned. The Driver Manager performs the required mappings when an ODBC 2.x application is working with an ODBC 3.x driver or when an ODBC 3.x application is working with an ODBC 2.x driver.

When ColumnNumber is equal to 0 (for a bookmark column), SQL_BINARY is returned in *DataTypePtr for variable-length bookmarks. (SQL_INTEGER is returned if bookmarks are used by an ODBC 3.x application working with an ODBC 2.x driver or by an ODBC 2.x application working with an ODBC 3.x driver.)

For more information on these data types, see SQL Data Types in Appendix D: Data Types. For information about driver-specific SQL data types, see the driver's documentation.

ColumnSizePtr
[Output] Pointer to a buffer in which to return the size (in characters) of the column on the data source. If the column size cannot be determined, the driver returns 0. For more information on column size, see Column Size, Decimal Digits, Transfer Octet Length, and Display Size in Appendix D: Data Types.

DecimalDigitsPtr
[Output] Pointer to a buffer in which to return the number of decimal digits of the column on the data source. If the number of decimal digits cannot be determined or is not applicable, the driver returns 0. For more information on decimal digits, see Column Size, Decimal Digits, Transfer Octet Length, and Display Size in Appendix D: Data Types.

NullablePtr
[Output] Pointer to a buffer in which to return a value that indicates whether the column allows NULL values. This value is read from the SQL_DESC_NULLABLE field of the IRD. The value is one of the following:

SQL_NO_NULLS: The column does not allow NULL values.

SQL_NULLABLE: The column allows NULL values.

SQL_NULLABLE_UNKNOWN: The driver cannot determine if the column allows NULL values.

Returns

SQL_SUCCESS, SQL_SUCCESS_WITH_INFO, SQL_STILL_EXECUTING, SQL_ERROR, or SQL_INVALID_HANDLE.

Diagnostics

When SQLDescribeCol returns either SQL_ERROR or SQL_SUCCESS_WITH_INFO, an associated SQLSTATE value can be obtained by calling SQLGetDiagRec with a HandleType of SQL_HANDLE_STMT and a Handle of StatementHandle. The following table lists the SQLSTATE values commonly returned by SQLDescribeCol and explains each one in the context of this function; the notation "(DM)" precedes the descriptions of SQLSTATEs returned by the Driver Manager. The return code associated with each SQLSTATE value is SQL_ERROR, unless noted otherwise.

SQLSTATE Error Description
01000 General warning Driver-specific informational message. (Function returns SQL_SUCCESS_WITH_INFO.)
01004 String data, right truncated The buffer *ColumnName was not large enough to return the entire column name, so the column name was truncated. The length of the untruncated column name is returned in *NameLengthPtr. (Function returns SQL_SUCCESS_WITH_INFO.)
07005 Prepared statement not a cursor-specification The statement associated with the StatementHandle did not return a result set. There were no columns to describe.
07009 Invalid descriptor index (DM) The value specified for the argument ColumnNumber was equal to 0, and the SQL_ATTR_USE_BOOKMARKS statement option was SQL_UB_OFF.

The value specified for the argument ColumnNumber was greater than the number of columns in the result set.
08S01 Communication link failure The communication link between the driver and the data source to which the driver was connected failed before the function completed processing.
HY000 General error An error occurred for which there was no specific SQLSTATE and for which no implementation-specific SQLSTATE was defined. The error message returned by SQLGetDiagRec in the *MessageText buffer describes the error and its cause.
HY001 Memory allocation failure The driver was unable to allocate memory required to support execution or completion of the function.
HY008 Operation canceled Asynchronous processing was enabled for the StatementHandle. The function was called, and before it completed execution, SQLCancel or SQLCancelHandle was called on the StatementHandle. Then the function was called again on the StatementHandle.

The function was called, and before it completed execution, SQLCancel or SQLCancelHandle was called on the StatementHandle from a different thread in a multithread application.
HY010 Function sequence error (DM) An asynchronously executing function was called for the connection handle that is associated with the StatementHandle. This asynchronous function was still executing when SQLDescribeCol was called.

(DM) SQLExecute, SQLExecDirect, or SQLMoreResults was called for the StatementHandle and returned SQL_PARAM_DATA_AVAILABLE. This function was called before data was retrieved for all streamed parameters.

(DM) An asynchronously executing function (not this one) was called for the StatementHandle and was still executing when this function was called.

(DM) The function was called prior to calling SQLPrepare, SQLExecute, or a catalog function on the statement handle.

(DM) SQLExecute, SQLExecDirect, SQLBulkOperations, or SQLSetPos was called for the StatementHandle and returned SQL_NEED_DATA. This function was called before data was sent for all data-at-execution parameters or columns.
HY013 Memory management error The function call could not be processed because the underlying memory objects could not be accessed, possibly because of low memory conditions.
HY090 Invalid string or buffer length (DM) The value specified for argument BufferLength was less than 0.
HY117 Connection is suspended due to unknown transaction state. Only disconnect and read-only functions are allowed. (DM) For more information about suspended state, see SQLEndTran Function.
HYT01 Connection timeout expired The connection timeout period expired before the data source responded to the request. The connection timeout period is set through SQLSetConnectAttr, SQL_ATTR_CONNECTION_TIMEOUT.
IM001 Driver does not support this function (DM) The driver associated with the StatementHandle does not support the function.
IM017 Polling is disabled in asynchronous notification mode Whenever the notification model is used, polling is disabled.
IM018 SQLCompleteAsync has not been called to complete the previous asynchronous operation on this handle. If the previous function call on the handle returns SQL_STILL_EXECUTING and if notification mode is enabled, SQLCompleteAsync must be called on the handle to do post-processing and complete the operation.

SQLDescribeCol can return any SQLSTATE that can be returned by SQLPrepare or SQLExecute when called after SQLPrepare and before SQLExecute, depending on when the data source evaluates the SQL statement associated with the statement handle.

For performance reasons, an application should not call SQLDescribeCol before executing a statement.

Comments

An application typically calls SQLDescribeCol after a call to SQLPrepare and before or after the associated call to SQLExecute. An application can also call SQLDescribeCol after a call to SQLExecDirect. For more information, see Result Set Metadata.

SQLDescribeCol retrieves the column name, type, and length generated by a SELECT statement. If the column is an expression, *ColumnName is either an empty string or a driver-defined name.

Note

ODBC supports SQL_NULLABLE_UNKNOWN as an extension, even though the Open Group and SQL Access Group Call Level Interface specification does not specify the option for SQLDescribeCol.

For information about See
Binding a buffer to a column in a result set SQLBindCol
Canceling statement processing SQLCancel
Returning information about a column in a result set SQLColAttribute
Fetching multiple rows of data SQLFetch
Returning the number of result set columns SQLNumResultCols
Preparing a statement for execution SQLPrepare

See Also

ODBC API Reference
ODBC Header Files