我的桌子看起来像这样

Table cards
|  CardId  |  cityName| eventName|  Colour|
|----------+----------+----------+--------|
|     29   |   "test" |   null   | "blue" |
|     2    |   null   |  "test   |  null  |

现在说我做
 "select * from card where CardId = 29"

我的返回 vector 将包含:“test”就是这样。因此,它将实质上包含eventName之前的所有数据。同样的情况适用于以下情况
 "select * from card where CardId = 2"

连接到数据库的C++代码:
void SqlConnection::sqlExecuteSelect(string *select) {

SQLCHAR DBName[20] = "PandemicMain";
SQLCHAR SQLStmt[4000] = { 0 };
SQLRETURN rc = SQL_SUCCESS;
ODBC_Class Example;
if (Example.ConHandle != NULL)

{

    rc = SQLConnect(Example.ConHandle, DBName, SQL_NTS, (SQLCHAR *) "concordia", SQL_NTS, (SQLCHAR *) "University4", SQL_NTS);

    // Allocate An SQL Statement Handle

    rc = SQLAllocHandle(SQL_HANDLE_STMT, Example.ConHandle, &Example.StmtHandle);

    rc = SQLExecDirect(Example.StmtHandle, SQLStmt, SQL_NTS);

    if (rc == SQL_SUCCESS)

    {

        // Define A SELECT SQL Statement
        char* finalSelect = new char[select->length() + 1];
        std::strcpy(finalSelect, select->c_str());

        strcpy((char *)SQLStmt, finalSelect);

        // Prepare And Execute The SQL Statement

            rc = SQLExecDirect(Example.StmtHandle, SQLStmt, SQL_NTS);

        // Display The Results Of The SQL Query
        if (!rc == SQL_SUCCESS) {
            cout << "*************************** failed ***************" << endl;
        }
        if (rc == SQL_SUCCESS)

        {

            Example.GetResultset();
            Connection.colData = Example.colData;



            // At this point you would want to do something

            // with the resultset, such as display it.

        }

    }

    // Free The SQL Statement Handle

    if (Example.StmtHandle != NULL)

        SQLFreeHandle(SQL_HANDLE_STMT, Example.StmtHandle);

    // Disconnect From The Northwind Sample Database
    rc = SQLDisconnect(Example.ConHandle);

}
}

Odbc.h
#pragma once

////////////////////////////////////////
#include <windows.h>
#include <sql.h>
#include<sqltypes.h>
#include<sqlext.h>
#include <string>
#include <vector>
#include <iostream>
using namespace std;



// Define The ODBC_Class Class
class ODBC_Class
{
  struct ColDescription
  {
    SQLSMALLINT colNumber;
    SQLCHAR colName[80];
    SQLSMALLINT nameLen;
    SQLSMALLINT dataType;
    SQLULEN colSize;
    SQLSMALLINT decimalDigits;
    SQLSMALLINT nullable;
  };
// Attributes
public:
    SQLHANDLE EnvHandle;
    SQLHANDLE ConHandle;
    SQLHANDLE StmtHandle;
    SQLRETURN rc;
    vector<ColDescription> cols;
    vector< vector<string> > colData;
    // Operations
public:
    ODBC_Class(); // Constructor
    ~ODBC_Class(); // Destructor
    SQLRETURN GetResultset();
    void DescribeColumns();
private:
    _inline SQLRETURN Describe(ColDescription& c);
    SQLRETURN GetColData(int colnum, string& str);
};

Odbc.cpp
 #include "ODBC.h"

//***************************CODE FROM
http://www.dreamincode.net/forums/topic/127959-odbc-c-example/
*************************************/
// Define The ODBC_Class Class


// Define The Class Constructor
ODBC_Class::ODBC_Class()
{
// Initialize The Return Code Variable
rc = SQL_SUCCESS;
// Allocate An Environment Handle
rc = SQLAllocHandle(SQL_HANDLE_ENV, SQL_NULL_HANDLE, &EnvHandle);
// Set The ODBC Application Version To 3.x
if (rc == SQL_SUCCESS)
    rc = SQLSetEnvAttr(EnvHandle, SQL_ATTR_ODBC_VERSION,
    (SQLPOINTER)SQL_OV_ODBC3, SQL_IS_UINTEGER);
// Allocate A Connection Handle
if (rc == SQL_SUCCESS)
    rc = SQLAllocHandle(SQL_HANDLE_DBC, EnvHandle, &ConHandle);
}

// Define The Class Destructor
ODBC_Class::~ODBC_Class()
{
// Free The Connection Handle
if (ConHandle != NULL)
    SQLFreeHandle(SQL_HANDLE_DBC, ConHandle);
// Free The Environment Handle
if (EnvHandle != NULL)
    SQLFreeHandle(SQL_HANDLE_ENV, EnvHandle);
}

// Get the data for one column and return the info in the form
// of a std::string.  The ODBC driver will make all necessary
// data conversions from whatever type the data is in the database
// to SQL_CHAR.  You could make this function more comples by
// getting the return type as it appears in the database then constructing
// a VARIANT object to hold the data.
SQLRETURN ODBC_Class::GetColData(int colnum, string& str)
{
  SQLCHAR buf[255] = { 0 };
  if ((rc = SQLGetData(StmtHandle, colnum, SQL_CHAR, buf, sizeof(buf), NULL)) == SQL_SUCCESS)
      str = reinterpret_cast<char*>(buf);
 return rc;
}

//
// Define The ShowResults() Member Function
SQLRETURN ODBC_Class::GetResultset()
{
   // Get all column description
   DescribeColumns();
   // erase anything that's in the colData vector
   colData.clear();
   // fetch a row from the resultset
   while (SQLFetch(StmtHandle) == SQL_SUCCESS)
   {
       // vector of strings to hold the column data
       vector<string> col;
       string data;
       // column counter
       int i = 1;
       // get the data for each column and add it to
       // the col vector
       while (GetColData(i, data) == SQL_SUCCESS)
       {
           col.push_back(data);
           ++i; // increment the column number
       }
       // add column data to the colData vector
       colData.push_back(col);
    }
   return SQL_SUCCESS;
  }

 // Get the description for one column in the resultset.
 // This was made a seprate function to simplify the coding
 SQLRETURN  ODBC_Class::Describe(ColDescription& c)
{
    return SQLDescribeCol(StmtHandle, c.colNumber,
         c.colName, sizeof(c.colName), &c.nameLen,
         &c.dataType, &c.colSize, &c.decimalDigits, &c.nullable);
}

// Get the description for all the columns in the resultset.
void ODBC_Class::DescribeColumns()
{
    ColDescription c;
    c.colNumber = 1;
     cols.clear();
    while (Describe(c) == SQL_SUCCESS)
   {
      cols.push_back(c);
       ++c.colNumber;
   }

 }

使用SQLGetDiagRec之后

我收到以下消息:



因此,我知道这意味着“指标变量是必需的但未提供”

无论如何要处理吗?

最佳答案

我有一阵子完全一样的错误。基本上,ODBC试图告诉您所检索的值为NULL。它将使用 SQLGetData()中的最后一个参数来执行此操作,您可以在此处找到更多信息:https://docs.microsoft.com/en-gb/sql/odbc/reference/syntax/sqlgetdata-function

该行:

  if ((rc = SQLGetData(StmtHandle, colnum, SQL_CHAR, buf, sizeof(buf), NULL)) == SQL_SUCCESS)
  str = reinterpret_cast<char*>(buf);

您需要将传入的NULL值更改为可以填充的值。这是ODBC将尝试填充的缓冲区(如果它是NULL值)。它将始终返回您在此字段中返回的列的长度(但如果传递NULL则不会失败),除非它返回NULL(在这种情况下需要它)。通过ref传递一个SQLLEN,它应该作为SQL_NULL_DATA返回。

07-24 09:38
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