C++ 数据库使用 SQL ODBC 从表中获取行数组

Question

参考此链接C++\SQL ODBC: Get row from table 我不理解这些文档，并尝试了许多变体来使某些东西起作用，但它只会输出完整的胡言乱语。基本上，我怎样才能从我的 Microsoft Access 数据库中获取数据到我的 C++ 程序中。我最终希望将整个表存储为数组或向量，也就是说，假设我的表中有 5 个字段，我想将所有内容存储（在 5 个不同的数组或向量中）到相应的字段。但是现在，我怎么能只拥有一个名为“示例表”的表，其中有一个名为“名称”的字段，并且类型是 Access 所称的字符串或“短文本”。然后如何将其加载到字符串数组或 char* 中以用于我的 GUI 或程序的其他部分？这是我当前的代码：

#include <stdio.h>
#include <tchar.h>
#include <Windows.h>
#include <sqlext.h>
#include <iostream>

WCHAR szDSN[] = L"Driver={Microsoft Access Driver (*.mdb, *.accdb)};DSN='';DBQ=C:\\Users\\Name\\Desktop\\databaseExample\\databaseExample\\Data.accdb";

int _tmain(int argc, _TCHAR* argv[])
{

    HENV    hEnv;
    HDBC    hDbc;

    /* ODBC API return status */
    RETCODE rc;

    int     iConnStrLength2Ptr;
    WCHAR    szConnStrOut[256];

    const WCHAR* query = L"select Names from Example_Table";

    HSTMT           hStmt;

    /* Allocate an environment handle */
    rc = SQLAllocEnv(&hEnv);
    /* Allocate a connection handle */
    rc = SQLAllocConnect(hEnv, &hDbc);

    /* Connect to the database */
    rc = SQLDriverConnect(hDbc, NULL, (WCHAR*)szDSN,
        SQL_NTS, (WCHAR*)szConnStrOut,
        255, (SQLSMALLINT*)&iConnStrLength2Ptr, SQL_DRIVER_NOPROMPT);
    if (SQL_SUCCEEDED(rc))
    {

        wprintf(L"Successfully connected to database. Data source name: \n  %s\n",
            szConnStrOut);

        /* Prepare SQL query */
        wprintf(L"SQL query:\n  %s\n", query);

        rc = SQLAllocStmt(hDbc, &hStmt);
        rc = SQLPrepare(hStmt, (SQLWCHAR*)query, SQL_NTS);

        /* Excecute the query */
        rc = SQLExecute(hStmt);
        if (SQL_SUCCEEDED(rc))
        {
            wprintf(L"SQL Success\n");
        }
        else {
            wprintf(L"SQL Failed\n");
        }
        
    }
    else
    {
        wprintf(L"Couldn't connect to %s.\n", szDSN);
    }

    /* Disconnect and free up allocated handles */
    SQLDisconnect(hDbc);
    SQLFreeHandle(SQL_HANDLE_DBC, hDbc);
    SQLFreeHandle(SQL_HANDLE_ENV, hEnv);

    getchar();
    printf("hi\n");
    return 0;
}

Answer 1

您需要使用SQLGetData，传递SQL_C_WCHAR 类型，例如，如果您的列是文本。下面是如何获取单个文本列的值并将它们放入向量的示例：

#include <iostream>
#include <string>
#include <vector>
#include <Windows.h>
#include <sqlext.h>

WCHAR szDSN[] = L"Driver={Microsoft Access Driver (*.mdb, *.accdb)};DSN='';DBQ=C:\\Test\\users.accdb";

void DisplayError(SQLSMALLINT t, SQLHSTMT h) {
    SQLWCHAR       SqlState[6], Msg[SQL_MAX_MESSAGE_LENGTH];
    SQLINTEGER    NativeError;
    SQLSMALLINT   i, MsgLen;
    SQLRETURN     rc;

    SQLLEN numRecs = 0;
    SQLGetDiagField(t, h, 0, SQL_DIAG_NUMBER, &numRecs, 0, 0);

    // Get the status records.
    i = 1;
    while (i <= numRecs && (rc = SQLGetDiagRec(t, h, i, SqlState, &NativeError,
        Msg, sizeof(Msg), &MsgLen)) != SQL_NO_DATA) {
        wprintf(L"Error %d: %s\n", NativeError, Msg);
        i++;
    }

}

std::vector<std::wstring> ExecuteSql(const WCHAR* sql) {

    std::vector<std::wstring> results = {};
    HENV    hEnv = NULL;
    HDBC    hDbc = NULL;
    HSTMT hStmt = NULL;
    int iConnStrLength2Ptr;
    WCHAR szConnStrOut[256];
    SQLINTEGER rowCount = 0;
    SQLSMALLINT fieldCount = 0;
    SQLWCHAR buf[128];
    SQLINTEGER ret;

    /* ODBC API return status */
    RETCODE rc;

    /* Allocate an environment handle */
    rc = SQLAllocEnv(&hEnv);
    /* Allocate a connection handle */
    rc = SQLAllocConnect(hEnv, &hDbc);

    /* Connect to the database */
    rc = SQLDriverConnect(hDbc, NULL, (WCHAR*)szDSN,
        SQL_NTS, (WCHAR*)szConnStrOut,
        255, (SQLSMALLINT*)&iConnStrLength2Ptr, SQL_DRIVER_NOPROMPT);

    if (SQL_SUCCEEDED(rc))
    {
        /* Prepare SQL query */
        rc = SQLAllocStmt(hDbc, &hStmt);
        rc = SQLPrepare(hStmt, (SQLWCHAR*)sql, SQL_NTS);

        /* Excecute the query */
        rc = SQLExecute(hStmt);
        if (SQL_SUCCEEDED(rc))
        {
            SQLNumResultCols(hStmt, &fieldCount);

            if (fieldCount > 0)
            {
                /* Loop through the rows in the result set */

                rc = SQLFetch(hStmt);

                while (SQL_SUCCEEDED(rc))
                {                   
                    //get data
                    rc = SQLGetData(hStmt, 1, SQL_C_WCHAR, buf, sizeof(buf), &ret);
                    
                    if (SQL_SUCCEEDED(rc) == FALSE) {
                        wprintf(L"SQLGetData failed\n");
                        continue;
                    }

                    //convert data to string
                    std::wstring str;
                    if (ret <= 0) {
                        str = std::wstring(L"(null");
                    }
                    else {
                        str = std::wstring(buf);
                    }
                    
                    results.push_back(str);
                    rc = SQLFetch(hStmt);
                    rowCount++;
                };

                rc = SQLFreeStmt(hStmt, SQL_DROP);

            }
            else
            {
                wprintf(L"Error: Number of fields in the result set is 0.\n");
            }

        }
        else {
            wprintf(L"SQL Failed\n");
            DisplayError(SQL_HANDLE_STMT, hStmt);
        }
    }
    else
    {
        wprintf(L"Couldn't connect to %s\n", szDSN);
        DisplayError(SQL_HANDLE_DBC, hDbc);
    }

    /* Disconnect and free up allocated handles */
    SQLDisconnect(hDbc);
    SQLFreeHandle(SQL_HANDLE_DBC, hDbc);
    SQLFreeHandle(SQL_HANDLE_ENV, hEnv);
    return results;
}

int main()
{
    auto results = ExecuteSql(L"SELECT name FROM Users");

    for (auto x : results) {
        std::wcout << x << std::endl;
    }

    getchar();
    return 0;
}