Connexion à un serveur MySQL à l'aide de C++ - Stack Overflow ~ Stackoverflow Blog

I'm attempting to connect to a MySQL server using C++ with the MySQL ODBC 5.1 Driver on Visual C++ 2008 Express Edition.

I'm following these instructions from MSDN:

SQLConnect

SQLGetData

SQLFetch

The only difference is that I have to convert all the SQLCHAR to SQLWCHAR, to match the function params, hopefully that doesn't affect the connection string.

Every time I connect I get SQL_ERROR as the return value. So I'm assuming there's something wrong with the connection string or the connection statement.

I've tried

DNS=TestConnection; UID=user; PSW=password

and

SERVER=localhost; DRIVER={MySQL ODBC 5.1 Driver}; PORT=3306; UID=user; PSW=password; DATABASE=dbo; and other similar connection strings.

The DNS that's called TestConnection has the same info as the latter connection string.

The schema is dbo, and have one table called testfire with the following column specs:

TEST_ID( INT(11), PRIMARY, AUTO INCREMENT)
TEST_STRING( VARCHAR(50) )
TEST_INTEGER( INT(11) )
TEST_FLOAT( FLOAT )
TEST_DATE( DATETIME )

With 3 rows:

  ID    STRING    INT   FLOAT           DATE
------------------------------------------------------
| 1  |  Test 1  |  1  |  0.1  |  2001-01-01 00:00:00 |
| 2  |  Test 2  |  2  |  0.2  |  2002-01-01 00:00:00 |
| 3  |  Test 3  |  3  |  0.3  |  2003-01-01 00:00:00 |
------------------------------------------------------

I've attempted to retrieve the data using an Excel connection, mostly to see if the driver works. Excel successfully retrieved the data without problem, so the DNS named TestConnection is valid, and so are the credentials.

What am I doing wrong?

What should I change?

Is it the conversion to MYSQLWCHAR * that messes up the connection string?

Is there a different, perhaps better and more efficient approach? (except perhaps class encapsulation, that's what I'm going to do after the test is successful)

Oh, and the compiler doesn't give any errors or warnings, the code is compiled and runs without any problems.

So, here's the test code, which returns "Query execution error":

#include <iostream>
#include <windows.h>
#include <sql.h>
#include <sqltypes.h>
#include <sqlext.h>

using namespace std;

int main(){
    SQLHENV henv;
    SQLHDBC hdbc;
    SQLHSTMT hstmt;
    SQLRETURN retcode;
    HWND desktopHandle = GetDesktopWindow();

    SQLWCHAR OutConnStr[255];
    SQLSMALLINT OutConnStrLen;
    SQLWCHAR szDNS[2048] ={0};

    // Allocate environment handle
    retcode = SQLAllocHandle(SQL_HANDLE_ENV, SQL_NULL_HANDLE, &henv);

    // Set the ODBC version environment attribute
    if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
        retcode = SQLSetEnvAttr(henv, SQL_ATTR_ODBC_VERSION, (void*)SQL_OV_ODBC3, 0); 

        // Allocate connection handle
        if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
            retcode = SQLAllocHandle(SQL_HANDLE_DBC, henv, &hdbc); 

             // Set login timeout to 5 seconds
            if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
                SQLSetConnectAttr(hdbc, SQL_LOGIN_TIMEOUT, (SQLPOINTER)5, 0);

                // Connect to data source
                retcode = SQLDriverConnect(
                    hdbc, 
                    desktopHandle, 
                    (SQLWCHAR*)"driver=MySQL Server", 
                    _countof("driver=MySQL Server"),
                    OutConnStr,
                    255, 
                    &OutConnStrLen,
                    SQL_DRIVER_PROMPT );

                // Allocate statement handle
                if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
                    retcode = SQLAllocHandle(SQL_HANDLE_STMT, hdbc, &hstmt); 

                    // Process data
                    retcode = SQLExecDirect(hstmt, (SQLWCHAR*)"SELECT TEST_STRING, TEST_INTEGER, TEST_FLOAT FROM dbo.testfire", SQL_NTS);

                    if (retcode == SQL_SUCCESS) {
                        SQLINTEGER sTestInt, cbTestStr, cbTestInt, cbTestFloat;
                        SQLFLOAT dTestFloat;
                        SQLCHAR szTestStr[200];
                        while (TRUE) {
                            cout<<"Inside loop";
                            retcode = SQLFetch(hstmt);
                            if (retcode == SQL_ERROR || retcode == SQL_SUCCESS_WITH_INFO) {
                                cout<<"An error occurred";
                            }
                            if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO){

                                SQLGetData(hstmt, 1, SQL_C_CHAR, szTestStr, 200, &cbTestStr);
                                SQLGetData(hstmt, 2, SQL_C_ULONG, &sTestInt, 0, &cbTestInt);
                                SQLGetData(hstmt, 3, SQL_C_FLOAT, &dTestFloat, 0,&cbTestFloat);

                                /* Print the row of data */
                                cout<<szTestStr<<endl;
                                cout<<sTestInt<<endl;
                                cout<<dTestFloat<<endl;
                            } else {
                                break;
                            }
                        }
                    }else{
                        cout<<"Query execution error."<<endl;
                        SQLWCHAR       SqlState[6], Msg[SQL_MAX_MESSAGE_LENGTH];
                        SQLINTEGER    NativeError;
                        SQLSMALLINT   i, MsgLen;
                        SQLRETURN     rc2;

                        // Get the status records.
                        i = 1;
                        while ((rc2 = SQLGetDiagRec(SQL_HANDLE_STMT, hstmt, i, SqlState, &NativeError,
                            Msg, sizeof(Msg), &MsgLen)) != SQL_NO_DATA) {
                          cout<<SqlState<<endl;
                          cout<<NativeError<<endl;
                          cout<<Msg<<endl;
                          cout<<MsgLen<<endl;
                          i++;
                        }
                    }
                    if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
                        SQLFreeHandle(SQL_HANDLE_STMT, hstmt);
                    }

                    SQLDisconnect(hdbc);
                }else{ 
                    cout<<"Connection error."<<endl;
                }
                SQLFreeHandle(SQL_HANDLE_DBC, hdbc);
            }
        }
        SQLFreeHandle(SQL_HANDLE_ENV, henv);
    }
    system("pause");
    return 0;
}

UPDATE

After updating the code (and post) using the correct arguments for the SQLDriverConnect from the documentation provided by Mat (see comments below), the connection works. How can I do the same thing without having to prompt for the DNS name? Put window handle as null and...?

Now it fails at the SQLExecDirect(hstmt, (SQLWCHAR*)"SELECT TEST_STRING, TEST_INTEGER, TEST_FLOAT FROM dbo.testfire", SQL_NTS), but the query is correct, so, what's the problem?

The exact error message returned is:

Sql State:      42000
Native Error:   1064
Message:        
Message Length: 211

42000: Syntax error or access violation

*StatementText contained an SQL statement that was not preparable or contained a syntax error.
The user did not have permission to execute the SQL statement contained in *StatementText.

So... what does that mean? How can I not have permission? How can that generate a syntax error, it's clearly a valid query?

With a little help from Mat, I was able to figure out what the problem was, but since he didn't give it in a form of an answer, I'll have to answer it so it can be shared for those who have the same problem, and also to mark as answered.

So, my problem was that I couldn't connect to the database. As Mat suggested, I should use the extended error info, known as SQLGetDiagRec and also fix the arguments according to the documentation. Took me a moment to learn how the SQLGetDiagRec function works, but once I managed to convert the wchar_t to char * I was able to see the error it was generating.

The connection attempt gave me the error Data source not found and no default driver specified. That gave me a clue, indicating I either wrote the incorrect connection string or that the text string was somehow misinterpreted or mangled.

Doing some searching on the net gave me the insight that the string was misinterpreted, and to fix it I had to make it a literal string. Surely enough, putting an L in front of the string solved it!

retcode = SQLDriverConnect(hdbc, 0, 
                           (SQLWCHAR*)L"DSN=TestConnection;SERVER=localhost;UID=user;PWD=password;DRIVER=MySQL Server;", 
                           _countof(L"DSN=TestConnection;SERVER=localhost;UID=user;PWD=password;DRIVER=MySQL Server;"), 
                           OutConnStr, 255, &OutConnStrLen, SQL_DRIVER_COMPLETE);

At the same time, I learned how to get rid of the prompt, which was quite easy to figure out after correcting the initial problem. Specify null for the window handle, set driver completion to SQL_DRIVER_COMPLETE and make sure you add all the information needed in the connection string.

So, the next problem I had with the query with SQLExecDirect was giving an error saying Syntax error or access violation. The problem was obviously the same as with the connection string. Surely enough

retcode = SQLExecDirect(hstmt, (SQLWCHAR*)L"SELECT TEST_STRING, TEST_INTEGER, TEST_FLOAT FROM dbo.testfire", SQL_NTS);

Worked like a charm.

Here's the code in its entirety, fully functional:

#include <iostream>
#include <windows.h>
#include <sql.h>
#include <sqltypes.h>
#include <sqlext.h>
#include <string>

using namespace std;

int main(){
    SQLHENV henv;
    SQLHDBC hdbc;
    SQLHSTMT hstmt;
    SQLRETURN retcode;

    SQLWCHAR OutConnStr[255];
    SQLSMALLINT OutConnStrLen;

    // Allocate environment handle
    retcode = SQLAllocHandle(SQL_HANDLE_ENV, SQL_NULL_HANDLE, &henv);

    // Set the ODBC version environment attribute
    if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
        retcode = SQLSetEnvAttr(henv, SQL_ATTR_ODBC_VERSION, (void*)SQL_OV_ODBC3, 0); 

        // Allocate connection handle
        if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
            retcode = SQLAllocHandle(SQL_HANDLE_DBC, henv, &hdbc); 

             // Set login timeout to 5 seconds
            if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
                SQLSetConnectAttr(hdbc, SQL_LOGIN_TIMEOUT, (SQLPOINTER)5, 0);

                // Connect to data source
                retcode = SQLDriverConnect(
                    hdbc, 
                    0,
                    (SQLWCHAR*)L"DSN=TestConnection;SERVER=localhost;UID=root;PWD=never140;DRIVER=MySQL Server;", 
                    _countof(L"DSN=TestConnection;SERVER=localhost;UID=root;PWD=never140;DRIVER=MySQL Server;"),
                    OutConnStr,
                    255, 
                    &OutConnStrLen,
                    SQL_DRIVER_COMPLETE );

                // Allocate statement handle
                if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
                    retcode = SQLAllocHandle(SQL_HANDLE_STMT, hdbc, &hstmt); 

                    // Process data
                    retcode = SQLExecDirect(hstmt, (SQLWCHAR*)L"SELECT TEST_STRING, TEST_INTEGER, TEST_FLOAT FROM dbo.testfire", SQL_NTS);

                    if (retcode == SQL_SUCCESS) {
                        SQLINTEGER sTestInt, cbTestStr, cbTestInt, cbTestFloat, iCount = 1;
                        SQLFLOAT dTestFloat;
                        SQLCHAR szTestStr[200];
                        while (TRUE) {
                            retcode = SQLFetch(hstmt);
                            if (retcode == SQL_ERROR || retcode == SQL_SUCCESS_WITH_INFO) {
                                cout<<"An error occurred";
                            }
                            if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO){

                                SQLGetData(hstmt, 1, SQL_C_CHAR, szTestStr, 200, &cbTestStr);
                                SQLGetData(hstmt, 2, SQL_C_ULONG, &sTestInt, 0, &cbTestInt);
                                SQLGetData(hstmt, 3, SQL_C_DOUBLE, &dTestFloat, 0,&cbTestFloat);

                                /* Print the row of data */
                                cout<<"Row "<<iCount<<":"<<endl;
                                cout<<szTestStr<<endl;
                                cout<<sTestInt<<endl;
                                cout<<dTestFloat<<endl;
                                iCount++;
                            } else {
                                break;
                            }
                        }
                    }else{
                        cout<<"Query execution error."<<endl;
                    }

                    SQLFreeHandle(SQL_HANDLE_STMT, hstmt);
                    SQLDisconnect(hdbc);
                }else{ 
                    cout<<"Connection error"<<endl;
                }
                SQLFreeHandle(SQL_HANDLE_DBC, hdbc);
            }
        }
        SQLFreeHandle(SQL_HANDLE_ENV, henv);
    }

        system("pause");
    return 0;
}

Just goes to show, even the tiniest thing can make everything fail.

Thank you Mat for your help.

change (SQLWCHAR*) to L. this works fine for me