Retrieving data

// Demonstrate running a query and getting results by querying the tables
// system table for a list of all tables in the current schema.
// Some standard headers
#include <stdlib.h>
#include <sstream>
#include <iostream>
#include <assert.h>
// Standard ODBC headers
#include <sql.h>
#include <sqltypes.h>
#include <sqlext.h>

// Use std namespace to make output easier
using namespace std;
// Helper function to print SQL error messages.
template <typename HandleT>
void reportError(int handleTypeEnum, HandleT hdl)
{
    // Get the status records.
    SQLSMALLINT   i, MsgLen;
    SQLRETURN ret2;
    SQLCHAR       SqlState[6], Msg[SQL_MAX_MESSAGE_LENGTH];
    SQLINTEGER    NativeError;
    i = 1;
    cout << endl;
    while ((ret2 = SQLGetDiagRec(handleTypeEnum, hdl, i, SqlState, &NativeError,
        Msg, sizeof(Msg), &MsgLen)) != SQL_NO_DATA) {
        cout << "error record #" << i++ << endl;
        cout << "sqlstate: " << SqlState << endl;
        cout << "detailed msg: " << Msg << endl;
        cout << "native error code: " << NativeError << endl;
    }
}

typedef struct {
    SQLHENV hdlEnv;
    SQLHDBC hdlDbc;
} DBConnection;

void connect(DBConnection *pConnInfo)
{
    // Set up the ODBC environment
    SQLRETURN ret;
    ret = SQLAllocHandle(SQL_HANDLE_ENV, SQL_NULL_HANDLE, &pConnInfo->hdlEnv);
    assert(SQL_SUCCEEDED(ret));
    // Tell ODBC that the application uses ODBC 3.
    ret = SQLSetEnvAttr(pConnInfo->hdlEnv, SQL_ATTR_ODBC_VERSION,
        (SQLPOINTER)SQL_OV_ODBC3, SQL_IS_UINTEGER);
    assert(SQL_SUCCEEDED(ret));

    // Allocate a database handle.
    ret = SQLAllocHandle(SQL_HANDLE_DBC, pConnInfo->hdlEnv, &pConnInfo->hdlDbc);
    assert(SQL_SUCCEEDED(ret));
    // Connect to the database
    cout << "Connecting to database." << endl;
    const char* dsnName = "ExampleDB";
    const char* userID = "dbadmin";
    const char* passwd = "password123";
    ret = SQLConnect(pConnInfo->hdlDbc, (SQLCHAR*)dsnName,
        SQL_NTS, (SQLCHAR*)userID, SQL_NTS,
        (SQLCHAR*)passwd, SQL_NTS);
    if (!SQL_SUCCEEDED(ret)) {
        cout << "Could not connect to database" << endl;
        reportError<SQLHDBC>(SQL_HANDLE_DBC, pConnInfo->hdlDbc);
        exit(EXIT_FAILURE);
    }
    else {
        cout << "Connected to database." << endl;
    }
}

void disconnect(DBConnection *pConnInfo)
{
    SQLRETURN ret;
    // Clean up by shutting down the connection
    cout << "Free handles." << endl;
    ret = SQLDisconnect(pConnInfo->hdlDbc);
    if (!SQL_SUCCEEDED(ret)) {
        cout << "Error disconnecting. Transaction still open?" << endl;
        exit(EXIT_FAILURE);
    }
    SQLFreeHandle(SQL_HANDLE_DBC, pConnInfo->hdlDbc);
    SQLFreeHandle(SQL_HANDLE_ENV, pConnInfo->hdlEnv);
}

void executeQuery(SQLHDBC hdlDbc, SQLCHAR* pQuery)
{
    SQLRETURN ret;
    // Set up a statement handle
    SQLHSTMT hdlStmt;
    SQLAllocHandle(SQL_HANDLE_STMT, hdlDbc, &hdlStmt);
    assert(SQL_SUCCEEDED(ret));

    // Execute a query to get the names and IDs of all tables in the schema
    // search p[ath (usually public).
    ret = SQLExecDirect(hdlStmt, pQuery, SQL_NTS);

    if (!SQL_SUCCEEDED(ret)) {
        // Report error an go no further if statement failed.
        cout << "Error executing statement." << endl;
        reportError<SQLHDBC>(SQL_HANDLE_STMT, hdlStmt);
        exit(EXIT_FAILURE);
    }
    else {
        // Query succeeded, so bind two variables to the two colums in the
        // result set,
        cout << "Fetching results..." << endl;
        SQLBIGINT table_id;       // Holds the ID of the table.
        SQLTCHAR table_name[256]; // buffer to hold name of table
        ret = SQLBindCol(hdlStmt, 1, SQL_C_SBIGINT, (SQLPOINTER)&table_id,
            sizeof(table_id), NULL);
        ret = SQLBindCol(hdlStmt, 2, SQL_C_TCHAR, (SQLPOINTER)table_name,
            sizeof(table_name), NULL);

        // Loop through the results,
        while (SQL_SUCCEEDED(ret = SQLFetchScroll(hdlStmt, SQL_FETCH_NEXT, 1))) {
            // Print the bound variables, which now contain the values from the
            // fetched row.
            cout << table_id << " | " << table_name << endl;
        }


        // See if loop exited for reasons other than running out of data
        if (ret != SQL_NO_DATA) {
            // Exited for a reason other than no more data... report the error.
            reportError<SQLHDBC>(SQL_HANDLE_STMT, hdlStmt);
        }
    }
    SQLFreeHandle(SQL_HANDLE_STMT, hdlStmt);
}

int main()
{
    DBConnection conn;

    connect(&conn);
    executeQuery(conn.hdlDbc,
        (SQLCHAR*)"SELECT table_id, table_name FROM tables ORDER BY table_name");
    executeQuery(conn.hdlDbc,
        (SQLCHAR*)"SELECT table_id, table_name FROM tables ORDER BY table_id");
    disconnect(&conn);
    exit(EXIT_SUCCESS);
}