使用 SQLBindCol

應用程式透過呼叫 SQLBindCol 來綁定欄位。此函式一次綁定一欄。藉此，應用程式會指定以下內容：

欄位編號。第 0 欄是書籤欄;此欄位未包含在某些結果集中。其他欄位皆以數字1開始編號。綁定比結果集中列數更高的欄位會造成錯誤；此錯誤直到結果集建立後才可偵測，因此由 SQLFetch 回傳，而非 SQLBindCol。
綁定於欄位的變數的 C 資料型態、位址與位元組長度。若指定 C 資料型別，而該欄位的 SQL 資料型別無法轉換，則屬錯誤;此錯誤可能要等到結果集建立後才會被偵測，因此由 SQLFetch 回傳，而非 SQLBindCol。有關支援的轉換清單，請參閱附錄D：資料型別中的「將資料從SQL轉換為C資料型別」。關於位元組長度的資訊，請參見資料緩衝區長度。
長度/指示緩衝區的位址。長度/指示緩衝區是可選的。它用來回傳二進位或字元資料的位元組長度，或在資料為 NULL 時回傳 SQL_NULL_DATA。欲了解更多資訊，請參閱使用長度/指示值。

當呼叫 SQLBindCol 時，驅動程式會將此資訊與該語句關聯起來。當每一列資料被擷取時，會利用這些資訊將每一欄的資料放入綁定的應用程式變數中。

例如，以下程式碼將變數綁定到 SalesPerson 和 CustID 欄位。欄位資料會以 SalesPerson 和 CustID 回傳。由於 SalesPerson 是字元緩衝區，應用程式會指定其位元組長度（11），讓驅動程式判斷是否要截斷資料。回傳標題的位元組長度，或是否為 NULL，會在 SalesPersonLenOrInd 中回傳。

由於 CustID 是整數變數且長度固定，因此無需指定其位元組長度;驅動程式假設它是 sizeof（SQLUINTEGER）。回傳的客戶 ID 資料的位元組長度，或是否為 NULL，會在 CustIDInd 中回傳。請注意，應用程式只關心薪資是否為 NULL，因為位元組長度總是 sizeof（SQLUINTEGER）。

SQLCHAR       SalesPerson[11];  
SQLUINTEGER   CustID;  
SQLINTEGER    SalesPersonLenOrInd, CustIDInd;  
SQLRETURN     rc;  
SQLHSTMT      hstmt;  
  
// Bind SalesPerson to the SalesPerson column and CustID to the   
// CustID column.  
SQLBindCol(hstmt, 1, SQL_C_CHAR, SalesPerson, sizeof(SalesPerson),  
            &SalesPersonLenOrInd);  
SQLBindCol(hstmt, 2, SQL_C_ULONG, &CustID, 0, &CustIDInd);  
  
// Execute a statement to get the sales person/customer of all orders.  
SQLExecDirect(hstmt, "SELECT SalesPerson, CustID FROM Orders ORDER BY SalesPerson",  
               SQL_NTS);  
  
// Fetch and print the data. Print "NULL" if the data is NULL. Code to   
// check if rc equals SQL_ERROR or SQL_SUCCESS_WITH_INFO not shown.  
while ((rc = SQLFetch(hstmt)) != SQL_NO_DATA) {  
   if (SalesPersonLenOrInd == SQL_NULL_DATA)   
            printf("NULL                     ");  
   else   
            printf("%10s   ", SalesPerson);  
   if (CustIDInd == SQL_NULL_DATA)   
         printf("NULL\n");  
   else   
            printf("%d\n", CustID);  
}  
  
// Close the cursor.  
SQLCloseCursor(hstmt);

以下程式碼執行使用者輸入的 SELECT 語句，並列印結果集中的每一列資料。由於應用程式無法預測由 SELECT 陳述式所產生的結果集形狀，因此無法像前述範例中那樣，將硬編碼變數綁定到結果集。應用程式會為該列的每一欄分配一個緩衝區來存放資料，以及一個長度/指示緩衝區。對於每一欄，它會計算該欄記憶體起始點的偏移量，並調整此偏移量，使該欄的資料及長度/指示緩衝區從對齊邊界開始。接著它會將從偏移位置開始的記憶體綁定到列。從驅動程式的角度來看，這個記憶體的位址與前述變數綁定的位址無法區分。欲了解更多關於對齊的資訊，請參見對齊。

// This application allocates a buffer at run time. For each column, this   
// buffer contains memory for the column's data and length/indicator.   
// For example:  
//      column 1         column 2      column 3      column 4  
// <------------><---------------><-----><------------>  
//      db1   li1   db2   li2   db3   li3   db4   li4  
//      |      |      |      |      |      |      |         |  
//      _____V_____V________V_______V___V___V______V_____V_  
// |__________|__|_____________|__|___|__|__________|__|  
//  
// dbn = data buffer for column n  
// lin = length/indicator buffer for column n  
  
// Define a macro to increase the size of a buffer so that it is a   
// multiple of the alignment size. Thus, if a buffer starts on an   
// alignment boundary, it will end just before the next alignment   
// boundary. In this example, an alignment size of 4 is used because   
// this is the size of the largest data type used in the application's   
// buffer--the size of an SDWORD and of the largest default C data type   
// are both 4. If a larger data type (such as _int64) was used, it would   
// be necessary to align for that size.  
#define ALIGNSIZE 4  
#define ALIGNBUF(Length) Length % ALIGNSIZE ? \  
                  Length + ALIGNSIZE - (Length % ALIGNSIZE) : Length  
  
SQLCHAR        SelectStmt[100];  
SQLSMALLINT    NumCols, *CTypeArray, i;  
SQLINTEGER *   ColLenArray, *OffsetArray, SQLType, *DataPtr;  
SQLRETURN      rc;   
SQLHSTMT       hstmt;  
  
// Get a SELECT statement from the user and execute it.  
GetSelectStmt(SelectStmt, 100);  
SQLExecDirect(hstmt, SelectStmt, SQL_NTS);  
  
// Determine the number of result set columns. Allocate arrays to hold   
// the C type, byte length, and buffer offset to the data.  
SQLNumResultCols(hstmt, &NumCols);  
CTypeArray = (SQLSMALLINT *) malloc(NumCols * sizeof(SQLSMALLINT));  
ColLenArray = (SQLINTEGER *) malloc(NumCols * sizeof(SQLINTEGER));  
OffsetArray = (SQLINTEGER *) malloc(NumCols * sizeof(SQLINTEGER));  
  
OffsetArray[0] = 0;  
for (i = 0; i < NumCols; i++) {  
   // Determine the column's SQL type. GetDefaultCType contains a switch   
   // statement that returns the default C type for each SQL type.  
   SQLColAttribute(hstmt, ((SQLUSMALLINT) i) + 1, SQL_DESC_TYPE, NULL, 0, NULL, (SQLPOINTER) &SQLType);  
   CTypeArray[i] = GetDefaultCType(SQLType);  
  
   // Determine the column's byte length. Calculate the offset in the   
   // buffer to the data as the offset to the previous column, plus the   
   // byte length of the previous column, plus the byte length of the   
   // previous column's length/indicator buffer. Note that the byte   
   // length of the column and the length/indicator buffer are increased   
   // so that, assuming they start on an alignment boundary, they will  
   // end on the byte before the next alignment boundary. Although this   
   // might leave some holes in the buffer, it is a relatively   
   // inexpensive way to guarantee alignment.  
   SQLColAttribute(hstmt, ((SQLUSMALLINT) i)+1, SQL_DESC_OCTET_LENGTH, NULL, 0, NULL, &ColLenArray[i]);  
   ColLenArray[i] = ALIGNBUF(ColLenArray[i]);  
   if (i)  
      OffsetArray[i] = OffsetArray[i-1]+ColLenArray[i-1]+ALIGNBUF(sizeof(SQLINTEGER));  
}  
  
// Allocate the data buffer. The size of the buffer is equal to the   
// offset to the data buffer for the final column, plus the byte length   
// of the data buffer and length/indicator buffer for the last column.  
void *DataPtr = malloc(OffsetArray[NumCols - 1] +  
               ColLenArray[NumCols - 1] + ALIGNBUF(sizeof(SQLINTEGER)));  
  
// For each column, bind the address in the buffer at the start of the   
// memory allocated for that column's data and the address at the start   
// of the memory allocated for that column's length/indicator buffer.  
for (i = 0; i < NumCols; i++)  
   SQLBindCol(hstmt,  
            ((SQLUSMALLINT) i) + 1,  
            CTypeArray[i],  
            (SQLPOINTER)((SQLCHAR *)DataPtr + OffsetArray[i]),  
            ColLenArray[i],  
            (SQLINTEGER *)((SQLCHAR *)DataPtr + OffsetArray[i] + ColLenArray[i]));  
  
// Retrieve and print each row. PrintData accepts a pointer to the data,   
// its C type, and its byte length/indicator. It contains a switch   
// statement that casts and prints the data according to its type. Code   
// to check if rc equals SQL_ERROR or SQL_SUCCESS_WITH_INFO not shown.  
while ((rc = SQLFetch(hstmt)) != SQL_NO_DATA) {  
   for (i = 0; i < NumCols; i++) {  
      PrintData((SQLCHAR *)DataPtr[OffsetArray[i]], CTypeArray[i],  
               (SQLINTEGER *)((SQLCHAR *)DataPtr[OffsetArray[i] + ColLenArray[i]]));  
   }  
}  
  
// Close the cursor.  
SQLCloseCursor(hstmt);

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Last updated on 2026-01-21

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