使用 SQLBindCol

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08/21/2023

應用程式藉由電話撥接 SQLBindCol.來繫結資料行。此函式一次只能繫結一個資料行。透過此函式，應用程式會指定以下內容：

資料行編號。第 0 欄為錨點資料行；此資料行未包含在部分結果。所有其他資料行都是從數字 1 開始編號。繫結的資料行編號高於結果集的資料行是錯誤的；在建立結果集之前無法偵測到此錯誤，因此 SQLFetch 會傳回此錯誤，而不是由 SQLBindCol 傳回。
變數的 C 資料類型、位址與位元組長度繫結至資料行。無法將資料行的 SQL 資料類型轉換為指定 C 資料類型是錯誤的；在建立結果集之前，可能不會偵測到此錯誤，因此 SQLFetch 會傳回此錯誤，而不是由 SQLBindCol 傳回。如需支援的轉換清單，請參閱附錄 D：資料類型的將資料從 SQL 轉換成 C 資料類型。如需位元組長度的相關資訊，請參閱資料緩衝區長度。
長度/指標緩衝區的位址。長度/指標緩衝區為選用。用於傳回二進位或字元資料的位元組長度，如果資料為 Null，則會傳回 SQL_Null_DATA。如需詳細資訊，請參閱使用長度/指標值。

當呼叫 SQLBindCol 時，驅動程式會將此資訊與語句產生關聯。當擷取每個資料列時，它使用該資訊將每個資料行的資料放置繫結於應用程式變數。

例如，下列程式碼會將變數繫結至 SalesPerson 及 CustID 資料行。資料行的資料將會在 SalesPerson 及 CustID 傳回。由於 SalesPerson 是字元緩衝區，因此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。傳回標題的位元組長度，或是否為 NULL，都會在 SalesPersonLenOrInd 傳回。

由於 CustID 是整數變數且長度固定，因此無須指定其位元組長度；驅動程式假設為 sizeof(SQLUINTEGER)。傳回客戶識別碼資料的位元組長度，或是否為 NULL，都會在 CustIDInd 傳回。請注意，應用程式只對薪資是否為 Null 感興趣，因為位元組長度一律為 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);

下列程式碼會執行使用者輸入的 SELECT 語句，並列印結果集的每個資料列。由於應用程式無法預測 SELECT 語句所建立結果集的圖形，因此無法如上述範例所示，將硬式編碼變數繫結至結果集。應用程式反而會為該資料列的每個資料行設定保存資料的緩衝區以及長度/指標緩衝區。對於每個資料行，它會計算到資料行記憶體開始的位移，並調整此位移，讓資料行的資料及長度/指標緩衝區從對齊界限開始。然後，它會從位移開始將記憶體繫結至資料行。從驅動程式的角度來看，此記憶體的位址與上述範例繫結的變數位址無法區分。如需對齊的詳細資訊，請參閱：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);

使用 SQLBindCol

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