使用 SQLBindCol

项目
08/30/2023

应用程序通过调用 SQLBindCol 绑定列。此函数一次绑定一列。通过此举，应用程序可以指定以下内容：

列号。第 0 列是书签列；该列不包括在某些结果集中。其他列用从 1 开始的数字编号。与结果集中存在的列相比，绑定编号更高的列是错误的；在创建结果集之前无法检测到此错误，因此由 SQLFetch 返回此错误，而不是 SQLBindCol。
绑定到列变量的 C 数据类型、地址和字节长度。如果列的 SQL 数据类型无法转换为指定的 C 数据类型，则会导致错误；在创建结果集之前可能无法检测到此错误，因此它由 SQLFetch 返回，而不是 SQLBindCol。有关受支持的转换列表，请参阅“附录 D：数据类型”中的将数据从 SQL 转换为 C 数据类型。有关字节长度的信息，请参阅数据缓冲区长度。
长度/指示器缓冲区的地址。长度/指示器缓冲区是可选的。它用于返回二进制或字符数据的字节长度，如果数据为 NULL，则返回 SQL_NULL_DATA。有关详细信息，请参阅使用长度/指示器值。

调用 SQLBindCol 时，驱动程序会将此信息与语句相关联。提取每行数据时，使用该信息将每列的数据放在绑定的应用程序变量中。

例如，以下代码将变量绑定到 SalesPerson 和 CustID 列。列的数据将在 SalesPerson 和 CustID 中返回。由于 SalesPerson 是字符缓冲区，因此应用程序会指定其字节长度 (11)，以便驱动程序可以确定是否截断数据。返回标题的字节长度，或是否为 NULL，都将在 SalesPersonLenOrInd 中返回。

由于 CustID 是一个整数变量并且具有固定长度，因此无需指定其字节长度;驱动程序假定其大小为 SQLUINTEGER。返回的客户 ID 数据的字节长度，或者是否为 NULL，都将在 CustIDInd 中返回。请注意，应用程序仅对工资是否为 NULL 感兴趣，因为字节长度始终为 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);

以下代码执行用户输入的 SELECT 语句，并输出结果集中的每一行数据。由于应用程序无法预测 SELECT 语句创建的结果集的形状，因此它无法将硬编码变量绑定到上一示例中的结果集。相反，应用程序会为该行中的每一列分配一个保存数据的缓冲区和一个长度/指示器缓冲区。对于每一列，它会计算到该列内存开始处的偏移量，并调整该偏移量，以便列的数据和长度/指示器缓冲区从对齐边界开始。然后，它将从偏移量处开始的内存绑定到列。从驱动程序的角度来看，这个内存的地址与前面示例中绑定的变量的地址是无法区分的。有关对齐的详细信息，请参阅对齐。

// 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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