请我在访问数据库方面需要帮助 [英] Please I need help with accessing databases

问题描述

我要求提供的材料将帮助我从使用C/C ++ i/o迁移到使用SQL Server，Oracle等数据库.我得到了一些很好的材料.但是我有
在理解某些代码时遇到问题.下面显示了一些代码示例.除了声明的代码行外，一切对我来说都是清晰易懂的.什么是fblob.什么是test_tbl.

谁能向我解释规定的代码行.

I requested for materials that will help me migrate from the use of C/C++ i/o to use of databases like sql server, oracle etc. I got some good materials. But I have
problems understanding some of the codes.Shown below some samples of the codes. Everything is clear and understandable to me except stated code line.What is fblob.What is test_tbl.

Can any one explain the stated code lines to me.

#include <stdio.h>  // for printf
#include <sqlapi.h> // main SQLAPI++ header

int main(int argc, char* argv[])
{
    SAConnection con; // connection object
    SACommand cmd;    // create command object
    
    try
    {
        // connect to database (Oracle in our example)
        con.Connect("test", "tester", "tester", SA_Oracle_Client);
        // associate a command with connection
        // connection can also be specified in SACommand constructor
        cmd.setConnection(&con);

        //I don't uderstand this line// create table
        cmd.setCommandText(
            "Create table test_tbl(fid integer, fvarchar20 varchar(20), fblob blob)");
        cmd.Execute();

        // insert value
        cmd.setCommandText(
            "Insert into test_tbl(fid, fvarchar20) values (1, 'Some string (1)')");
        cmd.Execute();

        // commit changes on success
        con.Commit();

        printf("Table created, row inserted!\n");
    }
    catch(SAException &x)
    {
        // SAConnection::Rollback()
        // can also throw an exception
        // (if a network error for example),
        // we will be ready
        try
        {
            // on error rollback changes
            con.Rollback();
        }
        catch(SAException &)
        {
        }
        // print error message
        printf("%s\n", (const char*)x.ErrText());
    }
    
    return 0;
}






#include <stdio.h>  // for printf
#include <sqlapi.h> // main SQLAPI++ header

int main(int argc, char* argv[])
{
    SAConnection con; // connection object
    SACommand cmd;    // command object
    
    try
    {
        // connect to database (Oracle in our example)
        con.Connect("test", "tester", "tester", SA_Oracle_Client);
        // associate a command with connection
        cmd.setConnection(&con);

        //(I don't understand this line)// Insert 2 rows
        cmd.setCommandText(
            "Insert into test_tbl(fid, fvarchar20) values(:1, :2)");

        // use first method of binding - param assignment
        cmd.Param(1).setAsLong() = 2;
        cmd.Param(2).setAsString() = "Some string (2)";
        // Insert first row
        cmd.Execute();

        // use second method of binding - stream binding
        cmd << (long)3 << "Some string (3)";
        // Insert second row
        cmd.Execute();

        // commit changes on success
        con.Commit();

        printf("Input parameters bound, rows inserted!\n");
    }
    catch(SAException &x)
    {
        // SAConnection::Rollback()
        // can also throw an exception
        // (if a network error for example),
        // we will be ready
        try
        {
            // on error rollback changes
            con.Rollback();
        }
        catch(SAException &)
        {
        }
        // print error message
        printf("%s\n", (const char*)x.ErrText());
    }
    
    return 0;
}

#include <stdio.h>  // for printf
#include <sqlapi.h> // main SQLAPI++ header

int main(int argc, char* argv[])
{
    SAConnection con; // connection object
    SACommand cmd(
        &con,
        "Select fid, fvarchar20 from test_tbl");    // command object
    
    try
    {
        // connect to database (Oracle in our example)
        con.Connect("test", "tester", "tester", SA_Oracle_Client);

        // Select from our test table
        cmd.Execute();
        // fetch results row by row and print results
        while(cmd.FetchNext())
        {
            printf("Row fetched: fid = %ld, fvarchar20 = '%s'\n", 
                cmd.Field("fid").asLong(),
                (const char*)cmd.Field("fvarchar20").asString());
        }

        // commit changes on success
        con.Commit();

        printf("Rows selected!\n");
    }
    catch(SAException &x)
    {
        // SAConnection::Rollback()
        // can also throw an exception
        // (if a network error for example),
        // we will be ready
        try
        {
            // on error rollback changes
            con.Rollback();
        }
        catch(SAException &)
        {
        }
        // print error message
        printf("%s\n", (const char*)x.ErrText());
    }
    
    return 0;
}


#include <stdio.h>  // for printf
#include <sqlapi.h> // main SQLAPI++ header

// forwards
void IntoFileReader(
	SAPieceType_t ePieceType,
	void *pBuf,
	unsigned int nLen,
	unsigned int nBlobSize,
	void *pAddlData);

int main(int argc, char* argv[])
{
    SAConnection con; // connection object
   
//I don't understand this line
 SACommand cmd(
        &con,
        "Select fblob from test_tbl");    // command object
    
    try
    {
        // connect to database (Oracle in our example)
        con.Connect("test", "tester", "tester", SA_Oracle_Client);

        // Usage 1. Read whole BLob(s) into internal buffers
		// Select BLob from our test table
        cmd.Execute();
        // fetch results row by row and print results
        while(cmd.FetchNext())
        {
			// after fetching a row all Long/Lob fields are automatically read into //internal buffers
			// just like other data types
			SAString s = cmd.Field("fblob").asBLob();
            printf("Size of BLob is %d bytes\n", s.GetLength());
        }

        // Usage 2. Read BLob in pieces providing user callback for BLob data processing
		// Select blob from our test table
        cmd.Execute();
		// do not automatically read this field into internal buffer (into corresponding SAField object)
		// we will provide a callback for BLob fetching after FetchNext
		cmd.Field("fblob").setLongOrLobReaderMode(SA_LongOrLobReaderManual);
        
		// fetch results row by row and print results
		SAString sFilename;
		int i = 0;
        while(cmd.FetchNext())
        {
			sFilename.Format("fblob%d.bin", ++i);
			// at that moment all fields are fetched except
			// those that set for manual retrieving
			// read them (fblob in our example) now
			if(!cmd.Field("fblob").isNull())
				cmd.Field("fblob").ReadLongOrLob(
					IntoFileReader,	// our callback to read BLob content into file
					10*1024,		// our desired piece size
					(void*)(const char*)sFilename	// additional data, filename in our example
					);
		}

        // commit changes on success
        con.Commit();

        printf("Rows with BLob field fetched!\n");
    }
    catch(SAException &x)
    {
        // SAConnection::Rollback()
        // can also throw an exception
        // (if a network error for example),
        // we will be ready
        try
        {
            // on error rollback changes
            con.Rollback();
        }
        catch(SAException &)
        {
        }
        // print error message
        printf("%s\n", (const char*)x.ErrText());
    }
    
    return 0;
}

static FILE *pFile = NULL;
static int nTotalRead;
void IntoFileReader(
	SAPieceType_t ePieceType,
	void *pBuf,
	unsigned int nLen,
	unsigned int nBlobSize,
	void *pAddlData)
{
	if(ePieceType == SA_FirstPiece || ePieceType == SA_OnePiece)
	{
		nTotalRead = 0;
		const char *sFilename = (const char *)pAddlData;
		pFile = fopen(sFilename, "wb");
		if(!pFile)
			SAException::throwUserException(-1, "Can not open file '%s' for 				writing", sFilename);
	}

	fwrite(pBuf, 1, nLen, pFile);
	nTotalRead += nLen;

	// show progress
	printf("%d bytes of %d read\n", nTotalRead, nBlobSize);

    if(ePieceType == SA_LastPiece || ePieceType == SA_OnePiece)
	{
		fclose(pFile);
		pFile = NULL;
	}
}

推荐答案

第一个代码示例使用创建表" SQL命令在Oracle数据库中创建一个名为"test_tbl"的表.该表由名为-
的三列组成

First code sample creates a table named "test_tbl" in Oracle database with "Create Table" SQL command. The table consists of three columns named-

fid, fvarchar20, fblob

与类型-

with types -

integer, varchar(20), blob

有关详细信息，请参见: Oracle命令 [ ^ ]
Blob-"Oracle支持20多种数据类型，您甚至可以定义自己的自定义数据类型.对于大型对象，Oracle提供了BLOB，CLOB和LONG RAW(已淘汰)." dba-oracle.com/t_read_blob.htm>如何读取BLOB [

For details see:Oracle commands[^]
Blob - "Oracle supports over 20 data types, and you can even define your own custom data types. For large objects, Oracle offers the BLOB, CLOB and LONG RAW (obsolete)."How to read a BLOB[^]
It then writes in first two columns of test_tbl -

1, ''Some string (1)''

每个命令都通过"Execute"执行.最后，成功就做出了改变.

Every command is executed with "Execute". And finally, changes are committed on success.

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