C/C++连接PostgreSQL数据库

2019年10月3日 197次阅读

本教程将使用libpqxx库，它是PostgreSQL的官方C++客户端API。 libpqxx的源代码可以在BSD许可证下使用，因此您可以免费下载它，将其传递给其他人，更改它，销售，将其包含在您自己的代码中，并与任何人分享您的更改/修改。

安装

最新版本的libpqxx可从链接下载：Libpqxx下载。所以下载最新版本，并按照以下步骤：

wget http://pqxx.org/download/software/libpqxx/libpqxx-4.0.tar.gz
tar xvfz libpqxx-4.0.tar.gz
cd libpqxx-4.0
./configure
make
make install

在开始使用C/C++的PostgreSQL接口之前，请在PostgreSQL安装目录中找到pg_hba.conf文件，并添加以下行：

# IPv4 local connections:
host    all         all         127.0.0.1/32          md5

可以启动/重新启动postgres服务器，使用以下命令运行：

[root@host]# service postgresql restart
Stopping postgresql service:                               [  OK  ]
Starting postgresql service:                               [  OK  ]

C/C++连接到PostgreSQL数据库

以下C代码段显示如何连接到端口5432上本地机器上运行的现有数据库。在这里，我使用反斜杠\行继续。

#include <iostream>
#include <pqxx/pqxx> 

using namespace std;
using namespace pqxx;

int main(int argc, char* argv[])
{
   try{
      connection C("dbname=testdb user=postgres password=cohondob \
      hostaddr=127.0.0.1 port=5432");
      if (C.is_open()) {
         cout << "Opened database successfully: " << C.dbname() << endl;
      } else {
         cout << "Can't open database" << endl;
         return 1;
      }
      C.disconnect ();
   }catch (const std::exception &e){
      cerr << e.what() << std::endl;
      return 1;
   }
}

现在，我们编译并运行上面的程序来连接到数据库testdb，它已经在你的架构中可用，可以使用用户postgres和密码为：pass123进行访问。您可以根据数据库设置使用用户名和密码。记住保持-lpqxx和-lpq在给定的顺序！否则，链接器将抱怨关于缺少以“PQ”开头的名称的函数。

$g++ test.cpp -lpqxx -lpq
$./a.out
Opened database successfully: testdb

创建表

以下C代码段将用于在之前创建的数据库(testdb)中创建一个表：

#include <iostream>
#include <pqxx/pqxx> 

using namespace std;
using namespace pqxx;

int main(int argc, char* argv[])
{
   char * sql;

   try{
      connection C("dbname=testdb user=postgres password=cohondob \
      hostaddr=127.0.0.1 port=5432");
      if (C.is_open()) {
         cout << "Opened database successfully: " << C.dbname() << endl;
      } else {
         cout << "Can't open database" << endl;
         return 1;
      }
      /* Create SQL statement */
      sql = "CREATE TABLE COMPANY("  \
      "ID INT PRIMARY KEY     NOT NULL," \
      "NAME           TEXT    NOT NULL," \
      "AGE            INT     NOT NULL," \
      "ADDRESS        CHAR(50)," \
      "SALARY         REAL );";

      /* Create a transactional object. */
      work W(C);

      /* Execute SQL query */
      W.exec( sql );
      W.commit();
      cout << "Table created successfully" << endl;
      C.disconnect ();
   }catch (const std::exception &e){
      cerr << e.what() << std::endl;
      return 1;
   }

   return 0;
}

当编译和执行上述程序时，它将在testdb数据库中创建一张COMPANY表，并显示以下语句：

Opened database successfully: testdb
Table created successfully

插入操作

以下C代码段显示了如何在上述示例中创建的COMPANY表中创建记录：

#include <iostream>
#include <pqxx/pqxx> 

using namespace std;
using namespace pqxx;

int main(int argc, char* argv[])
{
   char * sql;

   try{
      connection C("dbname=testdb user=postgres password=cohondob \
      hostaddr=127.0.0.1 port=5432");
      if (C.is_open()) {
         cout << "Opened database successfully: " << C.dbname() << endl;
      } else {
         cout << "Can't open database" << endl;
         return 1;
      }
      /* Create SQL statement */
      sql = "INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) "  \
      "VALUES (1, 'Paul', 32, 'California', 20000.00 ); " \
      "INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) "  \
      "VALUES (2, 'Allen', 25, 'Texas', 15000.00 ); "     \
      "INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY)" \
      "VALUES (3, 'Teddy', 23, 'Norway', 20000.00 );" \
      "INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY)" \
      "VALUES (4, 'Mark', 25, 'Rich-Mond ', 65000.00 );";

      /* Create a transactional object. */
      work W(C);

      /* Execute SQL query */
      W.exec( sql );
      W.commit();
      cout << "Records created successfully" << endl;
      C.disconnect ();
   }catch (const std::exception &e){
      cerr << e.what() << std::endl;
      return 1;
   }

   return 0;
}

当上述程序被编译和执行时，它将在COMPANY表中创建给定的记录，并显示以下两行：

Opened database successfully: testdb
Records created successfully

SELECT操作

以下C代码段显示了如何从上述示例中创建的COMPANY表中获取和显示记录：

#include <iostream>
#include <pqxx/pqxx> 

using namespace std;
using namespace pqxx;

int main(int argc, char* argv[])
{
   char * sql;

   try{
      connection C("dbname=testdb user=postgres password=cohondob \
      hostaddr=127.0.0.1 port=5432");
      if (C.is_open()) {
         cout << "Opened database successfully: " << C.dbname() << endl;
      } else {
         cout << "Can't open database" << endl;
         return 1;
      }
      /* Create SQL statement */
      sql = "SELECT * from COMPANY";

      /* Create a non-transactional object. */
      nontransaction N(C);

      /* Execute SQL query */
      result R( N.exec( sql ));

      /* List down all the records */
      for (result::const_iterator c = R.begin(); c != R.end(); ++c) {
         cout << "ID = " << c[0].as<int>() << endl;
         cout << "Name = " << c[1].as<string>() << endl;
         cout << "Age = " << c[2].as<int>() << endl;
         cout << "Address = " << c[3].as<string>() << endl;
         cout << "Salary = " << c[4].as<float>() << endl;
      }
      cout << "Operation done successfully" << endl;
      C.disconnect ();
   }catch (const std::exception &e){
      cerr << e.what() << std::endl;
      return 1;
   }

   return 0;
}

当上述程序被编译和执行时，将产生以下结果：

Opened database successfully: testdb
ID = 1
Name = Paul
Age = 32
Address = California
Salary = 20000
ID = 2
Name = Allen
Age = 25
Address = Texas
Salary = 15000
ID = 3
Name = Teddy
Age = 23
Address = Norway
Salary = 20000
ID = 4
Name = Mark
Age = 25
Address = Rich-Mond
Salary = 65000
Operation done successfully

更新操作

以下C代码段显示了如何使用UPDATE语句来更新指定记录，然后从COMPANY表中获取并显示更新的记录：

#include <iostream>
#include <pqxx/pqxx> 

using namespace std;
using namespace pqxx;

int main(int argc, char* argv[])
{
   char * sql;

   try{
      connection C("dbname=testdb user=postgres password=cohondob \
      hostaddr=127.0.0.1 port=5432");
      if (C.is_open()) {
         cout << "Opened database successfully: " << C.dbname() << endl;
      } else {
         cout << "Can't open database" << endl;
         return 1;
      }

      /* Create a transactional object. */
      work W(C);
      /* Create  SQL UPDATE statement */
      sql = "UPDATE COMPANY set SALARY = 25000.00 where ID=1";
      /* Execute SQL query */
      W.exec( sql );
      W.commit();
      cout << "Records updated successfully" << endl;

      /* Create SQL SELECT statement */
      sql = "SELECT * from COMPANY";

      /* Create a non-transactional object. */
      nontransaction N(C);

      /* Execute SQL query */
      result R( N.exec( sql ));

      /* List down all the records */
      for (result::const_iterator c = R.begin(); c != R.end(); ++c) {
         cout << "ID = " << c[0].as<int>() << endl;
         cout << "Name = " << c[1].as<string>() << endl;
         cout << "Age = " << c[2].as<int>() << endl;
         cout << "Address = " << c[3].as<string>() << endl;
         cout << "Salary = " << c[4].as<float>() << endl;
      }
      cout << "Operation done successfully" << endl;
      C.disconnect ();
   }catch (const std::exception &e){
      cerr << e.what() << std::endl;
      return 1;
   }

   return 0;
}

当上述程序被编译和执行时，将产生以下结果：

Opened database successfully: testdb
Records updated successfully
ID = 2
Name = Allen
Age = 25
Address = Texas
Salary = 15000
ID = 3
Name = Teddy
Age = 23
Address = Norway
Salary = 20000
ID = 4
Name = Mark
Age = 25
Address = Rich-Mond
Salary = 65000
ID = 1
Name = Paul
Age = 32
Address = California
Salary = 25000
Operation done successfully

删除操作

以下C代码段显示了如何使用DELETE语句删除指定记录，然后再COMPANY表中获取并显示剩余的记录：

#include <iostream>
#include <pqxx/pqxx> 

using namespace std;
using namespace pqxx;

int main(int argc, char* argv[])
{
   char * sql;

   try{
      connection C("dbname=testdb user=postgres password=cohondob \
      hostaddr=127.0.0.1 port=5432");
      if (C.is_open()) {
         cout << "Opened database successfully: " << C.dbname() << endl;
      } else {
         cout << "Can't open database" << endl;
         return 1;
      }

      /* Create a transactional object. */
      work W(C);
      /* Create  SQL DELETE statement */
      sql = "DELETE from COMPANY where ID = 2";
      /* Execute SQL query */
      W.exec( sql );
      W.commit();
      cout << "Records deleted successfully" << endl;

      /* Create SQL SELECT statement */
      sql = "SELECT * from COMPANY";

      /* Create a non-transactional object. */
      nontransaction N(C);

      /* Execute SQL query */
      result R( N.exec( sql ));

      /* List down all the records */
      for (result::const_iterator c = R.begin(); c != R.end(); ++c) {
         cout << "ID = " << c[0].as<int>() << endl;
         cout << "Name = " << c[1].as<string>() << endl;
         cout << "Age = " << c[2].as<int>() << endl;
         cout << "Address = " << c[3].as<string>() << endl;
         cout << "Salary = " << c[4].as<float>() << endl;
      }
      cout << "Operation done successfully" << endl;
      C.disconnect ();
   }catch (const std::exception &e){
      cerr << e.what() << std::endl;
      return 1;
   }

   return 0;
}

当上述程序被编译和执行时，将产生以下结果：

Opened database successfully: testdb
Records deleted successfully
ID = 3
Name = Teddy
Age = 23
Address = Norway
Salary = 20000
ID = 4
Name = Mark
Age = 25
Address = Rich-Mond
Salary = 65000
ID = 1
Name = Paul
Age = 32
Address = California
Salary = 25000
Operation done successfully