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📜  PostgreSQL-C / C++接口

📅  最后修改于: 2020-11-26 06:39:32             🧑  作者: Mango

本教程将使用libpqxx库，它是PostgreSQL的官方C++客户端API。 libpqxx的源代码在BSD许可下可用，因此您可以免费下载，将其传递给他人，进行更改，出售，将其包含在自己的代码中，并与选择的任何人共享您的更改。

安装

可以从下载Libpqxx链接下载libpqxx的最新版本。因此，下载最新版本并按照以下步骤操作：

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++ 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

您可以使用以下命令启动/重新启动postgres服务器，以防其未运行-

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

C / C++接口API

以下是重要的接口例程，可以满足您从C / C++程序使用PostgreSQL数据库的要求。如果您正在寻找更复杂的应用程序，则可以查阅libpqxx官方文档，也可以使用市售的API。

S. No. API & Description
1

pqxx::connection C( const std::string & dbstring )

This is a typedef which will be used to connect to the database. Here, dbstring provides required parameters to connect to the datbase, for example dbname = testdb user = postgres password=pass123 hostaddr=127.0.0.1 port=5432.

If connection is setup successfully then it creates C with connection object which provides various useful function public function.
2

C.is_open()

The method is_open() is a public method of connection object and returns boolean value. If connection is active, then this method returns true otherwise it returns false.
3

C.disconnect()

This method is used to disconnect an opened database connection.
4

pqxx::work W( C )

This is a typedef which will be used to create a transactional object using connection C, which ultimately will be used to execute SQL statements in transactional mode.

If transaction object gets created successfully, then it is assigned to variable W which will be used to access public methods related to transactional object.
5

W.exec(const std::string & sql)

This public method from transactional object will be used to execute SQL statement.
6

W.commit()

This public method from transactional object will be used to commit the transaction.
7

W.abort()

This public method from transactional object will be used to rollback the transaction.
8

pqxx::nontransaction N( C )

This is a typedef which will be used to create a non-transactional object using connection C, which ultimately will be used to execute SQL statements in non-transactional mode.

If transaction object gets created successfully, then it is assigned to variable N which will be used to access public methods related to non-transactional object.
9

N.exec(const std::string & sql)

This public method from non-transactional object will be used to execute SQL statement and returns a result object which is actually an interator holding all the returned records.

连接到数据库

以下C代码段显示了如何连接到本地计算机上运行的端口5432上的现有数据库。在这里，我使用反斜杠\进行行继续。

#include 
#include  

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，让我们编译并运行上述程序，以连接到我们的数据库testdb ，该数据库已在您的模式中提供，可以使用用户postgres和密码pass123进行访问。

您可以根据数据库设置使用用户ID和密码。切记按给定顺序保留-lpqxx和-lpq！否则，链接器将对缺少以“ PQ”开头的名称的功能感到痛苦。

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

建立表格

以下C代码段将用于在先前创建的数据库中创建表-

#include 
#include  

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;
}

编译并执行上述给定程序后，它将在您的testdb数据库中创建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例中创建的COMPANY表中创建记录-

#include 
#include  

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;
}

编译并执行上述给定程序时，它将在COMPANY表中创建给定记录，并显示以下两行-

Opened database successfully: testdb
Records created successfully

选择操作

以下C代码段显示了如何从上面示例中创建的COMPANY表中获取并显示记录-

#include 
#include  

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() << endl;
         cout << "Name = " << c[1].as() << endl;
         cout << "Age = " << c[2].as() << endl;
         cout << "Address = " << c[3].as() << endl;
         cout << "Salary = " << c[4].as() << endl;
      }
      cout << "Operation done successfully" << endl;
      C.disconnect ();
   } catch (const std::exception &e) {
      cerr << e.what() << std::endl;
      return 1;
   }

   return 0;
}

编译并执行上述给定程序时，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

UPDATE操作

以下C代码段显示了如何使用UPDATE语句更新任何记录，然后从COMPANY表中获取并显示更新的记录-

#include 
#include  

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() << endl;
         cout << "Name = " << c[1].as() << endl;
         cout << "Age = " << c[2].as() << endl;
         cout << "Address = " << c[3].as() << endl;
         cout << "Salary = " << c[4].as() << endl;
      }
      cout << "Operation done successfully" << endl;
      C.disconnect ();
   } catch (const std::exception &e) {
      cerr << e.what() << std::endl;
      return 1;
   }

   return 0;
}

编译并执行上述给定程序时，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语句删除任何记录，然后从COMPANY表中获取并显示剩余记录-

#include 
#include  

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() << endl;
         cout << "Name = " << c[1].as() << endl;
         cout << "Age = " << c[2].as() << endl;
         cout << "Address = " << c[3].as() << endl;
         cout << "Salary = " << c[4].as() << endl;
      }
      cout << "Operation done successfully" << endl;
      C.disconnect ();
   } catch (const std::exception &e) {
      cerr << e.what() << std::endl;
      return 1;
   }

   return 0;
}

编译并执行上述给定程序时，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