[poco] 访问数据库

From: https://blog.csdn.net/hl2015222050145/article/details/52335422?utm_source=blogxgwz6

poco访问数据基本步骤：
a. 创建会话(session)

b. 从DB中读写数据
c. 使用statements

d. 使用容器（Collection） (数据，集合...)
e. 使用limit限定
f. 如何使用复杂的数据类型(如何将一个C++对象映射到数据库的表

创建会话：Session create(const std::string& connectorKey, const std::string& connectionString);
参数1：SQLite；MySQL；或者ODBC，ODBC支持Oracle, SQLite, DB2, SQLServer和PostgreSQL。
参数2：就是数据库文件的路径或者连接字符

基本操作（增删查改）：

增：

   std::string aName("Peter");
   ses << "INSERT INTO 表名 VALUES(" << aName << ")", now;
   或者ses << "INSERT INTO 表名 VALUES(?)", use(aName), now;

查：

一般查找

   std::string aName;
   ses << "SELECT NAME FROM FORENAME", into(aName), now; // aName的默认值为空字符串
   ses << "SELECT NAME FROM FORENAME", into(aName, "default"), now;

匹配查找先into后use 且不能使用常量如:4

   std::string aName;
   std::string match("Peter")
   ses << "SELECT NAME FROM FORENAME WHERE NAME=?", into(aName), use(match), now;
   poco_assert (aName == match);

多字节匹配查找

   std::string firstName("Peter";
   std::string lastName("Junior");
   int age = 0;
   ses << "INSERT INTO PERSON VALUES (?, ?, ?)", use(firstName), use(lastName), use(age), now;
   ses << "SELECT (firstname, lastname, age) FROM Person", into(firstName), into(lastName), into(age), now;

into语句允许定义一个默认值,主要针对数据库查询里面包含空时候

ses << "SELECT (firstname, lastname, age) FROM Person", into(firstName), into(lastName), into(age, -1), now;

改：
session << "update Person set Address = ? WHERE Name= ?", use(addr),use(name),now;

使用Statements：

用于保存SQl语句，使得多次查询。

std::string aName("Peter");
Statement stmt = ( ses << "INSERT INTO FORENAME VALUES(?)", use(aName) );//括号必须

或者
Statement stmt(ses);
stmt << "INSERT INTO FORENAME VALUES(?)", use(aName);
eg：

   std::string aName("Peter");
   Statement stmt = ( ses << "INSERT INTO FORENAME VALUES(?)", use(aName) );
   .
   .
   stmt.execute();
   poco_assert (stmt.done());

容器使用：

默认支持以下容器类：
a. vector(数组): 无特别要求
b. set: 类型需要支持<操作符，注意：重复的key/value将忽略
c. multiset: 需支持<操作符
d. map: 需支持()操作符，并且按key返回对象，注意：重复的key/value将忽略
e. multimap: 需支持()操作符，并按key返加对象.

std::vector<std::string> names;
ses << "SELECT NAME FROM FORENAME", into(names), now;

Limit限定：针对查询结果的个数限定

       std::vector<std::string> names;
       ses << "SELECT NAME FROM FORENAME", into(names), limit(50), now;
以上代码将只返回50行的数据。（当然也可能什么都不返回）,并追加到 names这个容量中。如果想确保50行记录返回，需要设置limit的第二参数为true(默认为false):
       std::vector<std::string> names;
       ses << "SELECT NAME FROM FORENAME", into(names), limit(50, true), now;
       Statement stmt = (ses << "SELECT NAME FROM FORENAME", into(aName), lowerLimit(1), upperLimit(10));//限制范围
       Statement stmt = (ses << "SELECT NAME FROM FORENAME", into(aName), range(1,10));

类作为保存变量：这个就是将自定义类作为容器查询

       namespace Poco
       {
           namespace Data
               {
                   template <>
                   class TypeHandler<class Person>
                       {
                           .....
                       }
               }
       }

记录集(RecordSet)

a. 遍历数据表中所有的行与列
b. 获取各列的元信息，比如名称，类型，长度等.

           Statement select(session);
           select << "SELECT * FROM Person";
           select.execute();
           RecordSet rs(select);

遍历结果：

           bool more = rs.moveFirst();
           while (more)
           {
               for (std::size_t col = 0; col < cols; ++col)
               {
                   std::cout << rs[col].convert<std::string>() << " ";
               }
               std::cout << std::endl;
               more = rs.moveNext();
           }

Tuples：在数据库中的列类型已知的情阅下，Poco::Tuple以及Tuple数组提供了更简便的方法获取数据。

           typedef Poco::Tuple<std::string, std::string, int> Person;
           typedef std::vector<Person> People;

           People people;
           people.push_back(Person("Bart Simpson", "Springfield", 12));
           people.push_back(Person("Lisa Simpson", "Springfield", 10));

           Statement insert(session);
           insert << "INSERT INTO Person VALUES(?, ?, ?)",
           use(people), now;

           当然，tuple也可以用于查询:
           Statement select(session);
           select << "SELECT Name, Address, Age FROM Person", into(people), now;

           for (People::const_iterator it = people.begin(); it != people.end(); ++it)
           {
               std::cout << "Name: " << it->get<0>() <<
               ", Address: " << it->get<1>() <<
               ", Age: " << it->get<2>() <<std::endl;
           }

参考链接：http://pocoproject.org/docs/00200-DataUserManual.html

测试代码：

#include "Poco/Data/Session.h"
#include "Poco/Data/MySQL/Connector.h"
#include <vector>
#include <iostream>

using namespace Poco::Data::Keywords;
using Poco::Data::Session;
using Poco::Data::Statement;

struct Person
{
   std::string name;
   std::string address;
   int age;
};

int main(int argc, char** argv)
{
   Poco::Data::MySQL::Connector::registerConnector();

   // 创建 session
   Session session("MySQL", "host=127.0.0.1;port=3306;db=demo;user=admin;password=admin;compress=true;auto-reconnect=true");

   // 删除已存在的表
   session << "DROP TABLE IF EXISTS Person", now;

   // 创建新表
   session << "CREATE TABLE Person (Name char(20),Address char(30) ,Age integer)", now;//varchar 会报错
   // 插入数据
   Person person =
   {
       "Bart Simpson",
       "Springfield",
       12
   };

   Statement insert(session);
   insert << "INSERT INTO Person VALUES(?, ?, ?)",
       use(person.name),
       use(person.address),
       use(person.age);

   insert.execute();

   person.name = "Lisa Simpson";
   person.address = "Springfield";
   person.age = 10;

   insert.execute();

   //查询数据
   Statement select(session);
   select << "select Name, Address, Age FROM Person",
       into(person.name),
       into(person.address),
       into(person.age),
       range(0, 1); // 只查询前两行

   while (!select.done())
   {
       select.execute();
       std::cout << person.name << " " << person.address << " " << person.age << std::endl;
   }

   std::string addr = "hubeilichuan";
   std::string name = "Lisa Simpson";

   session << "update Person set Address = ? WHERE Name= ?", use(addr),use(name),now;
   // 另一种查询方式
   std::vector<std::string> names;
   session << "SELECT Address FROM Person",
       into(names),
       now;

   for (std::vector<std::string>::const_iterator it = names.begin(); it != names.end(); ++it)
   {
       std::cout << *it << std::endl;
   }
   getchar();
   return 0;
}