第三节 QBC查询

1、概念

QBC：Query By Criteria
03.QBC查询 - 图1

QBC查询最大的特点就是将SQL语句中的WHERE子句进行了组件化的封装，让我们可以通过调用Criteria对象的方法自由的拼装查询条件。

2、例子

    // 1.创建EmployeeExample对象<br />        EmployeeExample example = new EmployeeExample();
    // 2.通过example对象创建Criteria对象<br />        EmployeeExample.Criteria criteria01 = example.createCriteria();<br />        EmployeeExample.Criteria criteria02 = example.or();
    // 3.在Criteria对象中封装查询条件<br />        criteria01<br />            .andEmpAgeBetween(9, 99)<br />            .andEmpNameLike("%o%")<br />            .andEmpGenderEqualTo("male")<br />            .andEmpSalaryGreaterThan(500.55);
    criteria02<br />                .andEmpAgeBetween(9, 99)<br />                .andEmpNameLike("%o%")<br />                .andEmpGenderEqualTo("male")<br />                .andEmpSalaryGreaterThan(500.55);
    SqlSession session = factory.openSession();
    EmployeeMapper mapper = session.getMapper(EmployeeMapper.class);
    // 4.基于Criteria对象进行查询<br />        List<Employee> employeeList = mapper.selectByExample(example);
    for (Employee employee : employeeList) {<br />            System.out.println("employee = " + employee);<br />        }
    session.close();
    // 最终SQL的效果：<br />        // WHERE ( emp_age between ? and ? and emp_name like ? and emp_gender = ? and emp_salary > ? ) or( emp_age between ? and ? and emp_name like ? and emp_gender = ? and emp_salary > ? )