Mybatis学习笔记10 - 动态sql之if判断

示例代码：

接口定义：
package com.mybatis.dao;

import com.mybatis.bean.Employee;

import java.util.List;

public interface EmployeeMapper {
    //携带了哪个字段查询条件就带上这个字段的值
    public List<Employee> getEmpsByConditionIf(Employee employee);

    public List<Employee> getEmpsByConditionIfWhere(Employee employee);
}

mapper定义：
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mybatis.dao.EmployeeMapper">
    <!-- 查询员工，要求，携带了哪个字段查询条件就带上这个字段的值 -->
    <!-- public List<Employee> getEmpsByConditionIf(Employee employee); -->
    <select id="getEmpsByConditionIf" resultType="com.mybatis.bean.Employee">
        select * from tbl_employee
        where 1=1
        <if test="id!=null">
            and id=#{id}
        </if>
        <if test="lastName!=null && lastName!=''">
            and last_name like #{lastName}
        </if>
        <if test="email!=null and email.trim()!=""">
            and email=#{email}
        </if>
        <!-- ognl会进行字符串与数字的转换判断  "0"==0 -->
        <if test="gender==0 or gender==1">
            and gender=#{gender}
        </if>
    </select>

    <!--public List<Employee> getEmpsByConditionIfWhere(Employee employee);-->
    <select id="getEmpsByConditionIfWhere" resultType="com.mybatis.bean.Employee">
        select * from tbl_employee
        <where>
            <if test="id!=null">
                id=#{id}
            </if>
            <if test="lastName!=null && lastName!=''">
                and last_name like #{lastName}
            </if>
            <if test="email!=null and email.trim()!=""">
                and email=#{email}
            </if>
            <!-- ognl会进行字符串与数字的转换判断  "0"==0 -->
            <if test="gender==0 or gender==1">
                and gender=#{gender}
            </if>
        </where>
    </select>
</mapper>

测试代码：
package com.mybatis.demo;

import com.mybatis.bean.Employee;
import com.mybatis.dao.EmployeeMapper;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

public class MyTest {
    public SqlSessionFactory getSqlSessionFactory() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        return new SqlSessionFactoryBuilder().build(inputStream);
    }

    @Test
    public void test() throws IOException {
        SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
        SqlSession openSession = sqlSessionFactory.openSession(true);
        try {
            EmployeeMapper mapper = openSession.getMapper(EmployeeMapper.class);
            //查询的时候如果某些条件没带可能sql拼装会有问题
            //1、给where后面加上1=1，以后的条件都and xxx.
            //2、mybatis使用where标签来将所有的查询条件包括在内。mybatis就会将where标签中拼装的sql，多出来的and或者or去掉
            //where只会去掉第一个多出来的and或者or。
            List<Employee> emps = mapper.getEmpsByConditionIf(new Employee(null, "jetty", "jetty@126.com", 1));
            for (Employee emp : emps) {
                System.out.println(emp);
            }
            System.out.println("-------------------");
            List<Employee> employees = mapper.getEmpsByConditionIfWhere(new Employee(null, "jetty", "jetty@126.com", 1));
            for (Employee emp : employees) {
                System.out.println(emp);
            }
        } finally {
            openSession.close();
        }
    }
}