mybatis动态sql之初探学习where、if、trim标签

EmployeeMapperDynamicSql.java

package com.gong.mybatis.mapper;

import java.util.List;
 java.util.Map;

 org.apache.ibatis.annotations.MapKey;

 com.gong.mybatis.bean.Employee;

public interface EmployeeMapperDynamicSql {
    public List<Employee> getEmpByConditionIf(Employee employee);
}

EmployeeMapperDynamicSql.xml

<?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.gong.mybatis.mapper.EmployeeMapperDynamicSql">
    <!-- 查询，要查那个就带上那个条件 -->
    select id="getEmpByConditionIf" resultType="com.gong.mybatis.bean.Employee">
        select * from tbl_employee 
        where>
            if test="id!=null" 取值是从参数里面取 -->
                id=#{id}
            </if 遇见特殊符号应使用转义字符 -->
            ="lastName!=null &amp;&amp; lastName!=&quot;&quot;"
                and last_name like #{lastName}
            ="email!=null and email.trim()!=&quot;&quot;"
                and email=#{email}
             ognl会进行字符串和数字进行转换 ="gender==0 or gender==1"
                and gender=#{gender}
            >
        select>
mapper>

在TestMybatis3.java中进行测试：

 com.gong.mybatis.test;

 java.io.IOException;
 java.io.InputStream;
 org.apache.ibatis.io.Resources;
 org.apache.ibatis.session.SqlSession;
 org.apache.ibatis.session.SqlSessionFactory;
 org.apache.ibatis.session.SqlSessionFactoryBuilder;
 org.junit.Test;

 com.gong.mybatis.bean.Employee;
 com.gong.mybatis.mapper.EmployeeMapperDynamicSql;

class TestMybatis3 {
    
    public SqlSessionFactory getSqlSessionFactory() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream is = Resources.getResourceAsStream(resource);
        return new SqlSessionFactoryBuilder().build(is);
    }

    @Test
    void test()  IOException {
        SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
        SqlSession openSession = sqlSessionFactory.openSession();
        try {
            EmployeeMapperDynamicSql mapper = openSession.getMapper(EmployeeMapperDynamicSql.);
            Employee employee =  Employee();
            employee.setId(3);
//            employee.setLastName("%小%");
            employee.setEmail("[e-mail]");
            List<Employee> es = mapper.getEmpByConditionIf(employee);
            for(Employee e:es) {
                System.err.println(e);
            }
            openSession.commit();
        } finally {
            openSession.close();
        }
        
    }
    
}

首先是根据Id查询，结果为：

DEBUG 01-21 13:02:20,329 ==> Preparing: select * from tbl_employee WHERE id=? (BaseJdbcLogger.java:145)
Employee [id=3,lastName=小红,gender=0,email=[e-mail],dept=null]
DEBUG 01-21 13:02:20,375 ==> Parameters: 3(Integer) (BaseJdbcLogger.java:145)
DEBUG 01-21 13:02:20,452 <== Total: 1 (BaseJdbcLogger.java:145)

再根据姓名查询，结果为：

DEBUG 01-21 13:11:43,961 ==>  Preparing: select * from tbl_employee where last_name like ?   (BaseJdbcLogger.java:145) 
DEBUG 01-21 13:11:44,023 ==> Parameters: %小%(String)  (BaseJdbcLogger.java:145) 
DEBUG 01-21 13:11:44,086 ==      Total: 2  (BaseJdbcLogger.java:145) 
Employee [id=3,lastName=小红,1)">gender=0,1)">email=[e-mail],1)">dept=null]
Employee [id=4,1)">=小明,1)">=[e-mail],1)">=null]

说明：上述的动态sql会根据存在的字段进行查询。where标签可以去除掉第一个and。也就是说当我们根据姓名来查询时，原本sql拼接结果应该为select * from tbl-employee where and last_name like #{lastName}，由于使用了where标签，去掉了这里的第一个and。假设我们现在这么写：

>
                
                id=#{id} and
            
                last_name like #{lastName} and
            
                email=#{email} and
            
                gender=#{gender}
            >

即将and连接符放在if语句的最后，再进行按姓名查询就会报错：

DEBUG 01-21 13:08:17,137 ==>  Preparing: select * from tbl_employee WHERE last_name like ? and   (BaseJdbcLogger.java:145) 
DEBUG 01-21 13:08:17,191 ==> Parameters: %小%(String)  (BaseJdbcLogger.java:145) 

注意到日志里的sql语句最后存在一个and，这是不合法的，我们可以使用trim标签来解决：

trim prefix="where" prefixOverrides="" suffix suffixOverrides="and"trim>

trim标签中有四个属性：

prefix：前缀，为sql语句从该处开始加上指定字符串

prefixOverrides：去除掉sql语句从该处开始指定的字符串

suffix：后缀，为sql语句最后加上指定字符串

suffixOverrides：去除掉sql语句最后的指定的字符串

之后再进行测试：

=null]

发现sql语句正常，能够正确运行。