1 where标签

1.1若满足条件的首条sql语句前面没有‘and’或者‘or’，Mybatis会自动拼接sql语句，如果满足条件的首条sql语句前面有‘and’或者‘or’，Mybatis会自动的去掉‘and’或者‘or’

 <select id="getBlogIF" parameterType="map" resultType="blog">
        select * from blog
        <where>
            <if test="title != null">
                 title=#{title}
            </if>
            <if test="author != null">
                and author=#{author}
            </if>
        </where>
    </select>

1.2 当只有title不为空时,运行结果的sql语句

Preparing: select * from blog WHERE title=? 

1.3 当只有anthor不为空时,运行结果的sql语句

select * from blog WHERE author=? 

这里的and被自动去掉了

1.4 当title和anthor不为空时,运行结果的sql语句

select * from blog WHERE title=? and author=? 

这里的and就没有被删除，很智能化

2 choose标签(类似于Java中的swtich选择结构)

<select id="getBlogChoose" parameterType="map" resultType="blog">
        select * from blog
        <where>
            <choose>
                <when test="title != null">/*
                如果前面条件通过，后面的都不会实现
                  */
                    title=#{title}
                </when>
                <when test="author != null">
                    and author = #{author}
                </when>
                <otherwise>
                    and views=#{views}
                </otherwise>
            </choose>
        </where>
    </select>

3 set标签（主要用于数据更新） set标签会自动加上set并且会去除多余的‘，’

Mapper标签

<update id="updateBlog" parameterType="map">
        update blog
        <set>
            <if test="title != null">
                title=#{title},
            </if>
            <if test="author != null">
                author=#{author},
            </if>
        </set>
        where views=#{views}
    </update>

实现

 @Test
    public void updateBlog(){
        SqlSession sqlSession = sqlSessionFactory.getsqlSession();
        blogMapper mapper = sqlSession.getMapper(blogMapper.class);

        HashMap map = new HashMap();
        map.put("title","微服务11");
        map.put("author","小落11");
        map.put("views",1000);
        mapper.updateBlog(map);
        sqlSession.commit();//注意增删改要提交事务
        sqlSession.close();

sql结果（可以看到author后的，没有了）

update blog SET title=?, author=? where views=?