MyBatis——动态SQL
耳中所听恍若你呢喃,心之所向是指你为南,目之所及除你之外尽是荒野。
动态SQL
什么是动态SQL:
? 动态SQL就是根据不同的条件生成不同的SQL语句
- if
- choose(when,otherwise)
- trim(where,set)
- foreach
1、搭建环境
建表
CREATE TABLE `bolg`( `id` VARCHAR(50) NOT NULL COMMENT '博客id', `title` VARCHAR(100) not null comment '博客标题', `author` VARCHAR(30) not null comment '博客作者', `creat_time` datetime not null comment '创建时间', `views` int(30) not null comment '浏览量' )ENGINE=InnoDB DEFAULT CHARSET=utf8
创建一个基础工程
导包
编写配置文件
编写实体类
@Data public class Blog { private int id; private String title; private String author; private Date creatTime; private int views; }
编写实体类对应的Mapper接口和Mapper.xm
2、IF
<select id="queryBlogIF" parameterType="map" resultType="com.rui.pojo.Blog"> select * from mybatis.bolg where 1=1 <if test="title != null"> and title = #{title} </if> <if test="author != null"> and author = #{author} </if> </select>
@Test public void queryBlogIF(){ SqlSession sqlSession = MyBatisUtils.getSqlSession(); BlogMapper mapper = sqlSession.getMapper(BlogMapper.class); HashMap map = new HashMap(); map.put("author","尹锐"); List<Blog> blogs = mapper.queryBlogIF(map); for (Blog blog : blogs) { System.out.println(blog); } sqlSession.close(); }
3、choose(when,otherwise)
<select id="queryBlogChoose" parameterType="map" resultType="com.rui.pojo.Blog"> select * from mybatis.bolg <where> <choose> <when test="title != null"> title=#{title} </when> <when test="author!=null"> and author = #{author} </when> <otherwise> and views = #{views} </otherwise> </choose> </where> </select>
4、trim(where,set)
select * from mybatis.bolg <where> <if test="title != null"> title = #{title} </if> <if test="author != null"> and author = #{author} </if> </where>
<update id="updateBlog" parameterType="map"> update mybatis.bolg <set> <if test="title != null"> title = #{title}, </if> <if test="author != null"> author = #{author}, </if> </set> where id = #{id} </update>
所谓的动态SQL,本质还是SQL语句,只是我们可以在SQL层面,去执行一些逻辑代码
5、Foreach
select * from user where 1=1 and <foreach item="id" index="index" collection="ids" open="(" separator="or" close=")"> #{id} </foreach> (id=1 or id=2 or id=3)
<!-- select * from mybatis.bolg where 1=1 and (id=1 or id=2 or id=3) 我们现在传递一个万能的map,这个map中可以存在一个map --> <select id="queryBlogForeach" parameterType="map" resultType="com.rui.pojo.Blog"> select * from mybatis.bolg <where> <foreach collection="ids" item="id" open="(" close=")" separator="or"> id = #{id} </foreach> </where> </select>
动态SQL就是在拼接SQL语句,我们只要保证SQL的正确性,按照SQL的格式,去排列组合就可以了
建议:
- 先在MySQL中写出完整的SQL,再对应的去修改成为我们的动态SQL
SQL片段
有的时候,我们可能会将一些公共的部分抽取处理,方便复用
使用SQL标签抽取公共的部分
<sql id="if-title-author"> <if test="title != null"> title = #{title} </if> <if test="author != null"> and author = #{author} </if> </sql>
在需要使用的地方使用
Include
标签引用即可<select id="queryBlogIF" parameterType="map" resultType="com.rui.pojo.Blog"> select * from mybatis.bolg <where> <include refid="if-title-author"></include> </where> </select>
注意事项:
- 最好基于单表来定义SQL片段
- 不要存在where或者set标签,片段里尽量只有if就好了
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