动态SQL

动态SQL

什么是动态SQL:根据不同条件生成不同的SQL语句

动态 SQL 元素和 JSTL 或基于类似 XML 的文本处理器相似。在 MyBatis 之前的版本中，有很多元素需要花时间了解。MyBatis 3 大大精简了元素种类，现在只需学习原来一半的元素便可。MyBatis 采用功能强大的基于 OGNL 的表达式来淘汰其它大部分元素。

• if
• choose (when, otherwise)
• trim (where, set)
• foreach

搭建环境

CREATE TABLE `blog`(
`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',
`create_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

1. 导包

2. 编写配置文件

3. 编写实体类

   @Data
   public class Blog {
       private int id;
       private String title;
       private String author;
       private Date createTime;
       private int views;
   }
   

4. 编写实体类对应的Mapper接口和Mapper.xml文件

if - where

<select id="queryBlogIf" 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>

choose(when, otherwise) - where

<select id="queryBlogChoose" parameterType="map" resultType="blog">
    select * from blog
    <where>
        <choose>
            <when test="title != null">
                title = #{title}
            </when>
            <when test="author != null">
                and author = #{author}
            </when>
            <otherwise>
                views = #{views}
            </otherwise>
        </choose>
    </where>
</select>

if - set

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

where和set的本质都是trim

<trim prefix="WHERE" prefixOverrides="AND |OR ">
  ...
</trim>

<trim prefix="SET" suffixOverrides=",">
  ...
</trim>

SQL片段

<sql id="片段名">
	--片段代码
</sql>
...
	--插入片段
	<include refid="片段名"/>
...

foreach

<select id="selectPostIn" resultType="domain.blog.Post">
  SELECT *
  FROM POST P
  WHERE ID in
  <foreach item="item" index="index" collection="list"
      open="(" separator="," close=")">
        #{item}
  </foreach>
</select>

• 通过foreach查找id为1,2,4的数据:

  <select id="queryBlogForeach" parameterType="map" resultType="blog">
      select * from blog
      <where>
          <foreach item="id" collection="ids" open="and (" close=")" separator="or">
              id = #{id}
          </foreach>
      </where>
  </select>
  

  @Test
  public void queryBlogForeach() {
      SqlSession sqlSession = MybatisUtils.getSqlSession();
      BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);
      HashMap map = new HashMap();
      ArrayList<Integer> ids = new ArrayList<Integer>();
      ids.add(1);
      ids.add(2);
      ids.add(4);
      map.put("ids", ids);
      List<Blog> blogList = mapper.queryBlogForeach(map);
      for (Blog blog : blogList) {
          System.out.println(blog);
      }
      sqlSession.close();
  }