Mybatis: (八)动态sql

一、前言

动态sql是指根据不同的查询条件，生成不同的sql语句

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

二、实现

字段：id，title，author，create_time，views

1. 建表

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

2. IDutil工具类

public class IDUtil {

    public static String genId(){
        return UUID.randomUUID().toString().replaceAll("-","");
    }

}

3. 实体类

import java.util.Date;

public class Blog {

    private String id;
    private String title;
    private String author;
    private Date createTime;
    private int views;
    //set，get....
}

4. 创建Mapper接口

List<Blog> queryBlogIf(Map map);

5. 编写sql语句

   if语句

<!--需求1：
根据作者名字和博客名字来查询博客！
如果作者名字为空，那么只根据博客名字查询，反之，则根据作者名来查询
select * from blog where title = #{title} and author = #{author}
-->
<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>
</select>

测试

@Test
public void testQueryBlogIf(){
    SqlSession session = MybatisUtils.getSession();
    BlogMapper mapper = session.getMapper(BlogMapper.class);

    HashMap<String, String> map = new HashMap<String, String>();
    map.put("title","三体");
    map.put("author","刘慈欣");
    List<Blog> blogs = mapper.queryBlogIf(map);

    System.out.println(blogs);

    session.close();
}

where语句

<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>

“where”标签会知道如果它包含的标签中有返回值的话，它就插入一个‘where’。如果标签返回的内容是以AND 或OR 开头的，则它会剔除掉。

set语句

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

choose语句

List<Blog> queryBlogChoose(Map map);

<select id="queryBlogChoose" parameterType="map" resultType="blog">
    select * from blog
    <where>
        <choose>
        //when只要成立，后面的不再判断
            <when test="title != null">
                 title = #{title}
            </when>
            <when test="author != null">
                and author = #{author}
            </when>
            <otherwise>
                and views = #{views}
            </otherwise>
        </choose>
    </where>
</select>

sql片段

<sql id="if-title-author">
    <if test="title != null">
        title = #{title}
    </if>
    <if test="author != null">
        and author = #{author}
    </if>
</sql>

<select id="queryBlogIf" parameterType="map" resultType="blog">
    select * from blog
    <where>
        <!-- 引用 sql 片段，如果refid 指定的不在本文件中，那么需要在前面加上 namespace -->
        <include refid="if-title-author"></include>
    </where>
</select>

foreach

List<Blog> queryBlogForeach(Map map);

<select id="queryBlogForeach" parameterType="map" resultType="blog">
    select * from blog
    <where>
        <!--
        collection:指定输入对象中的集合属性
        item:每次遍历生成的对象
        open:开始遍历时的拼接字符串
        close:结束时拼接的字符串
        separator:遍历对象之间需要拼接的字符串
        select * from blog where 1=1 and (id=1 or id=2 or id=3)
      -->
        <foreach collection="ids"  item="id" open="and (" close=")" separator="or">
            id=#{id}
        </foreach>
    </where>
</select>

@Test
public void testQueryBlogForeach(){
    SqlSession session = MybatisUtils.getSession();
    BlogMapper mapper = session.getMapper(BlogMapper.class);

    HashMap map = new HashMap();
    List<Integer> ids = new ArrayList<Integer>();
    ids.add(1);
    ids.add(2);
    ids.add(3);
    map.put("ids",ids);

    List<Blog> blogs = mapper.queryBlogForeach(map);

    System.out.println(blogs);

    session.close();
}

bind

<select id="selectBlogsLike" resultType="Blog">
  <bind name="pattern" value="'%' + _parameter.getTitle() + '%'" />
  SELECT * FROM BLOG
  WHERE title LIKE #{pattern}
</select>