1 官网描述：
 2     MyBatis 的强大特性之一便是它的动态 SQL。如果你有使用 JDBC 或其它类似框架的经验，你就能体会到根据不同条件拼接 SQL 语句的痛苦。例如拼接时要确保不能忘记添加必要的空格，还要注意去掉列表最后一个列名的逗号。利用动态 SQL 这一特性可以彻底摆脱这种痛苦。
 3     虽然在以前使用动态 SQL 并非一件易事，但正是 MyBatis 提供了可以被用在任意 SQL 映射语句中的强大的动态 SQL 语言得以改进这种情形。
 4     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。
 5     
 6     -------------------------------
 7     - if
 8     - choose (when, otherwise)
 9     - trim (where, set)
10     - foreach
11     -------------------------------

1 CREATE TABLE `blog` (
2   `id` varchar(50) NOT NULL COMMENT ‘博客id‘,
3   `title` varchar(100) NOT NULL COMMENT ‘博客标题‘,
4   `author` varchar(30) NOT NULL COMMENT ‘博客作者‘,
5   `create_time` datetime NOT NULL COMMENT ‘创建时间‘,
6   `views` int(30) NOT NULL COMMENT ‘浏览量‘
7 ) ENGINE=InnoDB DEFAULT CHARSET=utf8

 1 import org.junit.Test;
 2 
 3 import java.util.UUID;
 4 
 5 public class IDUtils {
 6 
 7     public static String getId(){
 8         return UUID.randomUUID().toString().replaceAll("-","");
 9     }
10 
11     @Test
12     public void test(){
13         System.out.println(IDUtils.getId());
14         System.out.println(IDUtils.getId());
15         System.out.println(IDUtils.getId());
16     }
17 }

 1 import lombok.Data;
 2 
 3 import java.util.Date;
 4 
 5 @Data
 6 public class Blog {
 7 
 8     private String id;
 9     private String title;
10     private String author;
11     private Date createTime; //属性名和数据库字段名不一致，数据库字段名为create_time 需要开启驼峰命名转换
12     private int views;
13 }

1 import edu.ustc.wzh.pojo.Blog;
2 
3 public interface BlogMapper {
4 
5     //插入数据
6     int addBlog(Blog blog);
7 }

 1 <!--namespace用于绑定一个对应的Dao/Mapper接口-->
 2 <mapper namespace="edu.ustc.wzh.dao.BlogMapper">
 3 
 4     <insert id="addBlog" parameterType="Blog">
 5         insert into mybatis.blog(id, title, author, create_time, views)
 6         value (#{id},#{title},#{author},#{createTime},#{views});
 7     </insert>
 8 
 9 
10 </mapper>

 1 @Test
 2 public void addInitBlog(){
 3     SqlSession session = MybatisUtils.getSession();
 4     BlogMapper mapper = session.getMapper(BlogMapper.class);
 5 
 6     Blog blog = new Blog();
 7     blog.setId(IDUtils.getId());
 8     blog.setTitle("Mybatis如此简单");
 9     blog.setAuthor("狂神说");
10     blog.setCreateTime(new Date());
11     blog.setViews(9999);
12 
13     mapper.addBlog(blog);
14 
15     blog.setId(IDUtils.getId());
16     blog.setTitle("Java如此简单");
17     mapper.addBlog(blog);
18 
19     blog.setId(IDUtils.getId());
20     blog.setTitle("Spring如此简单");
21     mapper.addBlog(blog);
22 
23     blog.setId(IDUtils.getId());
24     blog.setTitle("微服务如此简单");
25     mapper.addBlog(blog);
26 
27     session.close();
28 }

1 //根据传入的属性查询博客（例如传入作者就查作者，传入title就查title）
2 List<Blog> queryBlogIF(Map map);

1 <select id="queryBlogIF" parameterType="map" resultType="blog">
2     select * from mybatis.blog where 1=1
3     <if test="title != null">
4         and title = #{title}
5     </if>
6     <if test="author != null">
7         and author = #{author}
8     </if>
9 </select>

 1 @Test
 2 public void queryBlogIFTest(){
 3     SqlSession session = MybatisUtils.getSession();
 4     BlogMapper mapper = session.getMapper(BlogMapper.class);
 5 
 6 
 7     Map<String,String> map = new HashMap<String,String>();
 8     map.put("title","Mybatis如此简单");
 9 
10     List<Blog> blogs = mapper.queryBlogIF(map);
11 
12     for (Blog blog : blogs) {
13 
14         System.out.println(blog);
15     }
16 
17     session.close();
18 }

 1 <select id="queryBlogIF" parameterType="map" resultType="blog">
 2     select * from mybatis.blog
 3     <where>
 4         <if test="title != null">
 5             and title = #{title}
 6         </if>
 7         <if test="author != null">
 8             and author = #{author}
 9         </if>
10     </where>
11 </select>

1 List<Blog> queryBlogChoose(Map map);

 1 <select id="queryBlogChoose" parameterType="map" resultType="blog">
 2     select * from mybatis.blog
 3     <where>
 4         <choose>
 5             <when test="title != null">
 6                 title = #{title}
 7             </when>
 8             <when test="author != null">
 9                 and author = #{author}
10             </when>
11             <otherwise>
12                 and views = #{views}
13             </otherwise>
14         </choose>
15     </where>
16 </select>

 1 @Test
 2 public void queryBlogChooseTest(){
 3     SqlSession session = MybatisUtils.getSession();
 4     BlogMapper mapper = session.getMapper(BlogMapper.class);
 5 
 6 
 7     Map<String,String> map = new HashMap<String,String>();
 8     map.put("title","Mybatis如此简单");
 9 
10     List<Blog> blogs = mapper.queryBlogChoose(map);
11 
12     for (Blog blog : blogs) {
13 
14         System.out.println(blog);
15     }
16 
17     session.close();
18 }

1 int updateBlogSet(Map map);

 1 <update id="updateBlogSet" parameterType="map">
 2     update mybatis.blog
 3     <set>
 4         <if test="title != null">
 5             title = #{title},
 6         </if>
 7         <if test="author != null">
 8             author = #{author}
 9         </if>
10     </set>
11     where id = #{id}
12 </update>

 1 @Test
 2 public void updateBlogSetTest(){
 3     SqlSession session = MybatisUtils.getSession();
 4     BlogMapper mapper = session.getMapper(BlogMapper.class);
 5 
 6 
 7     Map<String,String> map = new HashMap<String,String>();
 8     map.put("title","我学习Mybatis如此简单");
 9     map.put("id","5a95b31cb1494aa68c498c5c54243439");
10 
11     int res = mapper.updateBlogSet(map);
12 
13     if (res > 0){
14         System.out.println("更新成功！");
15     }
16 
17     session.close();
18 }

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

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

1 <sql id="if-title-author">
2     <if test="title != null">
3         title = #{title}
4     </if>
5     <if test="author != null">
6         and author = #{author}
7     </if>
8 </sql>

1 <select id="queryBlogIF" parameterType="map" resultType="blog">
2     select * from mybatis.blog
3     <where>
4         <!-- 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 -->
5         <include refid="if-title-author"></include>
6         <!-- 在这里还可以引用其他的 sql 片段 -->
7     </where>
8 </select>

1 List<Blog> queryBlogForeach(Map map);

 1 <select id="queryBlogForeach" parameterType="map" resultType="blog">
 2     select * from blog
 3     <where>
 4         <!--
 5         collection:指定输入对象中的集合属性
 6         item:每次遍历生成的对象
 7         open:开始遍历时的拼接字符串
 8         close:结束时拼接的字符串
 9         separator:遍历对象之间需要拼接的字符串
10         select * from blog where 1=1 and (id=1 or id=2 or id=3)
11       -->
12         <foreach collection="ids"  item="blogId" open="and (" close=")" separator="or">
13             id=#{blogId}
14         </foreach>
15     </where>
16 </select>

 1 @Test
 2     public void queryBlogForeachTest(){
 3         SqlSession session = MybatisUtils.getSession();
 4         BlogMapper mapper = session.getMapper(BlogMapper.class);
 5 
 6         HashMap map = new HashMap();
 7         List<Integer> ids = new ArrayList<Integer>();
 8         ids.add(1);
 9         ids.add(2);
10         ids.add(3);
11         map.put("ids",ids);
12 
13         List<Blog> blogs = mapper.queryBlogForeach(map);
14 
15         System.out.println(blogs);
16 
17         session.close();
18     }