狂神说Mybatis07：动态SQL
动态SQL
什么是动态SQL：动态SQL就是指根据不同的条件生成不同的SQL语句
利用动态SQL这一特性可以彻底摆脱这种痛苦
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(100) 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
1、创建一个基础maven子工程mybatis-08
2、导入Lombok包
3、编写配置文件（拷贝前面的配置文件和工具类）
4、编写实体类
@Data
public class Blog {
private String id;
private String title;
private String author;
private Date createTime;//属性名和字段名不一致
private int views;
}
5、工具类设置添加ID为随机
@SuppressWarnings("all")//抑制警告
public class IDutils {
public static String getId() {
return UUID.randomUUID().toString().replaceAll("-","");
}
@Test
public void test(){
System.out.println(IDutils.getId());
System.out.println(IDutils.getId());
System.out.println(IDutils.getId());
}
6、开启驼峰命名自动映射
<settings>
<!-- 是否开启驼峰命名自动映射-->
<setting name="mapUnderscoreToCamelCase" value="true"/>
</settings>
7、测试添加数据
<insert id="addBlog" parameterType="blog">
insert into mybatis.blog(id,title,author,create_time,views)
values (#{id},#{title},#{author},#{createTime},#{views});
</insert>
@Test
public void test() {
// System.out.println(IDutils.getId());
SqlSession sqlSession = MybatisUtils.getSqlSession();
BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);
Blog blog = new Blog();
blog.setId(IDutils.getId());
blog.setTitle("Mybatis");
blog.setAuthor("神");
blog.setCreateTime(new Date());
blog.setViews(9999);
mapper.addBlog(blog);
blog.setId(IDutils.getId());
blog.setTitle("Java");
mapper.addBlog(blog);
blog.setId(IDutils.getId());
blog.setTitle("Spring");
mapper.addBlog(blog);
blog.setId(IDutils.getId());
blog.setTitle("微服务");
mapper.addBlog(blog);
sqlSession.close();
}
8、编写实体类对应Mapper接口和Mapper.XML文件
1、IF
public interface BlogMapper {
//插入数据
int addBlog(Blog blog);
//查询博客
List<Blog> queryBlogIF(Map map);
}
<select id="queryBlogIF" parameterType="map" resultType="blog">
select * from blog where 1=1
<if test="title!=null">
and title =#{title}
</if>
<if test="author!=null">
and author =#{author}
</if>
</select>
测试查询
@Test
public void testqueryBlogIF() {
SqlSession sqlSession = MybatisUtils.getSqlSession();
BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);
HashMap hashMap = new HashMap();
hashMap.put("title","Mybatis");
List<Blog> blogs = mapper.queryBlogIF(hashMap);
for (Blog blog : blogs) {
System.out.println(blog);
}
sqlSession.close();
}
2、choose (when, otherwise)
<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>
and views=#{views}
</otherwise>
</choose>
</where>
</select>
3、trim (where,set)
select * from blog
<where>
<if test="title!=null">
and title =#{title}
</if>
<if test="author!=null">
and author =#{author}
</if>
</where>
<select 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}
</select>
所谓的动态SQL，本质还是SQL语句，只是我们可以在SQL层面，去执行一个逻辑代码
SQL片段
有的时候，我们可能会将一些公共的部分抽取出来，方便复用！
1、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">
author =#{author}
</if>
</sql>
2、在需要使用的地方使用Include标签引用即可
<select id="queryBlogIF" parameterType="map" resultType="blog">
select * from blog
<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标签
4、Foreach
select * from blog where 1=1 and
<foreach collection="ids" item="id"
open="(" separator="or" close=")" >
#{id}
</foreach>
(id=1 or id =2 or id=3)
@Test
public void testqueryBlogForeach(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);
HashMap hashMap = new HashMap();
ArrayList<Integer> ids = new ArrayList<Integer>();
ids.add(1);
ids.add(2);
hashMap.put("ids",ids);
List<Blog> blogs = mapper.queryBlogForeach(hashMap);
for (Blog blog : blogs) {
System.out.println(blog);
}
sqlSession.close();
}
<!-- select * from blog 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map中可以存在一个集合!-->
<select id="queryBlogForeach" parameterType="map" resultType="blog">
select * from blog
<where>
<foreach collection="ids" item="id" open="and (" 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：
现在Mysql中写出完整的SQL，再对应的去修改成为我们的动态SQL实现通用即可！