介绍
- 动态SQL指的是根据不同的查询条件 , 生成不同的Sql语句,也就是我们在jdbc代码中的sql拼接工作
- mybatis 动态SQL,通过 if, choose, when, otherwise, trim, where, set, foreach等标签,可组合成非常灵活的SQL语句,从而在提高 SQL 语句的准确性的同时,也大大提高了开发人员的效率。
![](https://img.laitimes.com/img/9ZDMuAjOiMmIsIjOiQnIsICM38FdsYkRGZkRG9lcvx2bjxiNx8VZ6l2cs0zY650dBRUT4t2MMBjVtJWd0ckW65UbM5WOHJWa5kHT20ESjBjUIF2X0hXZ0xCMx81dvRWYoNHLrdEZwZ1Rh5WNXp1bwNjW1ZUba9VZwlHdssmch1mclRXY39CXldWYtlWPzNXZj9mcw1ycz9WL49zZuBnL1YTN0UDO1kDM0ITNwEjMwIzLc52YucWbp5GZzNmLn9Gbi1yZtl2Lc9CX6MHc0RHaiojIsJye.png)
搭建环境
1、搭建数据库
字段:id,title,author,create_time,views
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
- 通过UUID类生成随机数
public class IDUtil {
public static String genId(){
return UUID.randomUUID().toString().replaceAll("-","");
}
}
3、实体类编写 【注意set方法作用】
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、编写Mapper接口及xml文件
public interface BlogMapper {
}
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.yang.mapper.BlogMapper">
</mapper>
5、mybatis核心配置文件
- 配置set使下划线驼峰自动转换
<settings>
<setting name="mapUnderscoreToCamelCase" value="true"/>
<setting name="logImpl" value="STDOUT_LOGGING"/>
</settings>
<!--注册Mapper.xml-->
<mappers>
<mapper resource="mapper/BlogMapper.xml"/>
</mappers>
6、插入初始数据
- 编写接口
//新增一个博客
int addBlog(Blog blog);
- sql配置文件
<insert id="addBlog" parameterType="blog">
insert into blog (id, title, author, create_time, views)
values (#{id},#{title},#{author},#{createTime},#{views});
</insert>
- 初始化博客
@Test
public void addInitBlog(){
SqlSession session = MybatisUtils.getSession();
BlogMapper mapper = session.getMapper(BlogMapper.class);
Blog blog = new Blog();
blog.setId(IDUtil.genId());
blog.setTitle("斗破苍穹");
blog.setAuthor("天蚕土豆");
blog.setCreateTime(new Date());
blog.setViews(9999);
mapper.addBlog(blog);
blog.setId(IDUtil.genId());
blog.setTitle("武动乾坤");
mapper.addBlog(blog);
blog.setId(IDUtil.genId());
blog.setTitle("魔兽剑圣异界纵横");
mapper.addBlog(blog);
blog.setId(IDUtil.genId());
blog.setTitle("大主宰");
mapper.addBlog(blog);
session.close();
}
if 标签
实现功能:根据作者名字和博客名字来查询博客
1、编写接口类
//需求1
List<Blog> queryBlogIf(Map map);
2、编写SQL语句
<!--
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>
3、测试
- 使用map传递两个参数
@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","大主宰");
map.put("author","天蚕土豆");
List<Blog> blogs = mapper.queryBlogIf(map);
System.out.println(blogs);
session.close();
}
- 这里当我们的书名为空时,sql语句就会出现错误了
- 为了处理这种错误,我们就需要使用下面的where标签
Where标签
- 同样功能实现,可以将sql写成如下
- where标签当有条件时自动插入where
- where会自动删除多余的and和or
<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>
Set标签
- set标签功能相似where标签,会自动帮我们增加set关键字
1、编写接口方法
2、sql配置文件
<!--注意set是用的逗号隔开-->
<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>
3、测试
@Test
public void testUpdateBlog(){
SqlSession session = MybatisUtils.getSession();
BlogMapper mapper = session.getMapper(BlogMapper.class);
HashMap<String, String> map = new HashMap<String, String>();
map.put("title","jjy的blog");
map.put("author","jjy");
map.put("id","123456789");
mapper.updateBlog(map);
session.close();
}
choose标签
- choose标签只会在下面的代码中选择一个分支进行
- 相当于java代码switch
1、编写接口方法
2、sql配置文件
<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、测试类
@Test
public void testQueryBlogChoose(){
SqlSession session = MybatisUtils.getSession();
BlogMapper mapper = session.getMapper(BlogMapper.class);
HashMap<String, Object> map = new HashMap<String, Object>();
map.put("title","武动乾坤");
map.put("author","天蚕土豆");
map.put("views",9999);
List<Blog> blogs = mapper.queryBlogChoose(map);
System.out.println(blogs);
session.close();
}
SQL片段
- 有时候可能某个 sql 语句我们用的特别多,为了增加代码的重用性,简化代码,我们需要将这些代码抽取出来,然后使用时直接调用。
- 提取SQL片段
<sql id="if-title-author">
<if test="title != null">
title = #{title}
</if>
<if test="author != null">
and author = #{author}
</if>
</sql>
- 引用SQL片段
<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>
<!-- 在这里还可以引用其他的 sql 片段 -->
</where>
</select>
注意
- 最好基于单表来定义 sql片段,提高片段的可重用性
- 在sql片段中不要包括 where
Foreach标签
将数据库中前三个数据的id修改为1,2,3
- 我们需要查询 blog 表中 id 分别为1,2,3的博客信息
1、编写接口
2、编写SQL语句
<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>
3、测试
@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();
}