动态SQL

一、环境准备

表blog

实体类

（使用了Lombok）

package com.zy.pojo; ​ import lombok.Data; ​ import java.util.Date; ​ @Data public class Blog { ​ private String id; private String title; private String author; private Date createTime; private int views; ​ }

 

jdbc.properties

driver=com.mysql.jdbc.Driver url=jdbc:mysql://localhost:3306/testmybatis?useSSL=false&useUnicode=true&characterEncoding=UTF-8
 username=root password=123456

 

mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
 <!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">
 <!--configuration核心配置文件-->
 <configuration>
     
     <properties resource="jdbc.properties"/>
     
     <settings>
         <setting name="logImpl" value="STDOUT_LOGGING"/>
         <!--是否开启自动驼峰命名规则(camel case)映射-->
         <setting name="mapUnderscoreToCamelCase" value="true"/>
     </settings> ​ <typeAliases>
         <!-- 方法1、定义一个alias别名，缺点在于需要一个实体类分别指定 <typeAlias type="com.zy.pojo.User" alias="user" />-->
         <!-- 方法2、也可以使用package来给某个包下面的所有实体类自动创建别名， 自动创建的别名规则是类的类名并将首字母改为小写 -->
         <package name="com.zy.pojo"/>
     </typeAliases> ​ <!--配置环境-->
     <environments default="mysql">
         <!--有多个环境时，通过修改default="id"来实现选择--> ​ <environment id="mysql">
             <!--JDBC事务管理-->
             <transactionManager type="JDBC"/>
             <dataSource type="POOLED">
                 <property name="driver" value="${driver}"/>
                 <property name="url" value="${url}"/>
                 <property name="username" value="{username}"/>
                 <property name="password" value="{password}"/>
             </dataSource>
         </environment>
     </environments> ​ <!--每一个Mapper.XML都需要Mybatis核心配置文件中注册-->
     <!--如果文件放在resources下，则这么写-->
     <mappers>
         <mapper resource="mapper/BlogMapper.xml"></mapper>
     </mappers> ​ </configuration>

 

 

二、< where >

当查询时，select * from xxx where id=#{id}

如果出现多条件时，比如 select * from xxx where id=#{id} and name=#{name}

但是当 id 或者 name 未知的情况下，就没有办法实现查询

所以使用动态SQL中的 < where > 进行查询

 

< if >

BlogMapper

package com.zy.mapper; import com.zy.pojo.Blog; import java.util.List; import java.util.Map; public interface BlogMapper { //查询博客
    List<Blog> queryBlogIF(Map map); }

 

BlogMapper.xml：

<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>
         <if test="views != null"> and views = #{views} </if>
     </where>
 </select>

 

测试类

@Test public void queryBlogIF() { ​ SqlSession sqlSession = MybatisUtils.getSqlSession(); BlogMapper blogMapper = sqlSession.getMapper(BlogMapper.class); ​ HashMap map = new HashMap(); map.put("title","45325"); map.put("author","efg"); List<Blog> blogs = blogMapper.queryBlogIF(map); ​ for (Blog blog : blogs) { System.out.println(blog); } ​ }

 

结果

 

如果不添加任何元素进入map

@Test public void queryBlogIF() { SqlSession sqlSession = MybatisUtils.getSqlSession(); BlogMapper blogMapper = sqlSession.getMapper(BlogMapper.class); ​ HashMap map = new HashMap(); List<Blog> blogs = blogMapper.queryBlogIF(map); ​ for (Blog blog : blogs) { System.out.println(blog); } ​ }

结果

 

< choose >

BlogMapper

package com.zy.mapper; ​ import com.zy.pojo.Blog; ​ import java.util.List; import java.util.Map; ​ public interface BlogMapper { ​ List<Blog> queryBlogChoose(Map map); ​ }

 

BlogMapper.xml

<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>

 

测试类

@Test public void queryBlogChoose() { ​ SqlSession sqlSession = MybatisUtils.getSqlSession(); BlogMapper blogMapper = sqlSession.getMapper(BlogMapper.class); ​ HashMap map = new HashMap(); map.put("title","45325"); map.put("author","efg"); map.put("views","322"); List<Blog> blogs = blogMapper.queryBlogChoose(map); for (Blog blog : blogs) { System.out.println(blog); } ​ }

 

结果

 

存在多个成立的条件

比如：这个sql中，不使用choose的话会查出3条记录

但只取choose中的第一个

 

使用 < trim >代替 < where > + SQL片段使用

BlogMapper.xml

<sql id="sql-title-author-views">
     <if test="title != null"> title = #{title} </if>
     <if test="author != null"> AND author = #{author} </if>
     <if test="views != null"> AND views = #{views} </if>
 </sql>
 <select id="queryBlogIF" parameterType="map" resultType="blog"> select * from blog <trim prefix="where" prefixOverrides="AND">
         <include refid="sql-title-author-views"></include>
     </trim>
 </select>

 

< set >

BlogMapper

package com.zy.mapper; ​ import com.zy.pojo.Blog; ​ import java.util.List; import java.util.Map; ​ public interface BlogMapper { ​ //更新博客
     int updateBlog(Map map); ​ }

 

BlogMapper.xml

<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>

 

测试类

@Test public void queryBlogSet() { ​ SqlSession sqlSession = MybatisUtils.getSqlSession(); BlogMapper blogMapper = sqlSession.getMapper(BlogMapper.class); ​ HashMap map = new HashMap(); map.put("title","6546"); map.put("author","fasfd"); map.put("views","6243"); map.put("id","d4303d0f-43f2-422e-8dbf-9a61ad872ff9"); ​ blogMapper.updateBlog(map); ​ List<Blog> blogs = blogMapper.queryBlogIF(map); for (Blog blog : blogs) { System.out.println(blog); } ​ }

 

原本的数据库

更新后

 

 

使用 < trim >代替 < set >

BlogMapper.xml

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

 

SQL片段

有时候，可以将一些功能相同的部分抽出，方便使用

抽取例子：

抽取前

<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>
         <if test="views != null"> and views = #{views} </if>
     </where>
 </select>

 

抽取后

<sql id="if-title-author-views">
     <if test="title != null"> title = #{title} </if>
     <if test="author != null"> and author = #{author} </if>
     <if test="views != null"> and views = #{views} </if>
 </sql> ​ <!--where标签 if-->
 <select id="queryBlogIF" parameterType="map" resultType="blog"> select * from blog <where>
         <include refid="if-title-author-views"></include>
     </where>
 </select>

 

测试

依旧能正常查出数据

注意点

　　1.最好基于单表定义SQL片段，尽量不要太复杂

　　2.不要存在 < where >

 

< ForEach >

对集合进行遍历使用 ForEach

对 blog表 的 id 修改一下

BlogMapper

 package com.zy.mapper; ​ import com.zy.pojo.Blog; ​ import java.util.List; import java.util.Map; ​ public interface BlogMapper { ​ //查询1-2-3号记录的博客
     List<Blog> queryBlogForeach(Map map); ​ }

 

BlogMapper.xml

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

 

测试类

@Test public void queryBlogForEach(){ ​ SqlSession sqlSession = MybatisUtils.getSqlSession(); BlogMapper blogMapper = sqlSession.getMapper(BlogMapper.class); ​ HashMap map = new HashMap(); ​ ArrayList<Integer> ids = new ArrayList<Integer>(); ​ map.put("ids",ids); ​ List<Blog> blogs = blogMapper.queryBlogForeach(map); ​ for (Blog blog : blogs) { System.out.println(blog); } ​ sqlSession.close(); }

 

结果

 

通过给ArrayList添加元素来实现想要的元素查询

比如想查 id 为1，2，3的

@Test public void queryBlogForEach(){ ​ SqlSession sqlSession = MybatisUtils.getSqlSession(); BlogMapper blogMapper = sqlSession.getMapper(BlogMapper.class); ​ HashMap map = new HashMap(); ​ ArrayList<Integer> ids = new ArrayList<Integer>(); ids.add(1); ids.add(2); ids.add(3); ​ map.put("ids",ids); ​ List<Blog> blogs = blogMapper.queryBlogForeach(map); ​ for (Blog blog : blogs) { System.out.println(blog); } ​ sqlSession.close(); }

 

结果