1. if

2. where

3. set

4. foreach

5. choose、when、otherwise

6. 完整项目实例

0. 简介

动态 SQL 是 MyBatis 的强大特性之一。如果你使用过 JDBC 或其它类似的框架，你应该能理解根据不同条件拼接 SQL 语句有多痛苦，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，可以彻底摆脱这种痛苦。

使用动态 SQL 并非一件易事，但借助可用于任何 SQL 映射语句中的强大的动态 SQL 语言，MyBatis 显著地提升了这一特性的易用性。

本篇文章要讲的mybatis元素主要有

if

choose (when, otherwise)

trim (where, set)

foreach

首先创建以下运行环境——数据库。

CREATE TABLE `blog` (`id` int(11) NOT NULL,`title` varchar(255) DEFAULT NULL,`author` varchar(30) DEFAULT NULL,`_time` datetime DEFAULT NULL,`read_count` int(11) DEFAULT NULL,`label` varchar(50) DEFAULT NULL,`_like` int(11) DEFAULT NULL,`visible` int(11) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;-- ----------------------------
-- Records of blog
-- ----------------------------
INSERT INTO `blog` VALUES ('116012859', '【Spring】Spring简介、组成及拓展', '牧心.', '2021-04-22 14:44:35', '3823', 'Spring', '1', '1');
INSERT INTO `blog` VALUES ('116017799', '【Spring】IOC理论推导、IOC本质', '牧心.', '2021-04-22 22:01:03', '3829', 'Spring', '1', '1');
INSERT INTO `blog` VALUES ('116492927', '【Spring】一个spring简单实例', '牧心.', '2021-05-07 15:01:39', '3154', 'Spring', '1', '1');
INSERT INTO `blog` VALUES ('116494838', '【Spring】IOC创建对象的方式', '牧心.', '2021-05-07 17:50:21', '3150', 'Spring', '1', '1');
INSERT INTO `blog` VALUES ('116502734', '【Spring】DI依赖注入', '牧心.', '2021-05-12 15:13:05', '2908', 'Spring', '2', '1');
INSERT INTO `blog` VALUES ('117374430', '【Spring】AOP（一）使用Spring的API接口', '牧心.', '2021-05-30 19:00:46', '228', 'Spring', '1', '1');
INSERT INTO `blog` VALUES ('118054659', '【Spring】AOP（二）自定义来实现AOP', '牧心.', '2021-06-19 15:50:23', '192', 'Spring', '1', '1');
INSERT INTO `blog` VALUES ('118058442', '【Spring】AOP（三）注解实现AOP', '牧心.', '2021-06-19 20:06:54', '203', 'Spring', '1', '1');
INSERT INTO `blog` VALUES ('118074003', '【Mybatis】Mybatis入门', '牧心.', '3921-07-20 20:03:04', '172', 'MyBatis', '1', '1');
INSERT INTO `blog` VALUES ('118074590', '【Mybatis】一个Mybatis程序', '牧心.', '2021-06-22 21:03:03', '129', 'MyBatis', '1', '1');
INSERT INTO `blog` VALUES ('118311033', '【Mybatis】增删改查的实现', '牧心.', '2021-06-28 20:20:29', '108', 'MyBatis', '2', '1');
INSERT INTO `blog` VALUES ('118343512', '【Mybatis】Map传参和模糊查询', '牧心.', '2021-06-29 21:22:36', '11', 'MyBatis', '1', '1');
INSERT INTO `blog` VALUES ('118344763', '【Mybatis】配置之属性优化', '牧心.', '2021-06-29 21:59:54', '21', 'MyBatis', '1', '1');

1. if

使用动态 SQL 最常见情景是根据条件包含 where 子句的一部分。比如：

<select id="getVisibleBlogListByLabel" resultType="com.company.org.pojo.Blog">select * from blogwhere visible = 1<if test="label != null and label != ''">AND label = #{label}</if>
</select>

这条语句提供了可选的查找文本功能。如果不传入label，那么只返回visible=1的博客；如果传入了label，那么就会对title一列进行查找并返回对应的结果。

如果希望通过 “title” 和 “label” 两个参数进行可选搜索该怎么办呢？很容易想到，我们可以利用下面的语句：

<select id="getBlogListByLabelAndTitle" parameterType="map" resultType="com.company.org.pojo.Blog">select * from blogwhere<if test="label != null">label = #{label}</if><if test="title != null">AND title like #{title}</if>
</select>

但是，这会产生一个问题，当label为空时，sql语句会变成：

select * from blog where and title like 'xxx'

甚至当label和title都为空时，sql语句则变成：

select * from blog where

很显然，这样会造成运行错误！

那么，如何解决上面所说的问题呢？mybatis给出了<where>元素。

2. where

上面提到的语句就可以改为以下形式：

<select id="getBlogListByLabelAndTitle2" parameterType="map" resultType="com.company.org.pojo.Blog">select * from blog<where><if test="label != null">label = #{label}</if><if test="title != null">AND title like #{title}</if></where>
</select>

where 元素只会在子元素返回任何内容的情况下才插入 “WHERE” 子句。而且，若子句的开头为 “AND” 或 “OR”，where 元素也会将它们去除。

如果 where 元素与你期望的不太一样，你也可以通过自定义 trim 元素来定制 where 元素的功能。比如，和 where 元素等价的自定义 trim 元素为：

<select id="getBlogListByLabelAndTitle3" parameterType="map" resultType="com.company.org.pojo.Blog">select * from blog<trim prefix="where" prefixOverrides="and |or "><if test="label != null">label = #{label}</if><if test="title != null">AND title like #{title}</if></trim>
</select>

到这里，你是否发现上面3个语句都包含：

<if test="label != null">label = #{label}
</if>
<if test="title != null">AND title like #{title}
</if>

这造成了代码的冗余，那我们是否可以将其提出来呢？答案是肯定的，mybatis提供了<sql>元素。

<sql id="if-label-title"><if test="label != null">label = #{label}</if><if test="title != null">AND title like #{title}</if>
</sql>

需要引入的地方使用<include>元素即可，（<include>就相当于）如：

    <select id="getBlogListByLabelAndTitle2" parameterType="map" resultType="com.company.org.pojo.Blog">select * from blog<where>
<!--            <if test="label != null">-->
<!--                label = #{label}-->
<!--            </if>-->
<!--            <if test="title != null">-->
<!--                AND title like #{title}-->
<!--            </if>--><include refid="if-label-title"></include></where></select>

3. set

用于动态更新语句的类似解决方案叫做 set。set 元素可以用于动态包含需要更新的列，忽略其它不更新的列。比如：

<update id="updateBlogById">update blog<set><if test="title != null">title = #{title},</if><if test="label != null">label = #{label},</if><if test="visible != null">visible = #{visible},</if></set>where id = #{id}
</update>

4. foreach

动态 SQL 的另一个常见使用场景是对集合进行遍历（尤其是在构建 IN 条件语句的时候）。

属性	描述
collection	表示迭代集合的名称，可以使用@Param注解指定，如下图所示该参数为必选
item	表示本次迭代获取的元素，若collection为List、Set或者数组，则表示其中的元素；若collection为map，则代表key-value的value，该参数为必选
open	表示该语句以什么开始，最常用的是左括弧’(’，注意:mybatis会将该字符拼接到整体的sql语句之前，并且只拼接一次，该参数为可选项
close	表示该语句以什么结束，最常用的是右括弧’)’，注意:mybatis会将该字符拼接到整体的sql语句之后，该参数为可选项
separator	mybatis会在每次迭代后给sql语句append上separator属性指定的字符，该参数为可选项
index	在list、Set和数组中,index表示当前迭代的位置，在map中，index代指是元素的key，该参数是可选项。

比如：表格引自：https://blog.csdn.net/wt_better/article/details/80941646

<select id="getBlogListByIds" parameterType="map" resultType="com.company.org.pojo.Blog">select * from blog<where><foreach collection="ids" item="id" separator="or">id = #{id}</foreach></where>
</select>

上面这个例子的作用是，查询id在ids（列表）中的博客。

相当于sql语句：

select * from blog where id = 116012859 or id = 117374430 or id = 118058442

上面的语句的功能等价于以下语句：

<select id="getBlogListByIds" parameterType="map" resultType="com.company.org.pojo.Blog">select * from blog<where>id in<foreach collection="ids" item="id" open="(" separator="," close=")">#{id}</foreach></where>
</select>

相当于sql语句：

select * from blog where id IN (116012859,117374430,118058442)

5. choose、when、otherwise

有时候，我们不想使用所有的条件，而只是想从多个条件中选择一个使用。针对这种情况，MyBatis 提供了 choose 元素，它有点像 Java 中的 switch 语句。如：

<select id="getBlogListByTitleOrLabel" parameterType="map" resultType="com.company.org.pojo.Blog">select * from blog<where><choose><when test="label != null">label = #{label}</when><when test="title != null">and title like #{title}</when><otherwise>and visible = #{visible}</otherwise></choose></where>
</select>

其中，when可以有多个，otherwise必须存在！

6. 完整项目实例

项目结构

（1）父工程pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><!-- 父工程 --><groupId>org.example</groupId><artifactId>mybatisProject</artifactId><packaging>pom</packaging><version>1.0-SNAPSHOT</version><modules><module>mybatis-01</module><module>mybatis-02</module></modules><!-- 导入依赖 --><dependencies><!--https://repo.maven.apache.org/maven2/--><!-- mysql驱动 --><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><version>8.0.23</version></dependency><!-- mubatis --><dependency><groupId>org.mybatis</groupId><artifactId>mybatis</artifactId><version>3.5.7</version></dependency><!-- junit --><dependency><groupId>junit</groupId><artifactId>junit</artifactId><version>4.12</version></dependency></dependencies><build><resources><resource><directory>src/main/resources</directory><includes><include>**/*.properties</include><include>**/*.xml</include></includes><filtering>true</filtering></resource><resource><directory>src/main/java</directory><includes><include>**/*.properties</include><include>**/*.xml</include></includes><filtering>true</filtering></resource></resources></build>
</project>

（2）子工程pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"><parent><artifactId>mybatisProject</artifactId><groupId>org.example</groupId><version>1.0-SNAPSHOT</version></parent><modelVersion>4.0.0</modelVersion><artifactId>mybatis-02</artifactId><build><resources><resource><directory>src/main/resources</directory><includes><include>**/*.properties</include><include>**/*.xml</include></includes><filtering>true</filtering></resource><resource><directory>src/main/java</directory><includes><include>**/*.properties</include><include>**/*.xml</include></includes><filtering>true</filtering></resource></resources></build></project>

（3）Blog.java

package com.company.org.pojo;import java.util.Date;public class Blog {private int id;private String title;private String author;private String time;private int read_count;private String label;private int like;private int visible;public Blog(){}public Blog(int id, String title, String author, String time, int read_count, String label, int like, int visible) {this.id = id;this.title = title;this.author = author;this.time = time;this.read_count = read_count;this.label = label;this.like = like;this.visible = visible;}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getTitle() {return title;}public void setTitle(String title) {this.title = title;}public String getAuthor() {return author;}public void setAuthor(String author) {this.author = author;}public String getTime() {return time;}public void setTime(String time) {this.time = time;}public int getRead_count() {return read_count;}public void setRead_count(int read_count) {this.read_count = read_count;}public String getLabel() {return label;}public void setLabel(String label) {this.label = label;}public int getLike() {return like;}public void setLike(int like) {this.like = like;}public int getVisible() {return visible;}public void setVisible(int visible) {this.visible = visible;}@Overridepublic String toString() {return "Blog{" +"id=" + id +", title='" + title + '\'' +", author='" + author + '\'' +", time='" + time + '\'' +", read_count=" + read_count +", label='" + label + '\'' +", like=" + like +", visible=" + visible +'}';}
}

（4）BlogMapper.java

package com.company.org.dao;import com.company.org.pojo.Blog;import java.util.List;
import java.util.Map;public interface BlogMapper {// insert blogint addBlog(Blog blog);// select all blogsList<Blog> getBlogList();// select visible blogs by labelList<Blog> getVisibleBlogListByLabel(String label);// select blogs by label and title <if>List<Blog> getBlogListByLabelAndTitle(Map<String, Object> map);// select blogs by label and title <where>List<Blog> getBlogListByLabelAndTitle2(Map<String, Object> map);// select blogs by label and title <where>List<Blog> getBlogListByLabelAndTitle3(Map<String, Object> map);// update blog by idint updateBlogById(Map<String, Object> map);// select blogs by id-listList<Blog> getBlogListByIds(Map<String, Object> map);// select blogs / choose、when、otherwiseList<Blog> getBlogListByTitleOrLabel(Map<String, Object> map);
}

（5）BlogMapper.xml

<?xml version="1.0" encoding="UTF8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><!--https://mybatis.org/mybatis-3/zh/getting-started.html-->
<!-- namespace=绑定一个对应的Dao/Mapper接口 -->
<mapper namespace="com.company.org.dao.BlogMapper"><insert id="addBlog" parameterType="com.company.org.pojo.Blog">insert into blog(id, title, author, _time, read_count, label, _like, visible) values (#{id}, #{title}, #{author}, #{time}, #{read_count}, #{label}, #{like}, #{visible})</insert><select id="getBlogList" resultType="com.company.org.pojo.Blog">select * from blog;</select><sql id="if-label-title"><if test="label != null">label = #{label}</if><if test="title != null">AND title like #{title}</if></sql><select id="getVisibleBlogListByLabel" resultType="com.company.org.pojo.Blog">select * from blogwhere visible = 1<if test="label != null and label != ''">AND label = #{label}</if></select><select id="getBlogListByLabelAndTitle" parameterType="map" resultType="com.company.org.pojo.Blog">select * from blogwhere<if test="label != null">label = #{label}</if><if test="title != null">AND title like #{title}</if></select><select id="getBlogListByLabelAndTitle2" parameterType="map" resultType="com.company.org.pojo.Blog">select * from blog<where>
<!--            <if test="label != null">-->
<!--                label = #{label}-->
<!--            </if>-->
<!--            <if test="title != null">-->
<!--                AND title like #{title}-->
<!--            </if>--><include refid="if-label-title"></include></where></select><select id="getBlogListByLabelAndTitle3" parameterType="map" resultType="com.company.org.pojo.Blog">select * from blog<trim prefix="where" prefixOverrides="and |or "><if test="label != null">label = #{label}</if><if test="title != null">AND title like #{title}</if></trim></select><update id="updateBlogById">update blog<set><if test="title != null">title = #{title},</if><if test="label != null">label = #{label},</if><if test="visible != null">visible = #{visible},</if></set>where id = #{id}</update><!--    <select id="getBlogListByIds" parameterType="map" resultType="com.company.org.pojo.Blog">-->
<!--        select * from blog-->
<!--        <where>-->
<!--            id in-->
<!--            <foreach collection="ids" item="id" open="(" separator="," close=")">-->
<!--               #{id}-->
<!--            </foreach>-->
<!--        </where>-->
<!--    </select>--><select id="getBlogListByIds" parameterType="map" resultType="com.company.org.pojo.Blog">select * from blog<where><foreach collection="ids" item="id"  separator="or">id = #{id}</foreach></where></select><select id="getBlogListByTitleOrLabel" parameterType="map" resultType="com.company.org.pojo.Blog">select * from blog<where><choose><when test="label != null">label = #{label}</when><when test="title != null">and title like #{title}</when><otherwise>and visible = #{visible}</otherwise></choose></where></select></mapper>

（6）MybatisUtils.java

package com.company.org.utils;import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;import java.io.IOException;
import java.io.InputStream;// sqlSessionFactory 来构建sqlSession
public class MybatisUtils {private static SqlSessionFactory sqlSessionFactory;static {try {// https://mybatis.org/mybatis-3/zh/getting-started.html// 使用Mybatis第一步：获取sqlSessionFactory对象String resource = "mybatis-config.xml";InputStream inputStream = Resources.getResourceAsStream(resource);sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);} catch (IOException e) {e.printStackTrace();}}// 既然有了 SqlSessionFactory，顾名思义，我们可以从中获得 SqlSession 的实例。// SqlSession 提供了在数据库执行 SQL 命令所需的所有方法。// 你可以通过 SqlSession 实例来直接执行已映射的 SQL 语句。public static SqlSession getSqlSession(){return sqlSessionFactory.openSession();}
}

（7）BlogMapperTest.java

package com.company.org.dao;import com.company.org.pojo.Blog;
import com.company.org.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;import java.text.SimpleDateFormat;
import java.util.*;public class BlogMapperTest {@Testpublic void addBlog(){SqlSession sqlSession = MybatisUtils.getSqlSession();Blog blog = new Blog(118074003, "【Mybatis】Mybatis入门", "牧心.", " 2021-06-20 20:03:04", 172, "MyBatis", 1, 1);Blog blog2 = new Blog(118074590, "【Mybatis】一个Mybatis程序", "牧心.", "2021-06-22 21:03:03", 129, "MyBatis", 1, 1);Blog blog3 = new Blog(118311033, "【Mybatis】增删改查的实现", "牧心.", "2021-06-28 20:20:29", 108, "MyBatis", 2, 1);Blog blog4 = new Blog(118343512, "【Mybatis】Map传参和模糊查询", "牧心.", "2021-06-29 21:22:36", 11, "MyBatis", 1, 1);Blog blog5 = new Blog(118344763, "【Mybatis】配置之属性优化", "牧心.", "2021-06-29 21:59:54", 21, "MyBatis", 1, 1);BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);mapper.addBlog(blog);mapper.addBlog(blog2);mapper.addBlog(blog3);mapper.addBlog(blog4);mapper.addBlog(blog5);sqlSession.commit();sqlSession.close();}@Testpublic void getBlogList(){SqlSession sqlSession = MybatisUtils.getSqlSession();BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);List<Blog> blogs = mapper.getBlogList();for (Blog blog : blogs) {System.out.println(blog);}sqlSession.close();}@Testpublic void getVisibleBlogListByLabel(){SqlSession sqlSession = MybatisUtils.getSqlSession();BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);List<Blog> blogs = mapper.getVisibleBlogListByLabel("");for (Blog blog : blogs) {System.out.println(blog);}sqlSession.close();}@Testpublic void getVisibleBlogListByLabelAndTitle(){SqlSession sqlSession = MybatisUtils.getSqlSession();BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);Map<String, Object> map = new HashMap<String, Object>();
//        map.put("label", "Spring");map.put("title", "%AOP%");List<Blog> blogs = mapper.getBlogListByLabelAndTitle(map);for (Blog blog : blogs) {System.out.println(blog);}sqlSession.close();}@Testpublic void getVisibleBlogListByLabelAndTitle2(){SqlSession sqlSession = MybatisUtils.getSqlSession();BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);Map<String, Object> map = new HashMap<String, Object>();map.put("label", "MyBatis");
//        map.put("title", "%AOP%");List<Blog> blogs = mapper.getBlogListByLabelAndTitle2(map);for (Blog blog : blogs) {System.out.println(blog);}sqlSession.close();}@Testpublic void getVisibleBlogListByLabelAndTitle3(){SqlSession sqlSession = MybatisUtils.getSqlSession();BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);Map<String, Object> map = new HashMap<String, Object>();
//        map.put("label", "Spring");map.put("title", "%AOP%");List<Blog> blogs = mapper.getBlogListByLabelAndTitle3(map);for (Blog blog : blogs) {System.out.println(blog);}sqlSession.close();}@Testpublic void updateBlogById(){SqlSession sqlSession = MybatisUtils.getSqlSession();BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);Map<String, Object> map = new HashMap<String, Object>();map.put("id", 116502734);map.put("title", "【Spring】DI依赖注入-new");map.put("visible", 0);mapper.updateBlogById(map);sqlSession.commit();sqlSession.close();}@Testpublic void getBlogListByIds(){SqlSession sqlSession = MybatisUtils.getSqlSession();BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);Map<String, Object> map = new HashMap<String, Object>();ArrayList<Integer> ids = new ArrayList<Integer>();ids.add(116012859);ids.add(118058442);ids.add(117374430);map.put("ids", ids);List<Blog> blogs = mapper.getBlogListByIds(map);for (Blog blog : blogs) {System.out.println(blog);}sqlSession.close();}@Testpublic void getBlogListByTitleOrLabel(){SqlSession sqlSession = MybatisUtils.getSqlSession();BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);Map<String, Object> map = new HashMap<String, Object>();
//        map.put("label", "Spring");
//        map.put("title", "%AOP%");map.put("visible", 0);List<Blog> blogs = mapper.getBlogListByTitleOrLabel(map);for (Blog blog : blogs) {System.out.println(blog);}sqlSession.close();}
}

（8）mybatis-config.xml

<?xml version="1.0" encoding="UTF8" ?>
<!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><environments default="development"><environment id="development"><transactionManager type="JDBC"/> <!--事务管理--><dataSource type="POOLED"><property name="driver" value="com.mysql.cj.jdbc.Driver"/><property name="url" value="jdbc:mysql://localhost:3306/mybatis?useSSL=true&amp;userUnicode=true&amp;characterEncoding=utf8&amp;autoReconnect=true&amp;failOverReadOnly=false"/><property name="username" value="root"/><property name="password" value="123456"/></dataSource></environment></environments><!-- 每一个Mapper.XML都需要在Mybatis核心配置文件中注册！ --><mappers><mapper resource="com/company/org/dao/BlogMapper.xml"/></mappers>
</configuration>

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【Mybatis】动态SQL 实例

1. if

2. where

3. set

4. foreach

5. choose、when、otherwise

6. 完整项目实例

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