文章目錄

• 1. 簡(jiǎn)介
• 2. 搭建環(huán)境
• • 2.1 在MySQL中創(chuàng)建blog表
  • 2.2 編寫實(shí)體類
  • 2.3 編寫實(shí)體類對(duì)應(yīng)Mapper接口
  • 2.4 編寫Mapper接口對(duì)應(yīng)的Mapper.xml文件
  • 2.5 編寫測(cè)試類
• 3. if
• 4. where
• 5. set
• 6. choose
• 7. foreach

1. 簡(jiǎn)介

動(dòng)態(tài)SQL就是指根據(jù)不同的條件生成不同的SQL語(yǔ)句

傳統(tǒng)的使用JDBC的方法，在組合復(fù)雜的的SQL語(yǔ)句的時(shí)候，需要去拼接，稍不注意哪怕少了個(gè)空格，都會(huì)導(dǎo)致錯(cuò)誤。Mybatis的動(dòng)態(tài)SQL功能正是為了解決這種問題， 其通過 if, choose, when, otherwise, trim, where, set, foreach標(biāo)簽，可組合成非常靈活的SQL語(yǔ)句，從而提高開發(fā)人員的效率。

2. 搭建環(huán)境

2.1 在MySQL中創(chuàng)建blog表

CREATE TABLE `blog` (`id` varchar(50) NOT NULL COMMENT '博客id',`title` varchar(100) NOT NULL COMMENT '博客標(biāo)題',`author` varchar(30) NOT NULL COMMENT '博客作者',`create_time` datetime NOT NULL COMMENT '創(chuàng)建時(shí)間',`views` int(30) NOT NULL COMMENT '瀏覽量' ) ENGINE=InnoDB DEFAULT CHARSET=utf8

2.2 編寫實(shí)體類

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

可選擇在idea中連接數(shù)據(jù)庫(kù)

解決辦法：在mybatis-config.xml中開啟駝峰命名
同時(shí)也開啟日志

<!--開啟日志和駝峰命名--><settings><setting name="mapUnderscoreToCamelCase" value="true"/><setting name="logImpl" value="STDOUT_LOGGING"/></settings>

2.3 編寫實(shí)體類對(duì)應(yīng)Mapper接口

package com.zz.mapper;import com.zz.pojo.Blog;public interface BlogMapper {//新增一個(gè)博客int addBlog(Blog blog); }

2.4 編寫Mapper接口對(duì)應(yīng)的Mapper.xml文件

<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="com.zz.mapper.BlogMapper"><insert id="addBlog" parameterType="Blog">insert into mybatis.blog (id, title, author, create_time, views)values (#{id},#{title},#{author},#{createDate},#{views}); </insert></mapper>

2.5 編寫測(cè)試類

可選擇在util目錄下創(chuàng)建IDUtils工具類，因?yàn)樵谡鎸?shí)開發(fā)中id一般是隨機(jī)的數(shù)字，調(diào)用IDUtils類，保證生成的id都不相同。

package com.zz.utils;import java.util.UUID;public class IDUtils {public static String getId(){return UUID.randomUUID().toString().replaceAll("-","");} }

編寫測(cè)試類，往數(shù)據(jù)庫(kù)中添加八篇博客

package com.zz.mapper;import com.zz.pojo.Blog; import com.zz.utils.IDUtils; import com.zz.utils.MyBatisUtils; import org.apache.ibatis.session.SqlSession; import org.junit.Test;import java.util.Date;public class BlogMapperTest {@Testpublic void testAddBlog(){SqlSession session = MyBatisUtils.getSession();BlogMapper mapper = session.getMapper(BlogMapper.class);Blog blog01 = new Blog();//調(diào)用IDUtils包，保證id都不相同blog01.setId(IDUtils.getId()); //真實(shí)開發(fā)中id一般是隨機(jī)的數(shù)字（uuid）blog01.setTitle("Hello MyBatis");blog01.setAuthor("大佬");blog01.setCreateDate(new Date());blog01.setViews(9999);mapper.addBlog(blog01);Blog blog02 = new Blog();//調(diào)用IDUtils包，保證id都不相同blog02.setId(IDUtils.getId()); //真實(shí)開發(fā)中id一般是隨機(jī)的數(shù)字（uuid）blog02.setTitle("Hello JDBC");blog02.setAuthor("大佬");blog02.setCreateDate(new Date());blog02.setViews(9999);mapper.addBlog(blog02);Blog blog03 = new Blog();//調(diào)用IDUtils包，保證id都不相同blog03.setId(IDUtils.getId()); //真實(shí)開發(fā)中id一般是隨機(jī)的數(shù)字（uuid）blog03.setTitle("Hello MySQL");blog03.setAuthor("大佬");blog03.setCreateDate(new Date());blog03.setViews(9999);mapper.addBlog(blog03);Blog blog04 = new Blog();//調(diào)用IDUtils類，保證id都不相同blog04.setId(IDUtils.getId()); //真實(shí)開發(fā)中id一般是隨機(jī)的數(shù)字（uuid）blog04.setTitle("Hello Spring");blog04.setAuthor("大佬");blog04.setCreateDate(new Date());blog04.setViews(9999);mapper.addBlog(blog04);session.commit(); //提交事務(wù)} }

運(yùn)行結(jié)果：

至此，環(huán)境搭建完畢！

3. if

接口BlogMapper的代碼：

package com.zz.mapper;import com.zz.pojo.Blog;import java.util.List; import java.util.Map;public interface BlogMapper {//新增一個(gè)博客int addBlog(Blog blog);//通過作者名和博客名來查詢博客//如果作者名為空，則根據(jù)博客名來查詢List<Blog> getBlogByIf(Map map); }

接口的配置文件BlogMapper.xml 代碼：

<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="com.zz.mapper.BlogMapper"><insert id="addBlog" parameterType="Blog">insert into mybatis.blog (id, title, author, create_time, views)values (#{id},#{title},#{author},#{createDate},#{views}); </insert><select id="getBlogByIf" resultType="Blog" parameterType="map">select * from mybatis.blog where<if test="title!=null">title=#{title}</if><if test="author!=null">and author=#{author}</if></select></mapper>

測(cè)試類BlogMapperTest 代碼：

• 當(dāng)在map中只添加title時(shí)

public class BlogMapperTest {//測(cè)試if@Testpublic void testGetBlogByIf(){SqlSession session = MyBatisUtils.getSession();BlogMapper mapper = session.getMapper(BlogMapper.class);Map<String, String> map = new HashMap<String, String>();map.put("title","Hello MyBatis");//map.put("author","小白");List<Blog> blogByIf = mapper.getBlogByIf(map);for (Blog blog : blogByIf) {System.out.println(blog);}} }

運(yùn)行結(jié)果：

• 當(dāng)在map中添加title和author時(shí)

public class BlogMapperTest {//測(cè)試if@Testpublic void testGetBlogByIf(){SqlSession session = MyBatisUtils.getSession();BlogMapper mapper = session.getMapper(BlogMapper.class);Map<String, String> map = new HashMap<String, String>();map.put("title","Hello MyBatis");map.put("author","小白");List<Blog> blogByIf = mapper.getBlogByIf(map);for (Blog blog : blogByIf) {System.out.println(blog);}} }

運(yùn)行結(jié)果：

可以看出：根據(jù)不同的條件生成不同的SQL語(yǔ)句，即動(dòng)態(tài)SQL

4. where

使用場(chǎng)景：如果我們需要拼接where條件，又不希望客戶端傳遞錯(cuò)誤信息，這時(shí)需要使用where 標(biāo)簽。
作用：如果后面有語(yǔ)句，就自動(dòng)添加where；如果后面語(yǔ)句開頭是and 或者or，它可以自動(dòng)去掉。

接口的配置文件BlogMapper.xml 代碼：
注意：在author 前添加了 and

<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="com.zz.mapper.BlogMapper"><insert id="addBlog" parameterType="Blog">insert into mybatis.blog (id, title, author, create_time, views)values (#{id},#{title},#{author},#{createDate},#{views}); </insert><select id="getBlogByIf" resultType="Blog" parameterType="map">select * from mybatis.blog<where><if test="title!=null">title=#{title}</if><if test="author!=null">and author=#{author}</if></where></select></mapper>

測(cè)試類BlogMapperTest 的代碼：

• 當(dāng)在map中什么都不添加，為空時(shí)

public class BlogMapperTest {//測(cè)試where@Testpublic void testGetBlogByIf(){SqlSession session = MyBatisUtils.getSession();BlogMapper mapper = session.getMapper(BlogMapper.class);Map<String, String> map = new HashMap<String, String>();//map.put("title","Hello MyBatis");//map.put("author","小白");List<Blog> blogByIf = mapper.getBlogByIf(map);for (Blog blog : blogByIf) {System.out.println(blog);}} }

運(yùn)行結(jié)果：

可以看出：查詢出了所有的博客

• 當(dāng)在map中只添加author時(shí)

public class BlogMapperTest { //測(cè)試where@Testpublic void testGetBlogByIf(){SqlSession session = MyBatisUtils.getSession();BlogMapper mapper = session.getMapper(BlogMapper.class);Map<String, String> map = new HashMap<String, String>();//map.put("title","Hello MyBatis");map.put("author","小白");List<Blog> blogByIf = mapper.getBlogByIf(map);for (Blog blog : blogByIf) {System.out.println(blog);}} }

運(yùn)行結(jié)果：

可以看出，把配置文件中的 and 去掉了

5. set

如果里面的條件滿足，自動(dòng)拼接set標(biāo)簽；后面如果有多余的逗號(hào)，可以自動(dòng)去除

接口BlogMapper的代碼：

//更新博客int updateBlog(Map map);

接口的配置文件BlogMapper.xml 代碼：

注意：在title 后添加了逗號(hào)

<insert id="updateBlog" parameterType="map">update mybatis.blog<set><if test="title !=null">title = #{title},</if><if test="author !=null">author = #{author}</if></set>where id= #{id}</insert>

測(cè)試類BlogMapperTest 的代碼：

map中必須添加id，否則會(huì)報(bào)錯(cuò)

• 當(dāng)在map中只添加id和author時(shí)

//測(cè)試set@Testpublic void testSet(){SqlSession session = MyBatisUtils.getSession();BlogMapper mapper = session.getMapper(BlogMapper.class);HashMap<String, String> map = new HashMap<String, String>();map.put("id","74205db5524c4e92942241e8b6443032");map.put("author","大佬");//map.put("title","大佬");mapper.updateBlog(map);session.commit();}

運(yùn)行結(jié)果：

• 當(dāng)在map中添加id，author和title時(shí)

//測(cè)試set@Testpublic void testSet(){SqlSession session = MyBatisUtils.getSession();BlogMapper mapper = session.getMapper(BlogMapper.class);HashMap<String, String> map = new HashMap<String, String>();map.put("id","74205db5524c4e92942241e8b6443032");map.put("author","大佬");map.put("title","大佬");mapper.updateBlog(map);session.commit();}

運(yùn)行結(jié)果：

• 當(dāng)在map中只添加id和title時(shí)

//測(cè)試set@Testpublic void testSet(){SqlSession session = MyBatisUtils.getSession();BlogMapper mapper = session.getMapper(BlogMapper.class);HashMap<String, String> map = new HashMap<String, String>();map.put("id","74205db5524c4e92942241e8b6443032");//map.put("author","大佬");map.put("title","大佬");mapper.updateBlog(map);session.commit();}

運(yùn)行結(jié)果：

可以發(fā)現(xiàn)：title 后面的逗號(hào)去掉了

where標(biāo)簽和set 標(biāo)簽的底層都是trim標(biāo)簽

6. choose

好比java中的switch

接口BlogMapper的代碼：

//查詢博客 但是只要有一個(gè)條件滿足即可 List<Blog> queryBlogByChoose(Map map);

接口的配置文件BlogMapper.xml 代碼：

<select id="queryBlogByChoose" parameterType="map" resultType="Blog">select * from mybatis.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>

測(cè)試類BlogMapperTest 的代碼：

• 當(dāng)在map中什么都不添加時(shí)

//測(cè)試choose@Testpublic void testChoose(){SqlSession session = MyBatisUtils.getSession();BlogMapper mapper = session.getMapper(BlogMapper.class);HashMap<String, String> map = new HashMap<String, String>();mapper.queryBlogByChoose(map);session.commit();}

運(yùn)行結(jié)果：

• 當(dāng)在map中只添加title時(shí)

//測(cè)試choose@Testpublic void testChoose(){SqlSession session = MyBatisUtils.getSession();BlogMapper mapper = session.getMapper(BlogMapper.class);HashMap<String, String> map = new HashMap<String, String>();map.put("title","Hello Spring");mapper.queryBlogByChoose(map);session.commit();}

運(yùn)行結(jié)果：

• 當(dāng)在map中只添加author時(shí)

//測(cè)試choose@Testpublic void testChoose(){SqlSession session = MyBatisUtils.getSession();BlogMapper mapper = session.getMapper(BlogMapper.class);HashMap<String, String> map = new HashMap<String, String>();//map.put("title","Hello Spring");map.put("author","大佬");mapper.queryBlogByChoose(map);session.commit();}

運(yùn)行結(jié)果：

• 當(dāng)在map中添加title和author時(shí)

//測(cè)試choose@Testpublic void testChoose(){SqlSession session = MyBatisUtils.getSession();BlogMapper mapper = session.getMapper(BlogMapper.class);HashMap<String, String> map = new HashMap<String, String>();map.put("title","Hello Spring");map.put("author","大佬");mapper.queryBlogByChoose(map);session.commit();}

運(yùn)行結(jié)果：

可以看出：雖然傳入了兩個(gè)參數(shù)，但只顯示了一個(gè)

7. foreach

相當(dāng)于子查詢 where in(1,2,3)

collection 表示輸入的參數(shù) map
item 表示遍歷出來的每一項(xiàng)
open 表示打開
close 表示關(guān)閉
separator 表示分割符
通過item 遍歷出來的標(biāo)簽可以在foreach中使用

接口BlogMapper的代碼：

List<Blog> queryBlogByForeach(Map map);

接口的配置文件BlogMapper.xml 代碼：

<!--子查詢 where in(1,2,3)--><select id="queryBlogByForeach" parameterType="map" resultType="Blog">select * from mybatis.blog<where><foreach collection="ids" item="id" open="and (" close=")" separator="or">id = #{id}</foreach></where></select>

測(cè)試類BlogMapperTest 代碼：

//測(cè)試foreach@Testpublic void testForeach(){SqlSession session = MyBatisUtils.getSession();BlogMapper mapper = session.getMapper(BlogMapper.class);Map<String,List> map = new HashMap();List<String> ids = new ArrayList<String>();ids.add("bac368d88b4a4d8eb6bd1b539a3338a0");ids.add("321c98e9beea45d092c39b2cfe0fc370");ids.add("cfbc34f0a1dd40fdba91e99dfe4c365a");map.put("ids",ids);mapper.queryBlogByForeach(map);session.commit();}

運(yùn)行結(jié)果：

可以看出：就相當(dāng)于SQL中的 select * from mybatis.blog where and (id=xxx or id=xxx or id=xxx)

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