动态SQL 动态SQL==根据不同的条件生成不同的SQL语句==ifchoose (when, otherwise)trim (where, set)foreach搭建环境CREATE TABLE `blog`( `id` VARCHAR(50) NOT NULL COMMENT 'id', `title` VARCHAR(100) NOT NULL COMMENT '标题', `author` VARCHAR(50) NOT NULL COMMENT '作者', `create_time` datetime NOT NULL COMMENT '创建时间', `views` int(11) NOT NULL COMMENT '浏览量' )ENGINE=INNODB DEFAULT CHARSET=utf8;1、导包2、编写配置文件3、创建实体类package com.sw.pojo; import lombok.Data; import java.io.Serializable; import java.util.Date; /** * @Author suaxi * @Date 2020/12/13 16:20 */ @Data public class blog implements Serializable { private String id; private String title; private String author; private Date createTime; private int views; } 4、编写实体类对应的Mapper接口和Mapper.xml文件IF<select id="findBlogIf" 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>chose(when,otherwise)<select id="findBlogChoose" 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>trim(where,set) <select id="findBlogIf" 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><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>SQL片段将部分SQl抽取出来，方便复用1、使用SQL标签抽取公共部分<sql id="if-title-author"> <if test="title != null"> title = #{title} </if> <if test="author != null"> and author = #{author} </if> </sql>2、在需要使用的地方用include标签引用、<select id="findBlogIf" parameterType="map" resultType="Blog"> select * from blog <where> <include refid="if-title-author"></include> </where> </select>注：最好基于单表查询，不要存在where标签Foreach<!--select * from bolg where 1=1 and (id=1 or id=2 or id=3)--> <select id="findBlogForeach" 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，先确保SQL的正确性，再按照SQL格式去排列组合==先在MySQL中写出完整的sql，再去修改成动态sql实现通用