动态sql
什么是动态sql，动态sql就是根据不同的条件产生不同的sql语句
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。
if
choose (when, otherwise)
trim (where, set)
foreach
搭建环境
CREATE TABLE `mybatis`.`blog` ( `id` INT(10) NOT NULL AUTO_INCREMENT COMMENT '博客id', `title` VARCHAR(30) 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', PRIMARY KEY (`id`) )
创建一个基础工程
导包
编写配置文件
<?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> <!-- 外部配置文件--> <properties resource="com/hyc/dao/db.properties"/> <!-- 可以给实体类起别名--> <settings> <setting name="logImpl" value="LOG4J"/> </settings> <typeAliases> <!-- <typeAlias type="com.hyc.pojo.user" alias="user"/>--> <package name="com.hyc.pojo"/> </typeAliases> <environments default="development"> <environment id="development"> <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> <mappers> <mapper resource="com/hyc/dao/BlogMapper.xml"/> </mappers> </configuration>
编写实体类
注意编写日期要用 util包下的date
package com.hyc.pojo; import lombok.Data; import java.util.Date; @Data public class Blog { private int id; private String title; private String author; private Date createTime; private int views; }
编写实体类对应的Mapper接口和对应的Mapper.xml文件
if
<select id="queryBlogif" 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>
choose
<select id="queryBlogChoose" resultType="blog" parameterType="map"> select * from blog <where> <choose> <when test="title != null"> and title = #{title} </when> <when test="author != null"> and author = #{author} </when> <otherwise> and views = #{views} </otherwise> </choose> </where> </select>
trim(where,set)
<select id="queryBlogif" parameterType="map" resultType="blog"> select * from blog <where> <if test="title != null"> and 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>
所谓的动态sqL,本质上就是sql语句，只是我们可以在sql层面，去执行一个逻辑代码
sql片段
有的时候，我们可能会将一些公共的部分抽取出来，方便使用
通过include标签的refid属性来调用sql片段
<select id="queryBlogif" parameterType="map" resultType="blog"> select * from blog <where> <include refid="if-title-author"></include> </where> </select>
设置id来让sql片段可以被调用，
<sql id="if-title-author"> <if test="title != null"> and title = #{title}; </if> <if test="author != null"> and author = #{author}; </if> </sql>
注意事项：
最好基于单表来定义sql片段
最好不在sql片段存在where
Foreach
select * from User where 1=1 and (id = 1 or id=2 or id=3) <select id="selectPostIn" resultType="domain.blog.Post"> SELECT * FROM POST P WHERE ID in <foreach item="item" index="index" collection="list" open="(" separator="," close=")"> #{item} </foreach> </select>
<!-- 就是拼接sql--> <!-- select * from blog where 1=1 and (id = 1 or id=2 or id=3) --> <select id="queryBlogForeach" resultType="blog" parameterType="map"> select * from blog <where> <foreach collection="ids" item="id" open="and (" close=")" separator="or"> id=#{id} </foreach> </where> </select>
建议：
现在mysql写出完整的sql，再对应的修改我们的动态sql再使用，
