Introduction
It is mentioned in the article Where Tag of MyBatis Dynamic SQL
: In the case of the if tag and choose-when-otherwise tag, the SQL statement adds a condition '1=1', which not only ensures the success of the condition behind where, but also avoids The first word that appears after where is a keyword such as and or or.
Assuming that the condition '1=1' is removed, the following statement can appear
select * from t_customer where and username like concat('%','#{username}','%')
The above statement is directly followed by and because where appears, and a syntax error will be reported when SQL is running.
At this time, you can use the where tag to process, and here you can also use the trim tag to process
the trim tag has four attributes : prefix, prefixOverrides, suffix, suffixOverrides
| prefix | The prefix for sql statement splicing |
| suffix | The suffix for sql statement splicing |
| prefixOverrides | Remove the keyword or character in front of the sql statement, the keyword or character is specified by the prefixOverrides attribute, assuming that the attribute is specified as "AND", when the beginning of the sql statement is "AND", the trim tag will remove the "AND" |
| suffixOverrides | Remove keywords or characters after the sql statement, which are specified by the suffixOverrides attribute |
grammatical format
trim is generally used to remove redundant AND keywords and commas in SQL statements, or to add suffixes such as where and set to SQL statements, and can be used for selective insert, update, delete, or conditional queries. The trim syntax is as follows.
<trim prefix="前缀" suffix="后缀" prefixOverrides="忽略前缀字符" suffixOverrides="忽略后缀字符">
SQL语句
</trim>
Network case
<select id="selectWebsite" resultType="net.biancheng.po.Website">
SELECT id,name,url,age,country
FROM website
<trim prefix="where" prefixOverrides="and">
<if test="name != null and name !=''">
AND name LIKE CONCAT ('%',#{name},'%')
</if>
<if test="url!= null">
AND url like concat ('%',#{url},'%')
</if>
</trim>
</select>
basic environment
1. Database preparation
# 创建一个名称为t_customer的表
CREATE TABLE t_customer (
id int(32) PRIMARY KEY AUTO_INCREMENT,
username varchar(50),
jobs varchar(50),
phone varchar(16)
);
# 插入3条数据
INSERT INTO t_customer VALUES ('1', 'joy', 'teacher', '13733333333');
INSERT INTO t_customer VALUES ('2', 'jack', 'teacher', '13522222222');
INSERT INTO t_customer VALUES ('3', 'tom', 'worker', '15111111111');
2. Create a new project or Module
3 Add in 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>mybatis</artifactId>
<groupId>com.example</groupId>
<version>1.0-SNAPSHOT</version>
</parent>
<modelVersion>4.0.0</modelVersion>
<groupId>com.biem</groupId>
<artifactId>dynamaicSql</artifactId>
<properties>
<maven.compiler.source>8</maven.compiler.source>
<maven.compiler.target>8</maven.compiler.target>
</properties>
<dependencies>
<!--1.引入mybatis包-->
<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.4.6</version>
</dependency>
<!--2.单元测试-->
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.12</version>
<scope>test</scope>
</dependency>
<!--3.mysql驱动-->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>8.0.18</version>
<scope>runtime</scope>
</dependency>
<!--4.log4j日志-->
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.17</version>
</dependency>
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
<version>1.18.16</version>
</dependency>
</dependencies>
</project>
4. Create package and folder
Create package com.biem.pojo under src/main/java/
com.biem.mapper
com.biem.util
Create folder
com/ biem/mapper under src/main/resources/
Create package
com.biem under src/test/java .test
5 Framework configuration files
5.1 mybatis core configuration file 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>
<!--引入properties文件-->
<properties resource="jdbc.properties"></properties>
<!--将下划线映射为驼峰-->
<settings>
<setting name="mapUnderscoreToCamelCase" value="true"/>
</settings>
<!--设置类型别名-->
<typeAliases>
<!--
以包为单位，将包下所有的类型设置设置默认的类型别名，即类名且不区分大小写
-->
<package name="com.biem.pojo"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${jdbc.driver}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.username}"/>
<property name="password" value="${jdbc.password}"/>
</dataSource>
</environment>
</environments>
<!-- 引入映射文件 -->
<mappers>
<!--
以包为单位引入映射文件
要求：
1. mapper接口所在的包要和映射文件所在的包一致
2. mapper接口要和映射文件的名字一致
-->
<package name="com.biem.mapper"/>
</mappers>
</configuration>
5.2 mybatis property file jdbc.properties
jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/mybatis?serverTimezone=UTC
jdbc.username=root
jdbc.password=root
5.3 log4j.xml file
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">
<appender name="STDOUT" class="org.apache.log4j.ConsoleAppender">
<param name="Encoding" value="UTF-8"/>
<layout class="org.apache.log4j.PatternLayout">
<param name="ConversionPattern" value="%-5p %d{MM-dd HH:mm:ss,SSS} %m (%F:%L) \n"/>
</layout>
</appender>
<logger name="java.sql">
<level value="debug"/>
</logger>
<logger name="org.apache.ibatis">
<level value="info"/>
</logger>
<root>
<level value="debug"/>
<appender-ref ref="STDOUT"/>
</root>
</log4j:configuration>
6 User Profiles
6.1 Entity class
package com.biem.pojo;
import lombok.*;
/**
* ClassName: Customer
* Package: com.biem.pojo
* Description:
*
* @Create 2023/4/5 22:17
* @Version 1.0
*/
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@Builder
@ToString
public class Customer {
private Integer id;
private String username;
private String jobs;
private String phone;
}
It is necessary to introduce lombok in pom.xml to simplify the code of the original entity class
6.2 mybatis interface class
package com.biem.mapper;
/**
* ClassName: CustomerMapper
* Package: com.biem.mapper
* Description:
*
* @Create 2023/4/5 22:19
* @Version 1.0
*/
public interface CustomerMapper {
}
6.3 mybatis user configuration file
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.biem.mapper.CustomerMapper">
<!-- namespace要和mapper接口的全类名保持一致，例如com.biem.mybatis.mapper.xxxMapper -->
<!-- sql语句要和接口的方法名保持一致 -->
</mapper>
6.4 mybatis tool class
package com.biem.util;
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;
/**
* ClassName: MybatisUtil
* Package: com.biem.utils
* Description:
*
* @Create 2023/4/5 22:23
* @Version 1.0
*/
public class MybatisUtil {
//利用static(静态)属于类不属于对象,且全局唯一
private static SqlSessionFactory sqlSessionFactory = null;
//利用静态块在初始化类时实例化sqlSessionFactory
static {
InputStream is= null;
try {
is = Resources.getResourceAsStream("mybatis-config.xml");
sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
} catch (IOException e) {
e.printStackTrace();
throw new ExceptionInInitializerError(e);
}
}
/**
* openSession 创建一个新的SqlSession对象
* @return SqlSession对象
*/
public static SqlSession openSession(boolean autoCommit){
return sqlSessionFactory.openSession(autoCommit);
}
public static SqlSession openSession(){
return sqlSessionFactory.openSession();
}
/**
* 释放一个有效的SqlSession对象
* @param session 准备释放SqlSession对象
*/
public static void closeSession(SqlSession session){
if(session != null){
session.close();
}
}
}
The project structure is as follows
Use the trim tag to remove redundant and keywords
1 Add in com.biem.mapper.CustomerMapper.class
public List<Customer> findCustomerByTrim(Customer customer);
2 Add in com/biem/mapper/CustomerMapper.xml
<!--public List<Customer> findCustomerByTrim(Customer customer);-->
<select id="findCustomerByTrim" parameterType="customer" resultType="customer">
select * from t_customer
<trim prefix="where" prefixOverrides="and">
<if test="username !=null and username != ''">
and username like concat('%', #{username}, '%')
</if>
<if test="jobs !=null and jobs != ''">
and jobs=#{jobs}
</if>
</trim>
</select>
3. Functional test com.biem.test.TestMybatis.java
@Test
public void testFindCustomerByTrim(){
// 通过工具类获取SqlSession对象
SqlSession session = MybatisUtil.openSession();
// 创建Customer对象，封装需要组合查询的条件
Customer customer = new Customer();
customer.setJobs("teacher");
CustomerMapper mapper = session.getMapper(CustomerMapper.class);
List<Customer> customers = mapper.findCustomerByTrim(customer);
System.out.println("customers = " + customers);
// 关闭SqlSession
session.close();
}