简介
where 标签主要用来简化 SQL 语句中的条件判断，可以自动处理 AND/OR 条件。
在if标签和choose-when-otherwise标签的案例中，SQL语句加入了一个条件’1=1’，它既保证了where后面的条件成，页避免了where后面出现的第一个词语是and 或者or之类的关键字。
假设把条件‘1=1’去掉，可以出现以下语句
select * from t_customer where and username like concat('%','#{username}','%')
上面语句因为出现了where后直接是and，在sql运行时会报语法错误。
这个时候可以使用where标签处理
语法
<where>
<if test="判断条件">
AND/OR ...
</if>
</where>
if 语句中判断条件为 true 时，where 关键字才会加入到组装的 SQL 里面，否则就不加入。where 会检索语句，它会将 where 后的第一个 SQL 条件语句的 AND 或者 OR 关键词去掉。
网络案例
<select id="selectWebsite" resultType="net.biancheng.po.Website">
select id,name,url from website
<where>
<if test="name != null">
AND name like #{name}
</if>
<if test="url!= null">
AND url like #{url}
</if>
</where>
</select>
where标签-完整案例
1.数据库准备
# 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.新建项目或Module
3 pom.xml中添加
<?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.创建package和文件夹
src/main/java/下创建package
com.biem.pojo
com.biem.mapper
com.biem.util
src/main/resources/下创建文件夹
com/biem/mapper
src/test/java下创建package
com.biem.test
5 框架配置文件
5.1 mybatis核心配置文件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属性文件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文件
<?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 用户配置文件
6.1 实体类
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;
}
需要在pom.xml中引入lombok，简化原来的实体类的代码
6.2 mybatis接口类
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用户配置文件
<?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工具类
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();
}
}
}
项目结构如下
7 标签功能测试
if 语句中判断条件为 true 时，where 关键字才会加入到组装的 SQL 里面，否则就不加入。where 会检索语句，它会将 where 后的第一个 SQL 条件语句的 AND 或者 OR 关键词去掉。
7.1 com.biem.mapper.CustomerMapper.class中添加
public List<Customer> findCustomerByIf(Customer customer);
public List<Customer> findCustomerByWhere(Customer customer);
7.2 com/biem/mapper/CustomerMapper.xml中添加
<!-- public List<Customer> findCustomerByIf(Customer customer);-->
<select id="findCustomerByIf" parameterType="customer" resultType="customer">
select * from t_customer where
<if test="username !=null and username != ''">
and username like concat('%', #{username}, '%')
</if>
<if test="jobs !=null and jobs != ''">
and jobs=#{jobs}
</if>
</select>
<!-- public List<Customer> findCustomerByWhere(Customer customer);-->
<select id="findCustomerByWhere" parameterType="customer" resultType="customer">
select * from t_customer
<where>
<if test="username !=null and username != ''">
and username like concat('%', #{username}, '%')
</if>
<if test="jobs !=null and jobs != ''">
and jobs=#{jobs}
</if>
</where>
8 功能测试
在src/test/java中创建类com.biem.test.TestCustomer.java，内容如下文章来源：https://www.toymoban.com/news/detail-668264.html
package com.biem.test;
import com.biem.mapper.CustomerMapper;
import com.biem.pojo.Customer;
import com.biem.util.MybatisUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import java.util.List;
/**
* ClassName: TestCustomer
* Package: com.biem.test
* Description:
*
* @Create 2023/4/5 22:32
* @Version 1.0
*/
public class TestCustomer {
@Test
public void testFindCustomerByIf(){
// 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.findCustomerByIf(customer);
System.out.println("customers = " + customers);
// 关闭SqlSession
session.close();
}
@Test
public void testFindCustomerByWhere(){
// 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.findCustomerByWhere(customer);
System.out.println("customers = " + customers);
// 关闭SqlSession
session.close();
}
}
结果分析：testFindCustomerByIf在username为null的时候会因为语法错误报错文章来源地址https://www.toymoban.com/news/detail-668264.html
com.biem.test.TestCustomer,testFindCustomerByIf
DEBUG 04-06 09:56:24,100 ==> Preparing: select * from t_customer where and jobs=? (BaseJdbcLogger.java:159)
DEBUG 04-06 09:56:24,152 ==> Parameters: teacher(String) (BaseJdbcLogger.java:159)
org.apache.ibatis.exceptions.PersistenceException:
### Error querying database. Cause: java.sql.SQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'and jobs='teacher'' at line 4
### The error may exist in com/biem/mapper/CustomerMapper.xml
### The error may involve com.biem.mapper.CustomerMapper.findCustomerByIf-Inline
### The error occurred while setting parameters
### SQL: select * from t_customer where and jobs=?
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