MyBatis 动态SQL之＜where＞标签-

• Jave

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

    
        AND/OR ...

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,name,url from website
    
        
            AND name like #{name}
        
        
            AND url like #{url}

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

在这里插入图片描述

3 pom.xml中添加


    
        mybatis
        com.example
        1.0-SNAPSHOT
    
    4.0.0

    com.biem
    dynamaicSql

    
        8
        8
    

    
        
        
            org.mybatis
            mybatis
            3.4.6
        
        
        
            junit
            junit
            4.12
            test
        

        
        
            mysql
            mysql-connector-java
            8.0.18
            runtime
        

        
        
            log4j
            log4j
            1.2.17
        

        
            org.projectlombok
            lombok
            1.18.16

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
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

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
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

在这里插入图片描述

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 findCustomerByIf(Customer customer);

    public List findCustomerByWhere(Customer customer);
7.2 com/biem/mapper/CustomerMapper.xml中添加
<!-- public List findCustomerByIf(Customer customer);-->
    
        select * from t_customer where
        
            and username like concat('%', #{username}, '%')
        
        
            and jobs=#{jobs}
        
    

    <!-- public List findCustomerByWhere(Customer customer);-->
    
        select * from t_customer
        
            
                and username like concat('%', #{username}, '%')
            
            
                and jobs=#{jobs}

8 功能测试

在src/test/java中创建类com.biem.test.TestCustomer.java，内容如下

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 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 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

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=?

本文来自网络，不代表协通编程立场，如若转载，请注明出处：https://www.net2asp.com/66a43c4dfd.html

赞 (0)
« 上一篇 1天前
下一篇 » 1天前

相关推荐