MyBatis 动态SQL之＜if＞标签-
if标签简介
if标签：条件判断
MyBatis if 类似于 Java 中的 if 语句，是 MyBatis 中最常用的判断语句。使用 if 标签可以节省许多拼接 SQL 的工作，把精力集中在 XML 的维护上。
if 语句使用方法简单，常常与 test 属性联合使用。语法如下:
if标签语法结构
<if test="判断条件">
SQL语句
if>
if标签使用场景
最常见的场景是在 if 语句中包含 where 子句
<select id="selectAllWebsite" resultMap="myResult">
select id,name,url from website
<if test="name != null">
where name like #{name}
if>
select>
以上代表表示根据网站名称去查找相应的网站信息，但是网站名称是一个可填可不填的条件，不填写的时候不作为查询条件。
可多个 if 语句同时使用。以下语句表示为可以按照网站名称（name）或者网址（url）进行模糊查询。如果您不输入名称或网址，则返回所有的网站记录。但是，如果你传递了任意一个参数，它就会返回与给定参数相匹配的记录：
<select id="selectAllWebsite" resultMap="myResult">
select id,name,url from website where 1=1
<if test="name != null">
AND name like #{name}
</if>
<if test="url!= null">
AND url like #{url}
</if>
</select>
DROP TABLE IF EXISTS `website`;
CREATE TABLE `website` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(20) COLLATE utf8_unicode_ci NOT NULL,
`url` varchar(30) COLLATE utf8_unicode_ci DEFAULT '',
`age` tinyint(3) unsigned NOT NULL,
`country` char(3) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
`createtime` timestamp NULL DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
if标签案例
1.数据库准备
use mybatis;
# 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
<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>mybatisartifactId>
<groupId>com.examplegroupId>
<version>1.0-SNAPSHOTversion>
parent>
<modelVersion>4.0.0modelVersion>
<groupId>com.biemgroupId>
<artifactId>ifartifactId>
<properties>
<maven.compiler.source>8maven.compiler.source>
<maven.compiler.target>8maven.compiler.target>
properties>
<dependencies>
<dependency>
<groupId>org.mybatisgroupId>
<artifactId>mybatisartifactId>
<version>3.4.6version>
dependency>
<dependency>
<groupId>junitgroupId>
<artifactId>junitartifactId>
<version>4.12version>
<scope>testscope>
dependency>
<dependency>
<groupId>mysqlgroupId>
<artifactId>mysql-connector-javaartifactId>
<version>8.0.18version>
<scope>runtimescope>
dependency>
<dependency>
<groupId>log4jgroupId>
<artifactId>log4jartifactId>
<version>1.2.17version>
dependency>
<dependency>
<groupId>org.projectlombokgroupId>
<artifactId>lombokartifactId>
<version>1.18.16version>
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.utils
src/main/resources/下创建文件夹
com/biem/mapper
5 框架配置文件
5.1 mybatis核心配置文件mybatis-config.xml
DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<properties resource="jdbc.properties">properties>
<settings>
<setting name="mapUnderscoreToCamelCase" value="true"/>
settings>
<typeAliases>
<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>
<mappers>
<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
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
DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.biem.mapper.CustomerMapper">
mapper>
6.4 mybatis工具类
package com.biem.utils;
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();
}
}
}
7 If标签功能测试
7.1 com.biem.mapper.CustomerMapper.class中添加
public List<Customer> findCustomerByNameAndJobs(Customer customer);
7.2 com/biem/mapper/CustomerMapper.xml中添加
<select id="findCustomerByNameAndJobs" parameterType="customer" resultType="customer">
select * from t_customer where 1=1
<if test="username !=null and username != ''">
and username like concat('%', #{username}, '%')
if>
<if test="jobs !=null and jobs != ''">
and jobs=#{jobs}
if>
select>
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 org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import com.biem.utils.MybatisUtil;
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 testFindAll(){
// 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();
CustomerMapper mapper = session.getMapper(CustomerMapper.class);
List<Customer> customers = mapper.findCustomerByNameAndJobs(customer);
System.out.println("customers = " + customers);
// 关闭SqlSession
session.close();
}
@Test
public void testFindCustomerByNameAndJobs(){
// 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.setUsername("jack");
customer.setJobs("teacher");
CustomerMapper mapper = session.getMapper(CustomerMapper.class);
List<Customer> customers = mapper.findCustomerByNameAndJobs(customer);
System.out.println("customers = " + customers);
// 关闭SqlSession
session.close();
}
}