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MyBatis最佳实践:动态 SQL

article 2025/1/26 17:29:12

第一章：MyBatis 映射文件 SQL 深入

配置 log4j.properrties 配置文件

该配置文件可打印输出 SQL 查询的日志

#将等级为DEBUG的日志信息输出到console和file这两个目的地，console和file的定义在下面的代码
log4j.rootLogger=DEBUG,console,file

#控制台输出的相关设置
log4j.appender.console = org.apache.log4j.ConsoleAppender
log4j.appender.console.Target = System.out
log4j.appender.console.Threshold=DEBUG
log4j.appender.console.layout = org.apache.log4j.PatternLayout
log4j.appender.console.layout.ConversionPattern=[%c]-%m%n

#文件输出的相关设置
log4j.appender.file = org.apache.log4j.RollingFileAppender
log4j.appender.file.File=./log/kuang.log
log4j.appender.file.MaxFileSize=10mb
log4j.appender.file.Threshold=DEBUG
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=[%p][%d{yy-MM-dd}][%c]%m%n

#日志输出级别
log4j.logger.org.mybatis=DEBUG
log4j.logger.java.sql=DEBUG
log4j.logger.java.sql.Statement=DEBUG
log4j.logger.java.sql.ResultSet=DEBUG
log4j.logger.java.sql.PreparedStatement=DEBUG

动态 SQL 语句之 if 标签：

之前做过拼接 SQL 语句查询条件的查询，需要动态的拼接 SQL 语句

UserMapper 接口的方法

public interface UserMapper {
    //条件查询
    public List<User> findByWhere(User user);
}a

UserMapper.xml 配置文件

<!-- 动态 SQL 条件查询 -->
<select id="findByWhere" resultType="com.qcby.model.User">
    select * from user where 1 = 1
    <if test="username != null and username != ''">
        and username like #{username}
    </if>
    <if test="sex != null and sex != ''">
        and sex = #{sex}
    </if>
</select>

测试方法：

@Test
public void test() throws IOException {
    //加载主配置文件
    InputStream in = Resources.getResourceAsStream("SqlMapConfig.xml");

    //创建 SqlSessionFactory 对象
    SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);

    SqlSession session = factory.openSession();

    UserMapper userMapper = session.getMapper(UserMapper.class);

    User user = new User();
    user.setSex("男");
    List<User> byWhere = userMapper.findByWhere(user);
    for (User u : byWhere){
        System.out.println(u);
    }

    session.close();
    in.close();
}

动态 SQL 语句之 where 标签：

where 标签的目的就是为了去掉 where 1 = 1 的拼接
where 标签使用在 if 标签的外面

代码：

<!-- 动态 SQL 条件查询 where 标签 -->
<select id="findByWhere" resultType="com.qcby.model.User">
    select * from user 
    <where>
        <if test="username != null and username != ''">
            and username like #{username}
        </if>
        <if test="sex != null and sex != ''">
            and sex = #{sex}
        </if>
    </where>
</select>

动态 SQL 语句之 foreach 标签：

需求一：

需求的 SQL 语句：select * from user where id = 1 or id = 2 or id = 3

在 User类中添加属性：

public class User implements Serializable {
    private static final long serialVersionUID = 525400707336671154L;
    private Integer id;
    private String username;
    private Date birthday;
    private String sex;
    private String address;

    private List<Integer> ids;

    public List<Integer> getIds() {
        return ids;
    }

    public void setIds(List<Integer> ids) {
        this.ids = ids;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", birthday=" + birthday +
                ", sex='" + sex + '\'' +
                ", address='" + address + '\'' +
                ", ids=" + ids +
                '}';
    }
}

UserMapper 接口中添加方法：

//动态 SQL  foreach 循环
public List<User> findByIds(User user);

编写配置文件：

<!-- 动态 SQL  foreach 循环 -->
<select id="findByIds" resultType="com.qcby.model.User">
    select * from user
    <where>
        <foreach collection="ids" open="id=" separator="or id =" item="i">
            #{i}
        </foreach>
    </where>
</select>

编写测试方法：

@Test
public void FindByIds() throws IOException {
    //加载配置文件
    InputStream inputStream = Resources.getResourceAsStream("SqlMapConfig.xml");

    //创建工厂对象
    SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);

    //创建 SqlSession 对象
    SqlSession session = factory.openSession();

    User user = new User();

    List<Integer> ids = new ArrayList<>();
    ids.add(1);
    ids.add(2);
    ids.add(44);
    user.setIds(ids);

    //查询
    UserMapper userMapper = session.getMapper(UserMapper.class);
    List<User> byIds = userMapper.findByIds(user);

    for(User u: byIds){
        System.out.println(u);
    }

    //关闭资源
    session.close();
    inputStream.close();
}

需求二：

需求 SQL： selelct * from user where id in (1，2，3)；

编写配置文件：

<select id="findByIdsIn" parameterType="com.qcby.model.User" resultType="com.qcby.model.User">
    select * from user
    <where>
        <foreach collection="ids" open="id in (" separator="," close=")" item="i">
            #{i}
        </foreach>
    </where>
</select>

提取公用的 SQL 语句：

<?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.qcby.mapper.UserMapper">

    <!-- 提取公共的 SQL -->
    <sql id="findAllSQL">
        select * from user
    </sql>

    <!-- 编写 sql 语句 -->
    <select id="findAll" parameterType="com.qcby.model.User">
        /* 引用公共 SQL */
        <include refid="findAllSQL"></include>
    </select>
    
    <!-- 使用 where 关键字 -->
    <select id="findByWhere" parameterType="com.qcby.model.User" resultType="com.qcby.model.User">
        <include refid="findAllSQL"/>
        <where>
            <if test="username != null and username != ''">
                username = #{username}
            </if>
            <if test="sex != null and sex != ''">
                and sex = #{sex}
            </if>
        </where>
    </select>
    
    <!-- 使用 foreach 语句-->
    <select id="findByIds" parameterType="com.qcby.model.User" resultType="com.qcby.model.User">
        <include refid="findAllSQL"></include>
        <where>
            <foreach collection="ids" open="id in (" separator="," close=")" item="i">
                #{i}
            </foreach>
        </where>
    </select>

</mapper>

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