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MyBatis的if标签的基本使用

news 文章来源：https://blog.csdn.net/weixin_45339291/article/details/144010448 2025/1/13 10:12:10

在MyBatis框架中，if标签用于在构建SQL语句时，根据参数条件判断的结果，动态地选择加入或不加where条件中。

一常见使用

在使用MyBatis处理查询逻辑的时候，常用的是判断一些参数是否为空，列举常用的几种情况展示

1.1 数据准备

1.1.1 创建表模型

CREATE TABLE `approval_info` (`id` BIGINT NOT NULL AUTO_INCREMENT,`info_name` VARCHAR(128) DEFAULT NULL,`info_type` VARCHAR(32) DEFAULT NULL,`info_size` VARCHAR(32) DEFAULT NULL,`approval_time` DATETIME DEFAULT NULL,`create_time` DATETIME DEFAULT NULL,PRIMARY KEY (`id`)
)

1.1.2 创建实体类

@Data
@AllArgsConstructor
@NoArgsConstructor
public class ApprovalInfo {private Long id;private String infoName;private String infoType;private String infoSize;private Date approvalTime;private Date createTime;
}

1.1.3 创建mapper层

public interface ApprovalInfoMapper {}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mango.mapper.ApprovalInfoMapper"></mapper>

1.1.4 插入测试数据

@Test
public void insertBatchObject() {List<ApprovalInfo> approvalInfoList = new ArrayList<>();// 使用工具类往集合中添加数据Collections.addAll(approvalInfoList, new ApprovalInfo(null, "cultureProjectOne", "culture", "A", assembleDate("2023-12-05 10:10:10"), assembleDate("2023-11-15 08:08:10")),new ApprovalInfo(null, "tourismProjectOne", "tourism", "A", assembleDate("2023-12-06 09:10:11"), assembleDate("2023-11-19 15:13:11")),new ApprovalInfo(null, "gameProjectOne", "game", "A", assembleDate("2023-12-08 17:10:13"), assembleDate("2023-11-21 01:03:16")),new ApprovalInfo(null, "cultureProjectTwo", "culture", "B", assembleDate("2023-12-11 01:07:15"), assembleDate("2023-11-25 20:23:40")),new ApprovalInfo(null, "cultureProjectThree", "culture", "B", assembleDate("2023-12-17 07:12:07"), assembleDate("2023-12-29 13:41:11")),new ApprovalInfo(null, "tourismProjectTwo", "tourism", "C", assembleDate("2023-12-21 13:01:19"), assembleDate("2023-12-01 11:31:19")),new ApprovalInfo(null, "gameProjectTwo", "game", "C", assembleDate("2023-12-25 19:13:10"), assembleDate("2023-12-03 17:51:15")),new ApprovalInfo(null, "gameProjectThree", "game", "B", assembleDate("2023-12-27 02:11:10"), assembleDate("2023-12-04 12:10:21")),new ApprovalInfo(null, "tourismProjectThree", "tourism", "B", assembleDate("2023-12-29 06:19:10"), assembleDate("2023-12-05 15:43:21")),new ApprovalInfo(null, "cultureProjectFour", "culture", "C", assembleDate("2023-12-31 01:09:10"), assembleDate("2023-12-08 20:21:37")));int effectLineNumber = approvalInfoMapper.saveBatchApprovalInfo(approvalInfoList);System.out.println("effectLineNumber is: " + effectLineNumber);
}private Date assembleDate(String strDate) {try {SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");return simpleDateFormat.parse(strDate);} catch (ParseException e) {e.printStackTrace();throw new RuntimeException(e.getMessage());}
}

public interface ApprovalInfoMapper {int saveBatchApprovalInfo(@Param("approvalInfoList") List<ApprovalInfo> approvalInfoList);
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mango.mapper.ApprovalInfoMapper"><insert id="saveBatchApprovalInfo">insert into approval_info(info_name, info_type, info_size, approval_time, create_time)values<foreach collection="approvalInfoList" item="approvalInfo" separator=",">(#{approvalInfo.infoName}, #{approvalInfo.infoType}, #{approvalInfo.infoSize},#{approvalInfo.approvalTime}, #{approvalInfo.createTime})</foreach></insert>
</mapper>

表中数据如下：

1.2 mapper方法参数是String类型

mapper方法参数是String类型，如果在查询条件中infoType的值不为空，那么就加上infoType的判断条件：

@Test
public void testQueryApprovalInfosByType() {//==>  Preparing: select id, info_name, info_type, info_size, approval_time, create_time from approval_info//==>  Parameters://<==  Total: 10// String infoType = null;// String infoType = "";//==>  Preparing: select id, info_name, info_type, info_size, approval_time, create_time from approval_info WHERE info_type = ?//==>  Parameters:   (String)//<==  Total: 0// String infoType = "  ";//==>  Preparing: select id, info_name, info_type, info_size, approval_time, create_time from approval_info WHERE info_type = ?//==>  Parameters: culture(String)//<==     Columns: id, info_name, info_type, info_size, approval_time, create_time//<==        Row: 1, cultureProjectOne, culture, A, 2023-12-05 10:10:10, 2023-11-15 08:08:10//<==        Row: 4, cultureProjectTwo, culture, B, 2023-12-11 01:07:15, 2023-11-25 20:23:40//<==        Row: 5, cultureProjectThree, culture, B, 2023-12-17 07:12:07, 2023-12-29 13:41:11//<==        Row: 10, cultureProjectFour, culture, C, 2023-12-31 01:09:10, 2023-12-08 20:21:37//<==  Total: 4//[// ApprovalInfo(id=1, infoName=cultureProjectOne, infoType=culture, infoSize=A, approvalTime=Tue Dec 05 10:10:10 CST 2023, createTime=Wed Nov 15 08:08:10 CST 2023),// ApprovalInfo(id=4, infoName=cultureProjectTwo, infoType=culture, infoSize=B, approvalTime=Mon Dec 11 01:07:15 CST 2023, createTime=Sat Nov 25 20:23:40 CST 2023),// ApprovalInfo(id=5, infoName=cultureProjectThree, infoType=culture, infoSize=B, approvalTime=Sun Dec 17 07:12:07 CST 2023, createTime=Fri Dec 29 13:41:11 CST 2023),// ApprovalInfo(id=10, infoName=cultureProjectFour, infoType=culture, infoSize=C, approvalTime=Sun Dec 31 01:09:10 CST 2023, createTime=Fri Dec 08 20:21:37 CST 2023)// ]String infoType = "culture";List<ApprovalInfo> approvalInfoList = approvalInfoMapper.queryApprovalInfosByType(infoType);System.out.println(approvalInfoList);
}

如果infoType的值为null，那么info_type = #{infoType}就不会加入到where条件中，查询出全部的数据；

如果infoType的值为""，那么info_type = #{infoType}就不会加入到where条件中，查询出全部的数据；

如果infoType的值为" "，那么info_type = #{infoType}就会加入到where条件中，且查询出来的结果当然是空的，MyBatis不会去除空格再进行计算；

如果infoType的值为culture，那么info_type = #{infoType}就会加入到where条件中，且只查询符合条件的结果，且从数据库中查出来的顺序就是往List有序集合中依次添加的顺序；

public interface ApprovalInfoMapper {List<ApprovalInfo> queryApprovalInfosByType(@Param("infoType") String infoType);
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mango.mapper.ApprovalInfoMapper"><resultMap id="approvalInfoMap" type="com.mango.domain.ApprovalInfo"><id property="id" column="id"/><result property="infoName" column="info_name"/><result property="infoType" column="info_type"/><result property="infoSize" column="info_size"/><result property="approvalTime" column="approval_time"/><result property="createTime" column="create_time"/></resultMap><select id="queryApprovalInfosByType" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="infoType != null and infoType != ''">info_type = #{infoType}</if></where></select>
</mapper>

在mapper映射文件中，要写resultMap标签用于在查询语句中，将表中的字段映射到Java对象的属性中；

在mapper映射文件中，where条件后面只有一个if标签，那么要写成<where>标签嵌套<if>标签；

mapper方法参数是String类型，那么在mapper映射文件中，就需要判断参数是否等于null，和是否等于空字符串；

1.3 mapper方法参数是Long类型

mapper方法参数是包装类Long类型或是long类型的区别；

如果在查询条件中(Long类型)id的值不为空，那么就加上id的判断条件；

@Test
public void testSelectApprovalInfoByBaseId() {// Long targetId = 5L;// long targetId = 5L;Long targetId = null;List<ApprovalInfo> approvalInfoList1 = approvalInfoMapper.queryApprovalInfoListByBaseId(targetId);List<ApprovalInfo> approvalInfoList2 = approvalInfoMapper.queryApprovalInfoListByWrapperId(targetId);System.out.println(approvalInfoList1);System.out.println(approvalInfoList2);System.out.println("集合中第一条数据: " + approvalInfoList1.get(0));System.out.println("集合中最后一条数据: " + approvalInfoList1.get(approvalInfoList1.size() - 1));
}

在服务层调用mapper层的queryApprovalInfoListByBaseId(@Param("id") long id)方法，对方法参数传值描述如下：
        如果对mapper层的方法参数id传基本数据类型，可以正常使用；
        如果对mapper层的方法参数id传包装类型，那么会先进行拆箱(由Long-->long)，传null值会抛异常；
在服务层调用mapper层queryApprovalInfoListByWrapperId(@Param("id") Long id)方法，对方法参数传值描述如下：
        如果对mapper层的方法参数id传基本数据类型，可以正常使用；
        如果对mapper层的方法参数id传包装类型，可以正常使用；
小结：mapper层接口参数有动态sql条件时，最好使用包装类型，而不是基本数据类型；

public interface ApprovalInfoMapper {/*** mapper层方法参数有参与到动态sql条件, 最好使用包装类型, 而不是基本数据类型*/List<ApprovalInfo> queryApprovalInfoListByBaseId(@Param("id") long id);List<ApprovalInfo> queryApprovalInfoListByWrapperId(@Param("id") Long id);
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mango.mapper.ApprovalInfoMapper"><resultMap id="approvalInfoMap" type="com.mango.domain.ApprovalInfo"><id property="id" column="id"/><result property="infoName" column="info_name"/><result property="infoType" column="info_type"/><result property="infoSize" column="info_size"/><result property="approvalTime" column="approval_time"/><result property="createTime" column="create_time"/></resultMap><select id="queryApprovalInfoListByBaseId" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="id != null">id >= #{id}</if></where></select><select id="queryApprovalInfoListByWrapperId" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="id != null">id >= #{id}</if></where></select>
</mapper>

mapper方法参数是Long类型，那么在mapper映射文件中，就只需要判断参数是否等于null；

番外篇

篇一：关于表中记录为空时，查询结果是null，譬如查询表中的最值，在mapper层的方法返回值中使用基本数据类型还是包装类型

@Test
public void testMaxIndex() {Long maxIndexWrapperType = approvalInfoMapper.getMaxIndexWrapperType();System.out.println("maxIndexWrapperType is: " + maxIndexWrapperType);long maxIndexBaseType = approvalInfoMapper.getMaxIndexBaseType();System.out.println("maxIndexBaseType is: " + maxIndexBaseType);
}

如果表中有数据，那么方法返回值使用long类型或是Long类型都可以；
如果表中没有数据，那么方法返回值使用long类型接收null值会报错，Long类型可以接收null值；
小结：如果表中记录为空，且查询结果为null，譬如mapper层获取表中字段最值时，方法返回值类型使用包装类型；

public interface ApprovalInfoMapper {/*** 获取表中字段的最值使用包装类型, long类型不能接收null值*/long getMaxIndexBaseType();Long getMaxIndexWrapperType();
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mango.mapper.ApprovalInfoMapper"><select id="getMaxIndexBaseType" resultType="long">select MAX(id) from approval_info</select><select id="getMaxIndexWrapperType" resultType="java.lang.Long">select MAX(id) from approval_info</select>
</mapper>

篇二：关于表中记录为空时，查询结果是0，譬如统计表中记录的数量，在mapper层的方法返回值中使用基本数据类型还是包装类型

@Test
public void testCount() {long baseCount = approvalInfoMapper.queryCountBaseType();System.out.println("基本数据类型接收的值: " + baseCount);Long wrapperCount = approvalInfoMapper.queryCountWrapperType();System.out.println("包装数据类型接收的值: " + wrapperCount);
}

如果表中没有数据，那么方法返回值0，使用long类型或是Long类型都可以接收；

如果表中有数据，那么方法返回值10，使用long类型或是Long类型都可以接收；
小结：如果表中记录为空，且查询结果为0，譬如mapper层统计表中记录的数量，方法返回值类型使用基本数据类型和包装类型都可以；

public interface ApprovalInfoMapper {/*** 获取表中记录的数量*/long queryCountBaseType();Long queryCountWrapperType();
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mango.mapper.ApprovalInfoMapper"><select id="queryCountBaseType" resultType="long">select count(*) from approval_info</select><select id="queryCountWrapperType" resultType="java.lang.Long">select count(*) from approval_info</select>
</mapper>

1.4 mapper方法参数是Date类型

mapper方法参数是包装类Date类型，如果在查询条件中date的值不为空，那么就加上date的判断条件：

@Test
public void testSelectApprovalInfoByDate() {// Date date = null;Date date = assembleDate("2023-12-03 17:51:15");List<ApprovalInfo> approvalInfoList = approvalInfoMapper.queryApprovalInfoListByDate(date);System.out.println(approvalInfoList);System.out.println("====");System.out.println(approvalInfoList.get(0));System.out.println(approvalInfoList.get(approvalInfoList.size() - 1));
}
private Date assembleDate(String strDate) {try {SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");return simpleDateFormat.parse(strDate);} catch (ParseException e) {e.printStackTrace();throw new RuntimeException(e.getMessage());}
}

public interface ApprovalInfoMapper {/*** 使用date类型筛选数据*/List<ApprovalInfo> queryApprovalInfoListByDate(@Param("date") Date date);
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mango.mapper.ApprovalInfoMapper"><resultMap id="approvalInfoMap" type="com.mango.domain.ApprovalInfo"><id property="id" column="id"/><result property="infoName" column="info_name"/><result property="infoType" column="info_type"/><result property="infoSize" column="info_size"/><result property="approvalTime" column="approval_time"/><result property="createTime" column="create_time"/></resultMap><!-- 方式一 --><select id="queryApprovalInfoListByDate" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="date != null">create_time >= #{date}</if></where></select><!-- 方式二 --><select id="queryApprovalInfoListByDate" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="date != null">create_time &gt;= #{date}</if></where></select><!-- 方式三 --><select id="queryApprovalInfoListByDate" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="date != null">create_time <![CDATA[>=]]> #{date}</if></where></select>
</mapper>

mapper方法参数是Date类型，那么在mapper映射文件中，就只需要判断参数是否等于null；

1.5 小结

如果MyBatis的mapper层方法参数是String类型，那么在mapper映射文件需要判断参数不等于null，和不等于空字符串；

如果MyBatis的mapper层方法参数是Long类型等基本数据类型对应的包装类型，那么在mapper映射文件只需要判断参数不等于null；

如果MyBatis的mapper层方法参数是Date类型，那么在mapper映射文件只需要判断参数不等于null；

在MyBatis表示关系运算符的方式有如下三种：

方式一，可以使用>关系运算符和>=关系运算符，不可以使用<关系运算符或<=关系运算符；

方式二，可以将关系运算符进行转义，使用&开头，和使用;结尾的方式

<	<
<=	<=
>	>
>=	>=
&	&
'	'
"	"

方式三，可以使用<![CDATA[ ]]>嵌套关系运算符，<![CDATA[>]]>，<![CDATA[>=]]>，<![CDATA[<]]>，和<![CDATA[<=]]>

二其他使用

2.1 if标签指定的单个字符

如果infoSize的值是指定字符A，那就查询指定字符A的ApprovalInfo集合，如果不是那就查询全部的ApprovalInfo集合：

@Test
public void testSelectApprovalInfoBySize() {String size = "A";List<ApprovalInfo> approvalInfoList = approvalInfoMapper.queryApprovalInfoListBySize(size);System.out.println(approvalInfoList);
}

public interface ApprovalInfoMapper {/*** 当参数值是单字符时, 对应着的动态sql标签的判断条件如何处理*/List<ApprovalInfo> queryApprovalInfoListBySize(@Param("size") String size);
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mango.mapper.ApprovalInfoMapper"><resultMap id="approvalInfoMap" type="com.mango.domain.ApprovalInfo"><id property="id" column="id"/><result property="infoName" column="info_name"/><result property="infoType" column="info_type"/><result property="infoSize" column="info_size"/><result property="approvalTime" column="approval_time"/><result property="createTime" column="create_time"/></resultMap><select id="queryApprovalInfoListBySize" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="size = 'A'">info_size = #{size}</if></where></select>
</mapper>

2.2 if标签指定的多个字符

如果infoType的值是指定的字符串culture，那就查询指定的字符串culture的ApprovalInfo集合，如果不是那就查询全部的ApprovalInfo集合：

@Test
public void testSelectApprovalInfoByType() {//==>  Preparing: select id, info_name, info_type, info_size, approval_time, create_time from approval_info//==>  Parameters://<==  Total: 10// String type = null;// String type = "culture12306";//==>  Preparing: select id, info_name, info_type, info_size, approval_time, create_time from approval_info WHERE info_type = ?//==>  Parameters: culture(String)//<==  Total: 4String type = "culture";List<ApprovalInfo> approvalInfoList = approvalInfoMapper.queryApprovalInfoListByType(type);System.out.println(approvalInfoList);System.out.println("====");System.out.println(approvalInfoList.get(0));System.out.println(approvalInfoList.get(approvalInfoList.size() - 1));
}

public interface ApprovalInfoMapper {/*** 当参数值是多字符时, 对应着的动态sql标签的判断条件如何处理* mapper接口中的方法参数, 只是用于mapper映射文件中的条件判断和组装sql字段值, 不一定要和表中字段名保持一致*/List<ApprovalInfo> queryApprovalInfoListByType(@Param("infoType") String infoType);
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mango.mapper.ApprovalInfoMapper"><resultMap id="approvalInfoMap" type="com.mango.domain.ApprovalInfo"><id property="id" column="id"/><result property="infoName" column="info_name"/><result property="infoType" column="info_type"/><result property="infoSize" column="info_size"/><result property="approvalTime" column="approval_time"/><result property="createTime" column="create_time"/></resultMap><!-- 方式一 --><select id="queryApprovalInfoListByType" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="infoType == 'culture'">info_type = #{infoType}</if></where></select><!-- 方式二 --><select id="queryApprovalInfoListByType" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test='infoType == "culture"'>info_type = #{infoType}</if></where></select>
</mapper>

在mapper映射文件中，判断方法参数是否等于指定的值，使用<if>标签的test属性值可以写如下两种书写的格式：

<if test=" infoType == 'culture' ">，双引号和单引号配合着使用；
<if test=' infoType == "culture" '>，单引号和双引号配合着使用；

2.3 if标签指定的数字

如果id的值是指定长整型5，那就查询infoType是culture的ApprovalInfo集合；

如果id的值是指定长整型7，那就查询infoType是tourism的ApprovalInfo集合；

@Test
public void testQueryApprovalInfoListById() {// Long id = 5L;// Long id = 6L;Long id = 7L;String type = assembleMapData().get(id);List<ApprovalInfo> approvalInfoList = approvalInfoMapper.queryApprovalInfoListById(id, type);System.out.println(approvalInfoList);
}
private Map<Long, String> assembleMapData() {Map<Long, String> map = new HashMap<>();map.put(5L, "culture");map.put(7L, "tourism");return map;
}

public interface ApprovalInfoMapper {/*** 当参数值是数字时, 对应着的动态sql标签的判断条件如何处理*/List<ApprovalInfo> queryApprovalInfoListById(@Param("id") Long id, @Param("infoType") String infoType);
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mango.mapper.ApprovalInfoMapper"><resultMap id="approvalInfoMap" type="com.mango.domain.ApprovalInfo"><id property="id" column="id"/><result property="infoName" column="info_name"/><result property="infoType" column="info_type"/><result property="infoSize" column="info_size"/><result property="approvalTime" column="approval_time"/><result property="createTime" column="create_time"/></resultMap><!-- 方式一 --><select id="queryApprovalInfoListById" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="id == 5">and info_type = #{infoType}</if><if test="id == 7">and info_type = #{infoType}</if></where></select><!-- 方式二 --><select id="queryApprovalInfoListById" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test='id == 5'>and info_type = #{infoType}</if><if test='id == 7'>and info_type = #{infoType}</if></where></select>
</mapper>

2.4 if标签的test属性介绍

在MyBatis中，<if>标签的test属性值用于设置条件判断：

如果test属性值为true，那么if标签的条件会加到where中，

如果test属性值为false，那么if标签的条件不会加到where中，

<if>标签的test属性值的条件判断的内容可以写基本数据类型，字符串常量，关系表达式和引用类型对象；

@Test
public void testSelectApprovalInfoByType() {String type = "culture";List<ApprovalInfo> approvalInfoList = approvalInfoMapper.queryApprovalInfoListByType(type);System.out.println(approvalInfoList);
}

public interface ApprovalInfoMapper {List<ApprovalInfo> queryApprovalInfoListByType(@Param("infoType") String infoType);
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mango.mapper.ApprovalInfoMapper"><resultMap id="approvalInfoMap" type="com.mango.domain.ApprovalInfo"><id property="id" column="id"/><result property="infoName" column="info_name"/><result property="infoType" column="info_type"/><result property="infoSize" column="info_size"/><result property="approvalTime" column="approval_time"/><result property="createTime" column="create_time"/></resultMap><!-- 方式一: boolean类型 --><select id="queryApprovalInfoListByType" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="true">info_type = #{infoType}</if></where></select><!-- 方式二: 数字(byte short int long float double)类型, 0为false, 非0(哪怕是0.0)为true --><select id="queryApprovalInfoListByType" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="1">info_type = #{infoType}</if></where></select><!-- 方式三: char类型, 任意char类型都表示为true --><select id="queryApprovalInfoListByType" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="'a'">info_type = #{infoType}</if></where></select><!--单引号+单字符会解析成char类型;双引号+单字符会解析成String类型;单引号+多字符会解析成String类型;多引号+多字符会解析成String类型;--><!-- 方式四: String类型, String类型都表示为true --><select id="queryApprovalInfoListByType" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="'abc'">info_type = #{infoType}</if></where></select><select id="queryApprovalInfoListByType" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test='"abc"'>info_type = #{infoType}</if></where></select><!-- 方式五: 关系表达式, 相等表达式 --><select id="queryApprovalInfoListByType" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="'abc'=='abc'">info_type = #{infoType}</if></where></select><!-- 方式六: 关系表达式, 不相等表达式 --><select id="queryApprovalInfoListByType" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="'abc' != 'cba'">info_type = #{infoType}</if></where></select><!-- 方式七: 引用类型对象, infoType是String类型 --><select id="queryApprovalInfoListByType" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="infoType">info_type = #{infoType}</if></where></select><!-- 方式八: 引用类型对象, infoType用在关系表达式中 --><select id="queryApprovalInfoListByType" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="infoType == 'culture'">info_type = #{infoType}</if></where></select>
</mapper>

小结：在if标签的test属性值表达式中，引用类型对象不加引号(双引号或单引号)，字符和字符串类型需要加引号(双引号或单引号)；

在mapper映射文件中，if标签的标签体是表字段、运算符和值表达式，其中值表达式和if标签的test属性值表达式的内容是一致的，只不过在使用引用类型时需要使用#{}或${}给包括起来；

在mapper接口的方法中，方法参数可以自定义需要的参数，不一定非得和表中字段一样或是匹配，只要能表述清楚就可以了；

@Test
public void testSelectApprovalInfoByCustomize() {String type = "culture";String code = "en";List<ApprovalInfo> approvalInfoList = approvalInfoMapper.queryApprovalInfoListByCustomize(type, code);System.out.println(approvalInfoList);
}

public interface ApprovalInfoMapper {List<ApprovalInfo> queryApprovalInfoListByCustomize(@Param("customizeType") String customizeType, @Param("customizeCode") String customizeCode);
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mango.mapper.ApprovalInfoMapper"><resultMap id="approvalInfoMap" type="com.mango.domain.ApprovalInfo"><id property="id" column="id"/><result property="infoName" column="info_name"/><result property="infoType" column="info_type"/><result property="infoSize" column="info_size"/><result property="approvalTime" column="approval_time"/><result property="createTime" column="create_time"/></resultMap><select id="queryApprovalInfoListByCustomize" resultMap="approvalInfoMap">select id, info_name, info_type, info_size, approval_time, create_timefrom approval_info<where><if test="customizeCode == 'en'">and info_type = #{customizeType}</if></where></select>
</mapper>

2.5 MyBatis中#{}常用属性

MyBatis中#{}占位符的常用属性，javaType、jdbcType、mode、numericScale、resultMap、typeHandler、jdbcTypeName、expression。
javaType属性：指定参数的Java类型；
jdbcType属性：指定参数的JDBC类型；
‌typeHandler‌属性：指定自定义的类型处理器，用于处理特定类型的参数或结果‌；
MyBatis在预编译(parsing)阶段会读取SQL中的字段类型，以确保预编译SQL的字段类型正确；
MyBatis在预编译(parsing)阶段由SqlSourceBuilder解析#{}参数，将#{}替换为?号(占位)，并将#{}中的内容解析为ParameterMapping的封装，ParameterMapping包含了参数的各个属性，解析是除了typeHandler和javaType外其它属性都直接从配置中获取到然后设定，若未指定则为null，ParameterMapping中的typeHandler和javaType是必须要的，因为在后面通过setter方法设置参数值时，从#{}中javaType属性指定的类型信息来选择对应的setter方法进行参数设定，最后执行sql；
在执行sql时，但是有时候在执行的时会报ClassCastException，譬如表中字段是单字符就会报String cannot be cast to Integer的错误，需要手动设置参数的Java类型为：info_size = #{size, javaType=String}；

一 常见使用