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Scenario : Use the test attribute of the <when></when> tag of the <choose></choose> tag of MyBatis to make a non-null (!=null) or non-empty (!='') judgment on the value of the input attribute , or make equal (==) or unequal (!=) judgments with constants.
Versions : JDK 1.8, Spring Boot 2.6.3, mybatis-3.5.9.
1. Basic knowledge
1.1 MyBatis label
(1) View the tags supported by MyBatis
Address: http://mybatis.org/dtd/mybatis-3-mapper.dtd
(2) View label usage
Take the <mapper></mapper> tag element as an example, as follows in mybatis-3-mapper.dtd:
<!ELEMENT mapper (cache-ref | cache | resultMap* | parameterMap* | sql* | insert* | update* | delete* | select* )+>
<!ATTLIST mapper
namespace CDATA #IMPLIED
><!ELEMENT mapper(...)+>, indicating that this is a label element mapper.
(..|insert*|update* | delete* | select*), indicating the list of elements that can be nested in the mapper element.
<!ATTLIST mapper>, indicating that this is a supported attribute of an element tag.
1.2 Use of MyBatis
(1) Configure the location of the xml file mapped by mybatis in the application.yml configuration file.
mybatis:
mapper-locations: classpath*:mapper/**/*.xml(2) Create a Java interface. Add methods to the interface.
(3) Create a Java interface mapping xml file. Use the namespace attribute of the <mapper></mapper> tag in xml to specify the full path of the Java interface. The Java interface and the xml mapping file complete the binding relationship.
(4) In the <mapper></mapper> tag, use the id attribute of the tags such as <insert><update><delete><select> to specify the Java method name. The method of the Java interface and the tags inside the <mapper></mapper> of the xml mapping file complete the binding relationship.
2. Use the <choose> </choose> tag element
Scenario : The <choose></choose> tag element is used in tag elements such as <insert><update><delete><select>.
一般组合：<choose><when></when><otherwise></otherwise></choose>
一般组合：<choose><when></when><when></when></choose>
Example function : The example uses <choose></choose> tags to assemble SQL for different queries according to different conditions.
2.1 Java interface
@Repository
public interface Label02ChooseMapper {
List<CityLabelPO> queryCity(CityLabelDTO cityLabelDTO);
}2.2 XML file for Java interface mapping
<?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.hub.example.mapper.Label02ChooseMapper">
<select id="queryCity" parameterType="com.hub.example.domain.CityLabelDTO"
resultType="com.hub.example.domain.CityLabelPO">
select CITY_ID AS "cityId",
<choose>
<when test="flagColumn !=null and flagColumn !='' and flagColumn == '01' ">
CITY_NAME AS "cityName",
LAND_AREA AS "landArea",
POPULATION AS "population",
GROSS AS "gross",
CITY_DESCRIBE AS "cityDescribe",
DATA_YEAR AS "dataYear",
UPDATE_TIME AS "updateTime"
</when>
<when test="flagColumn !=null and flagColumn !='' and flagColumn == '02' ">
CITY_NAME AS "cityName",
LAND_AREA AS "landArea"
</when>
<otherwise>
CITY_NAME AS "cityName",
LAND_AREA AS "landArea",
POPULATION AS "population",
GROSS AS "gross"
</otherwise>
</choose>
from
<choose>
<when test="flagFrom !=null and flagFrom !='' and flagFrom == '01' ">
t_city
</when>
<otherwise>
t_city_01
</otherwise>
</choose>
aa
where
<choose>
<when test="cityId !=null and cityId !='' and flagWhere == '01' ">
aa.CITY_ID = #{cityId}
</when>
<when test="cityName !=null and cityName !='' and flagWhere == '02' ">
aa.CITY_NAME = #{cityName}
</when>
</choose>
</select>
</mapper>3. Test
3.1 Test code
@Slf4j
@RestController
@RequestMapping("/hub/example/cityLabel")
public class CityLabelController {
@Autowired
private Label02ChooseMapper label02ChooseMapper;
@GetMapping("/load02")
public Object load02() {
log.info("测试开始...");
// 示例一
CityLabelDTO labelDTO = CityLabelDTO.builder()
.cityId(1L).flagColumn("01")
.flagFrom("01").flagWhere("01").build();
List<CityLabelPO> labelPOList = label02ChooseMapper.queryCity(labelDTO);
// 示例二
labelDTO = CityLabelDTO.builder()
.cityName("上海").flagColumn("02")
.flagFrom("02").flagWhere("02").build();
labelPOList = label02ChooseMapper.queryCity(labelDTO);
log.info("测试结束...");
return "执行成功";
}
}3.2 Test request
URL：http://127.0.0.1:18080/hub-example/hub/example/cityLabel/load02
3.3 Execute SQL
The example uses the <choose></choose> tag to assemble SQL for different queries according to different conditions to adapt to different business scenarios.
Example one:
SELECT
CITY_ID AS "cityId",
CITY_NAME AS "cityName",
LAND_AREA AS "landArea",
POPULATION AS "population",
GROSS AS "gross",
CITY_DESCRIBE AS "cityDescribe",
DATA_YEAR AS "dataYear",
UPDATE_TIME AS "updateTime"
FROM
t_city aa
WHERE aa.CITY_ID = ?;Example two:
SELECT
CITY_ID AS "cityId",
CITY_NAME AS "cityName",
LAND_AREA AS "landArea"
FROM
t_city_01 aa
WHERE aa.CITY_NAME = ?;4. Support
4.1 Entity objects
(1) Encapsulate the result object CityLabelPO
@Data
@Builder
@NoArgsConstructor
@AllArgsConstructor
public class CityLabelPO {
private Long cityId;
private String cityName;
private Double landArea;
private Long population;
private Double gross;
private String cityDescribe;
private String dataYear;
private Date updateTime;
}(2) Encapsulation parameter CityLabelDTO
@Data
@Builder
@NoArgsConstructor
@AllArgsConstructor
public class CityLabelDTO {
private Long cityId;
private String cityName;
private Double landArea;
private Long population;
private Double gross;
private String cityDescribe;
private String dataYear;
private Date updateTime;
// 标记查询的字段
private String flagColumn;
// 标记查询的表
private String flagFrom;
// 标记where条件
private String flagWhere;
}4.2 Create table statement
CREATE TABLE t_city (
CITY_ID BIGINT(16) NOT NULL COMMENT '唯一标识',
CITY_NAME VARCHAR(64) COLLATE utf8_bin NOT NULL COMMENT '城市名',
LAND_AREA DOUBLE DEFAULT NULL COMMENT '城市面积',
POPULATION BIGINT(16) DEFAULT NULL COMMENT '城市人口',
GROSS DOUBLE DEFAULT NULL COMMENT '生产总值',
CITY_DESCRIBE VARCHAR(512) COLLATE utf8_bin DEFAULT NULL COMMENT '城市描述',
DATA_YEAR VARCHAR(16) COLLATE utf8_bin DEFAULT NULL COMMENT '数据年份',
UPDATE_TIME DATETIME DEFAULT NULL COMMENT '更新时间'
) ENGINE=INNODB DEFAULT CHARSET=utf8 COLLATE=utf8_bin COMMENT='城市信息表';Above, thanks.
April 23, 2023