backend_code/target/classes/mapper/template/SmartWarningMapper.xml

<?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.template.mapper.SmartWarningMapper">

    <select id="warningType" resultType="java.lang.String">
        SELECT type FROM `smart_warning` GROUP BY type
    </select>
</mapper>