在Config(MybatisConfig或者MybatisPlusConfig)中添加，例如添加Mysql和DM

    @Beanpublic DatabaseIdProvider databaseIdProvider(){VendorDatabaseIdProvider databaseIdProvider = new VendorDatabaseIdProvider();Properties properties = new Properties();properties.put("MySQL","mysql");properties.put("DM DBMS","dm");databaseIdProvider.setProperties(properties);return databaseIdProvider;}

或者在mybatis-config.xml中添加

<databaseIdProvider type="DB_VENDOR"><property name="MySQL" value="mysql"/>       <property name="DM DBMS" value="dm" />
</databaseIdProvider>

然后Mybatis的xml中就可以使用

<?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.elements.user.dao.dbMapper" ><select id="SelectTime"   resultType="String" databaseId="mysql">SELECT  NOW() FROM dual </select><select id="SelectTime"   resultType="String" databaseId="dm">SELECT  'oralce'||to_char(sysdate,'yyyy-mm-dd hh24:mi:ss')  FROM dual </select>
</mapper>

或者

<select xxxx>       
<if test="_databaseId == 'mysql'">create_time between date_add(now(), interval - 10 minute) and now()</if><if test="_databaseId == 'dm'">create_time between sysdate() - 10/24/60 and sysdate()</if>
</select>

用来判断数据库类型