mybatis-config.xml

    <databaseIdProvider type="DB_VENDOR">
        <property name="Oracle" value="oracle"/>
        <property name="MySQL" value="mysql"/>
    </databaseIdProvider>

value的值可以改动。（type、name都是对应的别名，别改动）

usersMapper.xml

    <select id="getUser" parameterType="int" resultMap="usersMapper" databaseId="mysql">
        select id, username, password, hireDate, sex FROM users where id = #{id}
    </select>

databaseId的值对应上面的值（oracle、mysql......）。

下面介绍自定义系统规则：

    <typeAliases>
        <typeAlias type="com.mzs.databaseIdProvider.MyDatabaseIdProvider" alias="myDatabaseIdProvider"/>
    </typeAliases>

    <databaseIdProvider type="myDatabaseIdProvider">
        
    </databaseIdProvider>

自定义的DatabaseIdProvider

package com.mzs.databaseIdProvider;

import org.apache.ibatis.mapping.DatabaseIdProvider;

import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.SQLException;
import java.util.Properties;
import java.util.logging.Logger;

public class MyDatabaseIdProvider implements DatabaseIdProvider {

    private static String DATABASE_TYPE_MYSQL = "MySQL";
    private static String DATABASE_TYPE_ORACLE = "Oracle";
    private static Logger logger = Logger.getLogger("com.mzs.databaseIdProvider.MyDatabaseIdProvider");

    @Override
    public void setProperties(Properties properties) {
        logger.info(properties.toString());
    }

    @Override
    public String getDatabaseId(DataSource dataSource) throws SQLException {
        Connection connection = dataSource.getConnection();
        String databaseProductName = connection.getMetaData().getDatabaseProductName();
        if (DATABASE_TYPE_MYSQL.equals(databaseProductName)) {
            return "mysql";
        } else if (DATABASE_TYPE_ORACLE.equals(databaseProductName)) {
            return "oracle";
        } else {
            return null;
        }
    }
}

UsersMapper.xml

    <select id="getUser" parameterType="int" resultMap="usersMapper" databaseId="mysql">
        select id, username, password, hireDate, sex FROM users where id = #{id}
    </select>