mybatis mapper.xml中如何根据数据库类型选择对应SQL语句

  <bean id="vendorProperties" class="org.springframework.beans.factory.config.PropertiesFactoryBean">
    <property name="properties">
      <props>
        <prop key="DB2">db2</prop>
        <prop key="Oracle">oracle</prop>
        <prop key="MySQL">mysql</prop>
      </props>
    </property>
   </bean>
   <bean id="databaseIdProvider" class="org.apache.ibatis.mapping.VendorDatabaseIdProvider">
    <property name="properties" ref="vendorProperties"/>
  </bean>

<!-- sessionFactory 将spring和mybatis整合 -->
 <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
 <property name="dataSource" ref="dataSource" />
 <property name="databaseIdProvider" ref="databaseIdProvider" />
 <property name="configLocation" value="classpath:mybatis-config.xml" />
 <property name="mapperLocations"
 value="classpath*:/com/sunyard/cop/IF/mybatis/mapping/*.xml" /> 
 <property name="plugins">
     <array>
       <bean class="com.github.pagehelper.PageInterceptor">
         <property name="properties">
           <!--使用下面的方式配置参数，一行配置一个，后面会有所有的参数介绍 -->
           <value>
 　　　　　　　　helperDialect=oracle
 　　　　　　　　reasonable=true
 　　　　　　　　supportMethodsArguments=true
 　　　　　　　　params=count=countSql
 　　　　　　　　autoRuntimeDialect=true 
 　　　　　　</value>
         </property>
       </bean>
     </array>
         </property>
 </bean>

    <select id="selectByUserNo" databaseId="mysql"   parameterType="java.lang.String" resultMap="UserResultMap">
 select * from SM_USERS_TB
    </select>
    <select id="selectByUserNo"  parameterType="java.lang.String" resultMap="UserResultMap">
 select * from SM_USERS_TB
    </select>

<update id="upda" parameterType="User">
		update smbms_user
		<set>
                <!--修改时可以判断userCode是否是空的如果不为空就把数据库中这一列的值更改掉
                如果为空就不修改这一列数据库这一列的值也不会为Null-->
			<if test="userCode!=null and userCode!=''">
				userCode=#{userCode},
			</if>
			<if test="userName!=null and userName!=''">
				userName=#{userName},
			</if>
			<if test="userPassword!=null and userPassword!=''">
				userPassword=#{userPassword},
			</if>
			<if test="gender!=null and gender!=''">
				gender=#{gender},
			</if>
			<if test="phone!=null and phone!=''">
				phone=#{phone},
			</if>
			<if test="address!=null and address!=''">
				address=#{address},
			</if>
			<if test="userRole!=null and userRole!=''">
				userRole=#{userRole},
			</if>
			<if test="createdBy!=null and createdBy!=''">
				createdBy=#{createdBy},
			</if>
		</set>
		where id=#{id}
	</update>

<update id ="modify" parameterType="User">
update smbms_user
<trim prefix="set" suffixOverrides="," suffix="where id = #{id}">	
	<if test="userCode != null">userCode = #{userCode},</if>
	<if test="userName!= null">userCode = #{userName },</if>
	<if test="userPassword!= null">userPassword=#{userPassword },</if>
</trim>
</update>
 

    
SELECT u.*,r.roleName,r.roleCode FROM smbms_user u INNER JOIN smbms_role r ON u.userrole=r.id 
    <where>
            <!-- where相当于  select * from pet where id=1 中的where一样  -->
			<if test="usercode !=null and usercode !=''">
				AND userCode LIKE CONCAT('%',#{usercode},'%')
			</if>
			<if test="userrole!=0">
				AND userRole=#{userrole}
			</if>
			<if test="gender!=null and gender!=0">
				AND gender=#{gender} 
			</if>
	</where>

<update id="upda" parameterType="User">
		update smbms_user
		<set><!-- 与修改时搭配使用我们修改set的sql语句的‘,'的最后一列会被自动省略 -->
			<if test="userCode!=null and userCode!=''">
				userCode=#{userCode},
			</if>
			<if test="userName!=null and userName!=''">
				userName=#{userName},
			</if>
			<if test="userPassword!=null and userPassword!=''">
				userPassword=#{userPassword},
			</if>
			<if test="gender!=null and gender!=''">
				gender=#{gender},
			</if>
			<if test="phone!=null and phone!=''">
				phone=#{phone},
			</if>
			<if test="address!=null and address!=''">
				address=#{address},
			</if>
			<if test="userRole!=null and userRole!=''">
				userRole=#{userRole},
			</if>
			<if test="createdBy!=null and createdBy!=''">
				createdBy=#{createdBy},
			</if>
		</set>
		where id=#{id}
	</update>

<choose>
	<when test ="条件1"> …</when>
	<when test ="条件2"> …</when>
	<when test ="条件3"> …</when>
	…
	<otherwise>…</otherwise>
</choose>	

<select id="getUserName" resultType="User" parameterType="java.util.List">
		select * from smbms_user where userCode in
		<foreach collection="list" open="(" close=")" item="userCode" separator=",">
			#{userCode}
		</foreach>
	</select>