<choose>
            <when test="username != null and username != ''">and username = #{username}</when>
            <when test="usersex != null and usersex != ''">and usersex = #{usersex}</when>
            <otherwise>and userid = #{userid}</otherwise>
        </choose>

--         将传过来的collection(list,set)集合 遍历，以(开始， 逗号分割每个元素，)结束，结果传给userid
        <foreach collection="collection" item="userid" open="(" separator="," close=")">
--             获取userid的值
            #{userid}
        </foreach>

<!--    foreach 将map存储的值，遍历-->
        select count(*) from users where
--         index 存放每个属性值对应的  key
        <foreach collection="suibian" item="value" index="key" separator="and">
--         ${}这个不用编译位？#{}会被编译为？
            ${key} = #{value}
        </foreach>