sql 了解一下

<select id="entranceCardList" resultType="****************************************************************">
   SELECT 
        g.id,
        g.cardCode,
       c.cardType,
       c.status,
        c.userType,
            CONCAT(DATE_FORMAT(c.beginTime,'%Y-%m-%d %H:%i'),' 至 ',DATE_FORMAT(c.endTime,'%Y-%m-%d %H:%i')) AS authorizeTime,
       CASE WHEN c.userType = '01' THEN u.employeeName
           WHEN c.userType = '02' THEN v.visitorName 
            END as usePerson
       FROM rece_guard_door as g LEFT JOIN rece_card_authorize as c ON g.id = c.cardIds
                       LEFT JOIN info_employee_user as u ON u.id = c.personId AND c.userType = '01'
                       LEFT JOIN info_visitor_record as v ON v.id = c.personId AND c.userType = '02'          
   <where>
      c.`status` != '04'
   <if test="userType != null and userType !=''">
      and c.userType = #{userType}
   </if>
   <if test="cardCode != null and cardCode !=''">
      and g.cardCode = #{cardCode}
   </if>
   <if test="keyWord != null and keyWord !=''">
      and CONCAT(IFNULL(c.userType,''),IFNULL(u.employeeName,''),IFNULL(v.visitorName ,'')) LIKE CONCAT('%',#{keyWord},'%')
   </if>
   </where> 
   ORDER BY 
    <if test="orderStr != null and orderStr != ''">
           ${orderStr},
       </if>
   g.cardCode
   limit #{start},#{limit}
</select>