coalesce :返回参数（列名）中第一个非NULL值的字段值

示例 
下面的语句返回值 34。（只返回一个值就算后面不为NULL也直接丢弃） 
SELECT COALESCE( NULL, 34, 13, 0 ) 
备注 
如果所有参数均为 NULL，则 COALESCE 返回 NULL。 

SELECT bi.id,
               bp.id AS blockpositionid,
               COALESCE(bp.region, bi.defaultregion) AS region,
               COALESCE(bp.weight, bi.defaultweight) AS weight,
               COALESCE(bp.visible, 1) AS visible,
               bi.configdata
          FROM {block_instances} bi
     LEFT JOIN {block_positions} bp ON bp.blockinstanceid = bi.id
                                   AND ".$DB->sql_like('bp.pagetype', ':pagetype', false)."
         WHERE bi.blockname = 'progress'
           AND bi.parentcontextid = :contextid
      ORDER BY region, weight, bi.id