mysql 查询判断空值

select A.return_car_area, case when A.return_car_area='' THEN 0 ELSE 1 END from t_base_Bq_branch A

select A.return_car_area, case A.return_car_area when '' THEN 0 ELSE 1 END from t_base_Bq_branch A

当这个字段return_car_area为空串的时候显示 0 否则显示1   case when 字段=条件  then 赋值 else 赋值 end

select IFNULL(b.tobranchID,9999) from t_base_task b

当tobranchID为null时显示 9999 否则显示数据库值   IFNULL(字段，赋值) 当字段为空是赋值

  IFNULL(
  CASE c.tasktype
  WHEN 0 THEN (SELECT bpl.name FROM t_base_Bq_branch bpl WHERE c.stationId = bpl.rail_id )
  WHEN 1 THEN (SELECT bpl.name FROM t_base_Bq_branch bpl WHERE c.stationId = bpl.rail_id ) ELSE '' END ,'') toPark