联合查询union注意事项

SELECT * FROM   (select id,gdname,restricted_purchase,original_price,price,pic,text,seckill,seckill_start,seckill_end,roof_placement,number from yh_goods 
                    where 
                        seckill_start<= '1586241657' 
                    and seckill_end >= '1586241657' 
                    and `del` = 0 
                    AND `status` = 1 
                    AND `seckill` = 1 
                    AND `roof_placement` = 1  
                    order by roof_placement asc,update_time desc limit 9999999999) as a
                union 
                (select id,gdname,restricted_purchase,original_price,price,pic,text,seckill,seckill_start,seckill_end,roof_placement,number from yh_goods 
                    where 
                        seckill_start<= '1586241657' 
                    and seckill_end >= '1586241657' 
                    and `del` = 0 
                    AND `status` = 1 
                    AND `seckill` = 1 
                    AND `roof_placement` != 1 
                    order by seckill_end asc,create_time desc limit 9999999999) as b
                union 
                (select id,gdname,restricted_purchase,original_price,price,pic,text,seckill,seckill_start,seckill_end,roof_placement,number from yh_goods 
                    where 
                         `del` = 0 
                    AND `status` = 1 
                    AND `seckill` = 1 
                    order by seckill_end desc,create_time desc limit 9999999999
                )   limit 0,20

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1、在使用union的子查询的时候只能给第一个取别名其他的取别名会报语法错误

2、在子查询排序的时候如果不写limit的话会过滤掉排序因为写上limit 9999999方可解决