怎么优化下面SQL： select distinct bid from ( select row_number() over(order by bid,start_date) as num,bid,start_date,end_date from t1 )a inner join ( select row_number() over(order by bid,start_date) as num,bid,start_date,end_date from t1 )b on a.bid = b.bid where a.num = b.num+1

可以考虑使用CTE（公共表表达式）来优化该SQL语句。具体做法如下：

WITH cte AS (
  SELECT ROW_NUMBER() OVER (ORDER BY bid, start_date) AS num, bid, start_date, end_date
  FROM t1
)
SELECT DISTINCT a.bid
FROM cte a
INNER JOIN cte b ON a.bid = b.bid AND a.num = b.num + 1

使用公共表表达式的好处是可以避免重复计算，使得SQL语句更加简洁和易于理解。在这个例子中，我们只需要计算一次ROW_NUMBER()函数，然后在CTE中引用它即可。同时，使用WITH语句也可以使得SQL语句更加可读性强，更加易于维护。