Write a SQL query to find all numbers that appear at least three times consecutively.
+----+-----+| Id | Num |+----+-----+| 1 | 1 || 2 | 1 || 3 | 1 || 4 | 2 || 5 | 1 || 6 | 2 || 7 | 2 |+----+-----+
For example, given the above Logs
table, 1
is the only number that appears consecutively for at least three times.
思路:
自己没看题解做出来的第一道middle难度的题目,这个题和Rasing Temprature很像,考察的都是一个表中不同元组之间的某些属性的比较
那个easy的题目只要比较连续的两行就好,而这个题目要比较三行
将easy题目的两种解法套用过来,其中一个会超时,及O(2n^2)时间复杂度的那个嵌套选择算法,我分析这种算法之所以超时是因为每一次子select都要遍历整张表来判断where a.Id = c.Id - 2和where a.Id = b.Id-1,而当表的数据非常大的时候,速度就会很慢
第二种AC了的算法,感觉是一种消耗空间节省时间的方法,我们建立了表格之后,只需要遍历一遍就可以得出最后的答案。
AC代码: select distinct a.Num as consecutiveNumsfrom Logs as a,Logs as b,Logs as cwhere a.Id = b.Id - 1 and a.Id = c.Id - 2 and a.Num = b.Num and a.Num = c.Num
TLE代码:select distinct a.Num as ConsecutiveNumsfrom Logs as awhere a.Num = (select b.Num from Logs as bwhere a.Id = b.Id -1)anda.Num = (select c.Numfrom Logs as cwhere a.Id = c.Id -2)