【sql】連續出現至少3次的數 Consecutive Numbers

問題：

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.

+-----------------+
| ConsecutiveNums |
+-----------------+
| 1               |
+-----------------+

解決：

① 找出連續出現3次以上的數的值。因爲須要找三次相同數字，因此咱們須要創建三個表的實例，咱們能夠用l1分別和l2, l3內交，l1和l2的Id下一個位置比，l1和l3的下兩個位置比，而後將Num都相同的數字返回便可。1822 ms

SELECT DISTINCT l1.Num ConsecutiveNums FROM Logs l1
JOIN Logs l2 ON l1.Id = l2.Id - 1
JOIN Logs l3 ON l1.Id = l3.Id - 2
WHERE l1.Num = l2.Num AND l2.Num = l3.Num;

② 直接在三個表的實例中查找，而後把四個條件限定上，就能夠返回正確結果了。 2803 ms

SELECT DISTINCT l1.Num ConsecutiveNums FROM Logs l1,Logs l2,Logs l3
WHERE l1.Id = l2.Id - 1 AND l2.Id = l3.Id - 1
AND l1.Num = l2.Num AND l2.Num = l3.Num;

③ 用到了變量count和pre，分別初始化爲0和-1。2438 ms

SELECT DISTINCT Num ConsecutiveNums FROM (
    SELECT Num,@count := IF(@pre = Num,@count + 1,1) As n,@pre := Num
    FROM Logs,(SELECT @count := 0,@pre := -1) As init
    ) As t WHERE t.n >= 3;