基於前一個和下排的MySQL選擇行

Question

假設我有一個表，看起來像：

  mytable 
category |  begintime 
---------|---------------------- 
cat1  | 2016-09-25 15:00:00 
cat2  | 2016-09-25 16:00:00 
cat1  | 2016-09-25 17:30:00 
cat3  | 2016-09-25 19:00:00 
cat1  | 2016-09-25 20:00:00 
    :     : 

注意，它沒有一個ID號，begintime列是我的主鍵。最後，我想選擇被某個類別包圍的所有行，即選擇所有行，使得前一行的類別爲@catBefore，下一行的類別爲@catAfter。

例如，我想是這樣的：導致

SELECT * FROM ... 
category |  begintime 
---------|---------------------- 
cat2  | 2016-09-25 16:00:00 
cat3  | 2016-09-25 19:00:00 
    :     : 

的previousRow和nextRow在此

SELECT * FROM mytable WHERE previousRow.category = 'cat1' AND nextRow.category = 'cat1' 

，似乎並沒有被定義。

理念

我嘗試過的一些事情，但沒有奏效出來呢。我的一個想法是，先選擇一個和下一個類別作爲新列，所以像：

SELECT mytable.*, 
     previousRow.category AS prevCat, 
     nextRow.category AS nextCat 
FROM mytable, [stuff-I-don't-know] 

導致

 SELECT mytable.* ... 
category |  begintime  | prevCat | nextCat 
---------|----------------------|---------|--------- 
cat1  | 2016-09-25 15:00:00 | null | cat2 
cat2  | 2016-09-25 16:00:00 | cat1 | cat1 
cat1  | 2016-09-25 17:30:00 | cat2 | cat3 
cat3  | 2016-09-25 19:00:00 | cat1 | cat1 
cat1  | 2016-09-25 20:00:00 | cat3 | ... 
    :     : 

，然後用WHERE條款過濾。

這個想法是可能的，還是可以用其他方式完成？

Answer 1

一種方法使用相關的子查詢。這應該是沒關係的性能，明智的，如果你確實有主鍵聲明：

select t.* 
from (select t.*, 
      (select t2.category 
       from mytable t2 
       where t2.begintime < t.begintime 
       order by begintime desc 
       limit 1 
      ) as prev_category,   
      (select t2.category 
       from mytable t2 
       where t2.begintime > t.begintime 
       order by begintime asc 
       limit 1 
      ) as next_category 
    from mytable t 
) t 
where prev_category = @cat1 and next_category = @cat2; 

編輯：

你可以用變量做到這一點：

select t.* 
from (select t.*, 
      (@pn := (case when (@pcy := @pn) = NULL then -1 -- never gets here 
          when (@pn := category) = NULL then -1 -- never gets here 
          else @pcy 
         end) 
        ) as next_category 
     from (select t.*, 
        (@pc := (case when (@pcx := @pc) = NULL then -1 -- never gets here 
           when (@pc := category) = NULL then -1 -- never gets here 
           else @pcx 
          end) 
        ) as prev_category 
      from t cross join 
       (select @pc := '') params 
      order by t.begintime 
      ) t cross join 
      (select @pn := '') params 
     order by t.begintime desc 
    ) t 
where prev_category = @cat1 and next_category = @cat2; 

Answer 2

我這樣做的..它似乎工作，但它實際上似乎太慢... = \我會盡力改善它。請檢查它是否適用於您：

SELECT 
    t1.category, 
    t1.begintime 
FROM myTable t1 -- current 
INNER JOIN myTable t2 ON 1=1 -- prev 
    AND t2.begintime < t1.begintime 
INNER JOIN myTable t3 ON 1=1 -- next 
    AND t3.begintime > t1.begintime 
LEFT JOIN myTable t4 ON 1=1 -- between current and prev 
    AND t4.begintime < t1.begintime 
    AND t4.begintime > t2.begintime 
LEFT JOIN myTable t5 ON 1=1 -- between current and next 
    AND t5.begintime > t1.begintime 
    AND t5.begintime < t3.begintime 
WHERE 1=1 
    AND t2.category = 'cat1' -- prev cat 
    AND t3.category = 'cat1' -- next cat 
    AND t4.begintime IS NULL -- nothing between current and prev 
    AND t5.begintime IS NULL -- nothing between current and next 
;