Question

我有一张表格

select rowid,* from t;
rowid f
----- -----
1     aaa
2     bbb
3     ccc
4     ddd
5     eee
6     fff
7     ggg
8     aaa
9     bbb
10    ccc
11    ddd
12    eee
13    fff
14    ggg

我想在给定的行匹配之前选择n行和m行，例如对于匹配f ='ccc'且n = m = 1的行我想要

2     bbb
3     ccc
4     ddd
9     bbb
10    ccc
11    ddd

rowid在我的设置中是连续的，所以我想我们可以玩它。我尝试了

这一行

select rowid,f from t where rowid between 
  (select rowid-1 from t where f='ccc') and 
  (select rowid+1 from t where f='ccc');

rowid f
----- -----
2     bbb
3     ccc
4     ddd

但结果显然是错误的，我只得到了'ccc'比赛的第一次出现。我想我要加入或者可能是递归cte，但我很害怕它是迄今为止我的知识分子:) Thanx提前。

Answer 1

scalar subquery只能返回一个值。

你可以做两个自连接，但使用set操作会更简单：

SELECT * FROM t
WHERE rowid IN (SELECT rowid - 1 FROM t WHERE f = 'ccc'
                UNION ALL
                SELECT rowid     FROM t WHERE f = 'ccc'
                UNION ALL
                SELECT rowid + 1 FROM t WHERE f = 'ccc');

n和m的较大值需要更多子查询。如果太多，您可以使用联接：

SELECT *
FROM t
WHERE rowid IN (SELECT t.rowid
                FROM t
                JOIN (SELECT rowid - ? AS n,
                             rowid + ? AS m
                      FROM t
                      WHERE f = 'ccc'
                ) AS ranges
                ON t.rowid BETWEEN ranges.n AND ranges.m);

Answer 2

我提出了一个不是最优的解决方案，但我无法简化（删除）临时（中间）表。

select rowid,f from t;
rowid f
----- -----
1     aaa
2     bbb
3     ccc
4     ddd
5     eee
6     fff
7     ggg
8     aaa
9     bbb
10    ccc
11    ddd
12    eee
13    fff
14    ggg

create table u as
    select t2.rowid x,t1.rowid+2 y from t t1 // +2 ==> 2 rows after 'ccc'
    join t t2 on t1.rowid=t2.rowid+1         // +1 ==> 1 row before 'ccc'
    where t1.f='ccc';

select * from u;
x     y
----- -----
2     5
9     12

select t.rowid,t.f from t inner
join u on t.rowid>=u.x and t.rowid<=u.y'
rowid f
----- -----
2     bbb     1 before
3     ccc <== match
4     ddd     2 after
5     eee
9     bbb     1 before
10    ccc <== match
11    ddd     2 after
12    eee

我认为我已经满足了我的需求，但是优化非常好：）

sqlite：选择在匹配行周围显示几行

2 个答案: