SQL::自联接表以满足特定条件?
我有下表:SQL::自联接表以满足特定条件?,sql,mysql,Sql,Mysql,我有下表: mysql> SELECT * FROM temp; +----+------+ | id | a | +----+------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | | 4 | 4 | +----+------+ 我正在尝试获得以下输出: +----+------+------+ | id | a | a | +----+------+------+ | 1 | 1 | 2 |
mysql> SELECT * FROM temp;
+----+------+
| id | a |
+----+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
+----+------+
我正在尝试获得以下输出:
+----+------+------+
| id | a | a |
+----+------+------+
| 1 | 1 | 2 |
| 2 | 2 | 3 |
| 3 | 3 | 4 |
+----+------+------+
但是我有一个小问题。我编写了以下查询:
mysql> SELECT A.id, A.a, B.a FROM temp A, temp B WHERE B.a>A.a;
但我的输出如下:
+----+------+------+
| id | a | a |
+----+------+------+
| 1 | 1 | 2 |
| 1 | 1 | 3 |
| 2 | 2 | 3 |
| 1 | 1 | 4 |
| 2 | 2 | 4 |
| 3 | 3 | 4 |
+----+------+------+
有人能告诉我如何将其转换为所需的输出吗?我试图得到一个只生成连续值的表单。我的意思是,如果2大于1,3大于2,我不希望3大于1。选项1:“三角连接”-二次复杂度
SELECT A.id, A.a, MIN(B.a) AS a
FROM temp A
JOIN temp B ON B.a>A.a
GROUP BY A.id, A.a;`
select a_numbered.id, a_numbered.a, b_numbered.a
from
(
select id,
a,
@rownum := @rownum + 1 as rn
from temp
join (select @rownum := 0) r
order by id
) a_numbered join (
select id,
a,
@rownum2 := @rownum2 + 1 as rn
from temp
join (select @rownum2 := 0) r
order by id
) b_numbered
on b_numbered.rn = a_numbered.rn+1
选项2:“伪行数()”-线性复杂度
SELECT A.id, A.a, MIN(B.a) AS a
FROM temp A
JOIN temp B ON B.a>A.a
GROUP BY A.id, A.a;`
select a_numbered.id, a_numbered.a, b_numbered.a
from
(
select id,
a,
@rownum := @rownum + 1 as rn
from temp
join (select @rownum := 0) r
order by id
) a_numbered join (
select id,
a,
@rownum2 := @rownum2 + 1 as rn
from temp
join (select @rownum2 := 0) r
order by id
) b_numbered
on b_numbered.rn = a_numbered.rn+1
a
中的数字是连续的,没有空格,还是只是排序,可能有空格?@Martin:Oh。。不,它们不必是连续的。他们可能有差距。@OMG:是的。如果这就是他们所说的:)从temp A中选择A.id、A.A、B.A,temp B其中B.A>A.A按A.A分组代码>似乎给了我想要的,但不确定这是否正确。。。有什么评论吗?呃,实际上我认为选择A.id,A.A,MIN(B.A)作为A从临时A加入临时B.A>A.A组A.id,A.A当我执行你的查询时,我只得到一行:4,4,1。我做错什么了吗?@legend-我需要在选项2
中使用不同的变量名。我以为它会被重置,但显然不是。现在修好了!您可能会发现,如果您有一个较大的表,那么第二个版本的性能会更好,因为工作负载将线性增长,而不是像选项1中的二次增长。非常感谢您和+1的解释!)