Update with group by | 易学教程

问题

I'm stumped on what seemed to be a simple UPDATE statement.

I'm looking for an UPDATE that uses two values. The first (a) is used to group, the second (b) is used to find a local minimum of values within the respective group. As a little extra there is a threshold value on b: Any value 1 or smaller shall remain as it is.

drop table t1;
create table t1 (a number, b number);
insert into t1 values (1,0);
insert into t1 values (1,1);
insert into t1 values (2,1);
insert into t1 values (2,2);
insert into t1 values (3,1);
insert into t1 values (3,2);
insert into t1 values (3,3);
insert into t1 values (4,1);
insert into t1 values (4,3);
insert into t1 values (4,4);
insert into t1 values (4,5);


-- 1,0 -> 1,0
-- 1,1 -> 1,1
-- 2,1 -> 2,1
-- 2,2 -> 2,2
-- 3,1 -> 3,1
-- 3,2 -> 3,2
-- 3,3 -> 3,2  <-
-- 4,1 -> 4,1
-- 4,3 -> 4,3  <-
-- 4,4 -> 4,3  <-
-- 4,5 -> 4,3  <-

Obviously not sufficient is:

update t1 x
   set b = (select min(b) from t1 where b > 1)
;

Whatever more complicated stuff I try, e.g.

UPDATE t1 x
   set (a,b) = (select distinct a,b from (
                    select a, min(b) from t1 where b > 1 group by a)
               )
;

I get SQL-Fehler: ORA-01427: Unterabfrage für eine Zeile liefert mehr als eine Zeile 01427. 00000 - "single-row subquery returns more than one row"

which is not overly surprising as I need a row for each value of a.

Of course I could write a PL/SQL Procedure with a cursor loop but is it possible in a single elegant SQL statement? Maybe using partition by?

回答1:

Your question is a bit confusing. You say that you would like to set value b to a minimum value from partition a that column b is in row with, while the rows containing b = 1 should remain untouched.

From what I can see in your question as comments (I assume it's your expected output) you also want to get the minimum value that follows 1 within a partition - so you basically want the minimum value of b that is greater than 1.

Below is SQL query that does this

UPDATE t1 alias
  SET b = (
    SELECT min(b) 
    FROM t1 
    WHERE alias.a = t1.a 
      AND t1.b > 1 -- this would get the minimum value higher than 1
    GROUP BY a
  )
  WHERE alias.b > 1 -- update will not affect rows with b <= 1

Output after update

 a | b 
---+---
 1 | 0
 1 | 1
 2 | 1
 2 | 2
 3 | 1
 3 | 2
 3 | 2
 4 | 1
 4 | 3
 4 | 3
 4 | 3

来源：https://stackoverflow.com/questions/29692077/update-with-group-by

标签

Oracle

group-by

partition