How can I add a sequencial number without cursor in mysql? - mysql

Question

I have a table that can have some valid duplicated values so I need an additional column with the sequence number of appearance of said duplicate for future use.
A sample might be
ROW | COLUMN_A | COLUMN_B | COLUMN_C | SEQ_NUM <= Want this column
1 A B 1 1
2 A B 1 2
3 A B 2 1
4 A B 2 2
5 A B 2 3
The values are supposed to be unique like (COLUMN_A, COLUMB_B, COLUMN_C), but I cannot use a unique index because I need those duplicated values as well, I just need to keep track of the order of apparition. So I added a column SEQ_NUM to keep track of those repetitions.
And i fill it like this:
begin
declare done boolean default false;
declare _A varchar(1);
declare _B varchar(1);
declare _C integer unsigned;
declare cur cursor for
select COLUMN_A , COLUMN_B , COLUMN_C
from tmp_horario
group by COLUMN_A , COLUMN_B , COLUMN_C
having count(*) > 1; -- Here I loop throught the repeated values
declare continue handler for not found set done := true;
open cur;
loop_dup: loop
fetch cur into _A, _B, _C;
if done then
leave loop_dup;
end if;
set #_seq = 0; -- I initialize my sequence in 0 to start
update tmp_table h
set h.SEQ_NUM = (#_seq := #_seq + 1) -- Set the next sequential to the repeated values
where h.COLUMN_A = _A
and h.COLUMN_B = _B
and h.COLUMN_C = _C;
end loop loop_dup;
close cur;
end;
Note: The table has way more columns making the cursor (fetch into) a bigger pain.
As you can see that works like charm except that it takes my store from 20 s to 80 s which I find a little disappointing (already checked indexes and they are being properly used), I believe the problem lies in the use of the cursor.
My question then is: Is there a way of setting that famous sequential number in a single query without the cursor?.

Answer 1

Assuming you want this to happen when you insert a value to the table you could do this as such:
INSERT INTO tmp_horario(COLUMN_A, COLUMN_B, COLUMN_C, SEQ_NUM)
VALUE(A_VAL, B_VAL, C_VAL, (IFNULL((
SELECT MAX(SEQ_NUM)
FROM tmp_horario AS a
WHERE a.COLUMN_A = A_VAL AND a.COLUMN_B = B_VAL AND a.COLUMN_C = C_VAL), 0)+1));
The basic premise is you look for rows with the same values, get the maximum sequential value if one exists, and then add one to that for the new value. If no match is found then set the insert value to one. The IFNULL statement is really all you need to get the SEQ_NUM, should you need to adapt this query.

Answer 2

Yes pretty much like your cursor
DROP TABLE IF EXISTS T;
CREATE TABLE T(ROW INT, COLUMN_A VARCHAR(1), COLUMN_B VARCHAR(1), COLUMN_C VARCHAR(1), SEQ_NUM INT);
INSERT INTO T VALUES
(1 , 'A' , 'B' , 1,NULL),
(2 , 'A' , 'B' , 1,NULL),
(3 , 'A' , 'B' , 2,NULL),
(4 , 'A' , 'B' , 2,NULL),
(5 , 'A' , 'B' , 2,NULL);
UPDATE T
JOIN (
SELECT T.ROW,
IF(CONCAT(T.COLUMN_A,T.COLUMN_B,T.COLUMN_C) <> #P , #RN:=1,#RN:=#RN+1) RN,
#P:=CONCAT(T.COLUMN_A,T.COLUMN_B,T.COLUMN_C) P
FROM T , (SELECT #RN:=0,#P:=0) R
ORDER BY ROW
) S ON S.ROW = T.ROW
SET SEQ_NUM = S.RN
WHERE 1 = 1
MariaDB [sandbox]> SELECT * FROM T;
+------+----------+----------+----------+---------+
| ROW | COLUMN_A | COLUMN_B | COLUMN_C | SEQ_NUM |
+------+----------+----------+----------+---------+
| 1 | A | B | 1 | 1 |
| 2 | A | B | 1 | 2 |
| 3 | A | B | 2 | 1 |
| 4 | A | B | 2 | 2 |
| 5 | A | B | 2 | 3 |
+------+----------+----------+----------+---------+
5 rows in set (0.00 sec)