Mysql process data by groups - mysql

Question

I have a few groups of data. Each group has a some property field.
For example:
_________________________
| id | value | property |
--------------------------
| 1 | 2 | 3 |
--------------------------
| 2 | 2 | 3 |
--------------------------
| 3 | 2 | 3 |
--------------------------
| 4 | 2 | 4 |
-------------------------
| 5 | 2 | 4 |
--------------------------
| 6 | 2 | 4 |
--------------------------
How can I update two strings ordered by id ASC with property = 3, and 2 strings ordered by id ASC with property = 4 by one query?
I want to update 2 of 3 rows with property = 3 and update 2 of 3 rows with property = 4. For example: rows with id 1 and 2, and rows with id 4 and 5
i.e. i want update groups of data with different conditions by one query

Answer 1

You can do it using calculated rank field, e.g. -
SELECT p1.*, COUNT(*) rank FROM properties p1
LEFT JOIN properties p2
ON p2.property = p1.property AND p2.id <= p1.id
GROUP BY p1.property, p1.id
This query will return dataset with row-number by property:
+------+-------+----------+------+
| id | value | property | rank |
+------+-------+----------+------+
| 1 | 2 | 3 | 1 |
| 2 | 2 | 3 | 2 |
| 3 | 2 | 3 | 3 |
| 4 | 2 | 4 | 1 |
| 5 | 2 | 4 | 2 |
| 6 | 2 | 4 | 3 |
+------+-------+----------+------+
Then you should update records with rank < 3:
UPDATE properties p
JOIN (SELECT p1.*, COUNT(*) rank FROM properties p1
LEFT JOIN properties p2
ON p2.property = p1.property AND p2.id <= p1.id
GROUP BY p1.property, p1.id) r
ON p.id = r.id
SET p.value = 100 -- set new value here
WHERE r.rank < 3

Answer 2

Here's the solution, and see discussion following:
update
t,
(select GROUP_CONCAT(ids) as matching_ids from (
select
SUBSTRING_INDEX(GROUP_CONCAT(id order by id), ',', 2) AS ids
from
t
where
property in (3,4)
group by
property
) s1
) s2
set value=12345
where
FIND_IN_SET(id, matching_ids) > 0
;
To illustrate, and assuming your table is called t, and the initial state is:
root#mysql-5.1.51> select * from t;
+----+-------+----------+
| id | value | property |
+----+-------+----------+
| 1 | 2 | 3 |
| 2 | 2 | 3 |
| 3 | 2 | 3 |
| 4 | 2 | 4 |
| 5 | 2 | 4 |
| 6 | 2 | 4 |
+----+-------+----------+
The result of running this query is:
root#mysql-5.1.51> select * from t;
+----+-------+----------+
| id | value | property |
+----+-------+----------+
| 1 | 12345 | 3 |
| 2 | 12345 | 3 |
| 3 | 2 | 3 |
| 4 | 12345 | 4 |
| 5 | 12345 | 4 |
| 6 | 2 | 4 |
+----+-------+----------+
A brief explanation of the query:
I pick up the first two ids for each property using the SUBSTRING_INDEX(GROUP_CONCAT(id order by id), ',', 2) statement.
I combine the above using GROUP_CONCAT(ids) as matching_ids to get all valid ids.
Finally, I update all rows in the table where the id is within combined matching_ids text.
Notes:
You should verify your group_concat_max_len variable is long enough. Default is 1024. You most probably want to have this in the millions, anyhow (regardless of my answer).
The query is far from being optimal. It answers your question, but you can't have an optimal query here.
You are most probably better off with a transaction containing two or three queries.
Good luck!

Answer 3

I'm assuming you mean to limit your two updates to two rows each. You can use ORDER BY and LIMIT in your update statements:
UPDATE yourtable
SET property = 'new_value'
WHERE value=2 AND property = 4
ORDER BY id ASC LIMIT 2
UPDATE yourtable
SET property = 'new_value'
WHERE value=2 AND property = 3
ORDER BY id DESC LIMIT 2
Update:
To force this into one query, you would need to JOIN against a subquery which retrieves the ids to update via UNION. I think this is legal:
UPDATE yourtable
JOIN (
(SELECT id FROM yourtable WHERE value=2 AND property=4 ORDER BY id ASC LIMIT 2)
UNION ALL
(SELECT id FROM yourtable WHERE value=2 AND property=3 ORDER BY id DESC LIMIT 2)
) updaterows ON yourtable.id = updaterows.id
SET property = 'new value'