I want to create a small copy of a bigger table and link both of them.
Every time I make an update in the bigger one, I want that the small one updates too.
For example, I have this data:
Big table:
id | name | price
1 | a | 10
2 | b | 12
Small table:
id | name
1 | a
2 | b
---- UPDATING THE BIGGER ONE ---
Big table:
id | name | price
1 | y | 10
2 | b | 12
3 | c | 13
Small table should become AUTOMATICALLY (after I update the bigger one):
id | name
1 | y
2 | b
3 | c
Do you know how to do it?
To do exactly what you're asking you could use triggers. Triggers are SQL that execute automatically when certain events happen. To mirror the data you would need to create UPDATE, INSERT, and DELETE triggers. (I don't have access to a MySQL instance at the moment to test, so there could be typos)
CREATE TRIGGER big_to_small_insert
AFTER INSERT ON big
FOR EACH ROW
INSERT INTO small (id, name) VALUES (NEW.id, NEW.name)
CREATE TRIGGER big_to_small_update
AFTER UPDATE ON big
FOR EACH ROW
UPDATE small SET name = NEW.name WHERE id = NEW.id
CREATE TRIGGER big_to_small_delete
AFTER DELETE ON big
FOR EACH ROW
DELETE FROM small WHERE id = OLD.id
However, a View is probably a better option if the "small" table is truly just the big table with a subset of data. A View won't store a copy of the data, so if you update the table (big) it will be reflected in the view (small), but the opposite is also true. If you do an INSERT, UPDATE, or DELETE on the view (small) it will actually happen in the table (big).
CREATE VIEW small AS
SELECT id, name FROM big
I have a table which contains task list of persons. followings are columns
+---------+-----------+-------------------+------------+---------------------+
| task_id | person_id | task_name | status | due_date_time |
+---------+-----------+-------------------+------------+---------------------+
| 1 | 111 | walk 20 min daily | INCOMPLETE | 2017-04-13 17:20:23 |
| 2 | 111 | brisk walk 30 min | COMPLETE | 2017-03-14 20:20:54 |
| 3 | 111 | take medication | COMPLETE | 2017-04-20 15:15:23 |
| 4 | 222 | sport | COMPLETE | 2017-03-18 14:45:10 |
+---------+-----------+-------------------+------------+---------------------+
I want to find out monthly compliance in percentage(completed task/total task * 100) of each person like
+---------------+-----------+------------+------------+
| compliance_id | person_id | compliance | month |
+---------------+-----------+------------+------------+
| 1 | 111 | 100 | 2017-03-01 |
| 2 | 111 | 50 | 2017-04-01 |
| 3 | 222 | 100 | 2017-03-01 |
+---------------+-----------+------------+------------+
Here person_id 111 has 1 task in month 2017-03-14 and which status is completed, as 1 out of 1 task is completed in march then compliance is 100%
Currently, I am using separate table which stores this compliance but I have to calculate compliance update that table every time the task status is changed
I have tried creating a view also but it's taking too much time to execute view almost 0.5 seconds for 1 million records.
CREATE VIEW `person_compliance_view` AS
SELECT
`t`.`person_id`,
CAST((`t`.`due_date_time` - INTERVAL (DAYOFMONTH(`t`.`due_date_time`) - 1) DAY)
AS DATE) AS `month`,
COUNT(`t`.`status`) AS `total_count`,
COUNT((CASE
WHEN (`t`.`status` = 'COMPLETE') THEN 1
END)) AS `completed_count`,
CAST(((COUNT((CASE
WHEN (`t`.`status` = 'COMPLETE') THEN 1
END)) / COUNT(`t`.`status`)) * 100)
AS DECIMAL (10 , 2 )) AS `compliance`
FROM
`task` `t`
WHERE
((`t`.`isDeleted` = 0)
AND (`t`.`due_date_time` < NOW())
GROUP BY `t`.`person_id` , EXTRACT(YEAR_MONTH FROM `t`.`due_date_time`)
Is there any optimized way to do it?
The first question to consider is whether the view can be optimized to give the required performance. This may mean making some changes to the underlying tables and data structure. For example, you might want indexes and you should check query plans to see where they would be most effective.
Other possible changes which would improve efficiency include adding an extra column "year_month" to the base table, which you could populate via a trigger. Another possibility would be to move all the deleted tasks to an 'archive' table to give the view less data to search through.
Whatever you do, a view will always perform worse than a table (assuming the table has relevant indexes). So depending on your needs you may find you need to use a table. That doesn't mean you should junk your view entirely. For example, if a daily refresh of your table is sufficient, you could use your view to help:
truncate table compliance;
insert into compliance select * from compliance_view;
Truncate is more efficient than delete, but you can't use a rollback, so you might prefer to use delete and top-and-tail with START TRANSACTION; ... COMMIT;. I've never created scheduled jobs in MySQL, but if you need help, this looks like a good starting point: here
If daily isn't often enough, you could schedule this to run more often than daily, but better options will be triggers and/or "partial refreshes" (my term, I've no idea if there is a technical term for the idea.
A perfectly written trigger would spot any relevant insert/update/delete and then insert/update/delete the related records in the compliance table. The logic is a little daunting, and I won't attempt it here. An easier option would be a "partial refresh" on called within a trigger. The trigger would spot user targetted by the change, delete only the records from compliance which are related to that user and then insert from your compliance_view the records relating to that user. You should be able to put that into a stored procedure which is called by the trigger.
Update expanding on the options (if a view just won't do):
Option 1: Daily full (or more frequent) refresh via a schedule
You'd want code like this executed (at least) daily.
truncate table compliance;
insert into compliance select * from compliance_view;
Option 2: Partial refresh via trigger
I don't work with triggers often, so can't recall syntax, but the logic should be as follows (not actual code, just pseudo-code)
AFTER INSERT -- you may need one for each of INSERT / UPDATE / DELETE
FOR EACH ROW -- or if there are multiple rows and you can trigger only on the last one to be changed, that would be better
DELETE FROM compliance
WHERE person_id = INSERTED.person_id
INSERT INTO compliance select * from compliance_view where person_id = INSERTED.person_id
END
Option 3: Smart update via trigger
This would be similar to option 2, but instead of deleting all the rows from compliance that relate to the relevant person_id and creating them from scratch, you'd work out which ones to update, and update them and whether any should be added / deleted. The logic is a little involved, and I'm not not going to attempt it here.
Personally, I'd be most tempted by Option 2, but you'd need to combine it with option 1, since the data goes stale due to the use of now().
Here's a similar way of writing the same thing...
Views are of very limited benefit in MySQL, and I think should generally be avoided.
DROP TABLE IF EXISTS my_table;
CREATE TABLE my_table
(task_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY
,person_id INT NOT NULL
,task_name VARCHAR(30) NOT NULL
,status ENUM('INCOMPLETE','COMPLETE') NOT NULL
,due_date_time DATETIME NOT NULL
);
INSERT INTO my_table VALUES
(1,111,'walk 20 min daily','INCOMPLETE','2017-04-13 17:20:23'),
(2,111,'brisk walk 30 min','COMPLETE','2017-03-14 20:20:54'),
(3,111,'take medication','COMPLETE','2017-04-20 15:15:23'),
(4,222,'sport','COMPLETE','2017-03-18 14:45:10');
SELECT person_id
, DATE_FORMAT(due_date_time,'%Y-%m') yearmonth
, SUM(status = 'complete')/COUNT(*) x
FROM my_table
GROUP
BY person_id
, yearmonth;
person_id yearmonth x
111 2017-03 1.0
111 2017-04 0.5
222 2017-03 1.0
I am runnning a Postgres and Mysql server and I have table:
id | name | age | old_age
1 abc 20
I want to update column age with 21 with storing old value 20 into old_age column.
I can fetch the row and read the value of age and then update but that will require running 2 queries. 1 select and 1 update. Is there any way we can update in 1 query? I just want to copy age to old_age.
Just do
UPDATE TableName SET old_age=age, age=21 WHERE
Okay, talking millions of rows here..
Structure of like
EXAMPLE 1
some_data_before this| x_counter_total | y_counter_total | x_counter_week | y_counter_week | x_counter_year | y_counter_year
--------------------------------------------------------------------------------------------------------------------------------------
some_data_here... | 42142142....... | `241242142..... | 23214124...... | .............. | .............. |` ..............
And every of X and Y events to increment these columns vs this
EXAMPLE 2
table A
some_data_before this| x_counter_total | y_counter_total |
----------------------------------------------------------
some_data_here...... | 42142142....... | `241242142..... |
table B
key_connected_with_table_A | x_event | y_event | occured_timestamp
-------------------------------------------------------------------
id 21...................... | true | false | current_timestamp
My need is this. I need number of X and Y events in some time, past day/week/month/year etc.
My question is that Is it better to update(increment) multiple columns describing the time period i need, like in EXAMPLE 1 or is it better to
on each Event add a Row like in EXAMPLE 2 and then count total VOTES with same ID WHERE occured_timestamp - current_timestamo < TIMESTAMP_OF_A_WEEK for example. Which one is more efficient? talking millions of records, and thousands of request in a minute.
NO, I would keep them in a single table since then I would need to fire only one UPDATE statement but if you separate them to 2 tables then either you will need to execute 2 update statement (or) create a AFTER UPDATE TRIGGER to insert into the other table (or) probably do a update join to update all the respective values ion both tables which to me looks more performance hit than having all the columns in single table.
Is there a way to auto increment the id field of my database based on the values of two other columns in the inserted row?
I'd like to set up my database so that when multiple rows are inserted at the same time, they keep their tracknumber ordering. The ID field should auto increment based firstly on the automatically generated timestamp field, and then secondly the tracknumber contained within that timestamp.
Here's an example of how the database might look:
id | tracknumber | timestamp
________________________________________
1 | 1 | 2014-03-31 11:35:17
2 | 2 | 2014-03-31 11:35:17
3 | 3 | 2014-03-31 11:35:17
4 | 1 | 2014-04-01 09:10:14
5 | 2 | 2014-04-01 09:10:14
I've been reading up on triggers but not sure if that's appropriate here? I feel as though i'm missing an obvious function.
This is a bit long for a comment.
There is no automatic way to do this. You can do it with triggers, if you like. Note the plural, you will need triggers for insert, update, and delete, if you want the numbering to remain accurate as the data changes.
You can do this one the query side, if the goal is to enumerate the values. Here is one method using a subquery:
select t.*,
(select count(*) from table t2 where t2.timestamp = t.timestamp and t2.id <= t.id
) as tracknumber
from table t;
The performance of this might even be reasonable with an index on table(timestamp, id).
If the data is being created once, you can also populate the values using an update query.
If you are inserting them in one transaction and or script, then sort the data yourself in the server side according to these two fields (assuming you create timestamp on server side too because that would seem logical) and then insert the rows one after another. I don't think it is necessary to overthink this and look for a difficult approach in the database. Database will still be inserting rows one after another, not all at once so there is no way it will know that it needs to do some kind of sorting beforehand. It is you who has to do it.