I have a table which has composite key on type, entityId, userid. Now i want to alter table record for some reason. Issue with updating record that update statement gives error like duplicate key.
For example my query is like —
UPDATE iap SET type = 'tour', entityId = 3 WHERE type = 'bundle' AND
entityId = 2
But with update statement it look out for exiting record where already that user has tour and entityID is same which we wants to update.
Now in that case i want to delete that record with type bundle as i want all bundle with id 2 become tour with id 3.
So if tour with id 3 exist then simply i want to delete bundle with id 2 and no need to update.
Any possible query around, please?
Try this.
CREATE TEMPORARY TABLE IF NOT EXISTS tmp_iap AS (SELECT id, user_id, type, entityId FROM iap where type='tour' AND
entityId = 3);
DELETE from iap where type='bundle' AND entityId = 2 AND EXISTS (SELECT 1 FROM tmp_iap Where tmp_iap.user_id = iap.user_id)
Please take a backup first if there is any important data.
I've got a potential race condition in an application I'm developing, which I'd like to account for and avoid in my querying.
To summarise the application flow...
Create a new row in the entries table:
INSERT INTO entries ( name, email ) VALUES ( 'Foo Bar', 'foo#example.com' );
Find out if Mr Bar is a winner by checking a time-sensitive prizes table:
SELECT id FROM prizes WHERE various_time_conditions = 'met' AND id NOT IN ( SELECT prize_id FROM entries );
If he's a winner, update his entry row accordingly:
UPDATE entries SET prize_id = [prize id] WHERE id = [entry id];
As each prize can only be given out once, I need to eliminate any possibility of a race condition where another process can query the prizes table and update the entry table between steps 2 and 3 above.
I've been doing some research and have found a load of information about transactions (all my tables use InnoDB) and using MySQL's SELECT ... FOR UPDATE syntax but I'm confused as to which is the most suitable solution for me.
You're going to want to lock the prize record. So add some availability flag on the prizes table (perhaps with a default value) if you're not going to use something like a winner_id. Something like this:
SELECT id FROM prizes WHERE ... AND available = 1 FOR UPDATE
Then set the availability if you do assign the prize:
UPDATE prizes SET available = 0 WHERE id = ...
You'll need to wrap this inside a transaction of course.
Make sure that every time you check to see if the prize is available, you add AND available = 1 FOR UPDATE to the query because a SELECT without the FOR UPDATE is not going to wait for a lock.
We have an issue-tracking system for our software. Yesterday my boss gave me the task of finding all issues in the issue table submitted by this certain guy with an ID of 2992:
issue table:
issue_id (int)
submitter_id (int)
...other fields
Then for each issue in that result set I had to look at another table to find who was subscribed to it:
issue_subscribers table:
issue_id (int) [primary key to issue table]
subscriber_id (int)
...other fields
I had these ID numbers (3247, 4748, 7430, 8429) which represent other users, and if any of them were NOT subscribed, I had to add them as subscribers to the issue. This means simply adding a new row to issue_subscribers table.
This was way over my head. I ended up doing this with 3 different queries in a Java JDBC program instead of trying to figure it out with pure SQL. But now I'm curious how it could be done. Is this a simple procedure for professional SQL guys? Is it possible to write a single query to do this? How would it be done?
Supposing:
subscriber_id is a foreign key to some table that hold all possible subscribers.
You only need to do this with issue 2992 and subscribers 3247, 4748, 7430 and 8429.
You could write following SQL query:
Insert Into issue_subscribers
(
issue_id,
subscriber_id
-- and other fields if necessary
)
(
Select 2992 issue_id,
subscriber_id
-- and other fields if necessary
From subscribers subs -- table that holds all possible subscribers
Where subscriber_id In (3247, 4748, 7430, 8429)
And Not Exists (Select 1
From issue_subscribers iss
Where issue_id = 2992
And iss.subscriber_id = subs.subscriber_id)
);
In case you would need to use it for other issues and subscribers you would need to change the ids to parameters, and specify the subscriber_ids of the in condition some other way (perhaps a subselect of some kind).
I have a scenario that requires a value in a row of a table to be updated automatically whenever a row has been added or deleted in another table. I'm not sure how to do it.BTW I'm using phpmyadmin in order to manage my database. Thanks in advance.
pages Table
------------
page_no
no_of_choices
choices Table
-------------
page_no
choice_no
When I add a choice with choice number 1 and page_no, then the table page which has the row, page_no=1 should be updated with no_of_choices=no_of_choices+1
You can use triggers.
For example:
CREATE TRIGGER `test1`
AFTER INSERT ON `tbl1`
FOR EACH ROW SET NEW.upd_fld = new_value
Similarly could be done for delete.
You can also create triggers from phpMyAdmin
TABLE A: page_no, no_of_choices
TABLE B: page_no, choice_no...
With a relational database you very rarely want to have duplicate data. If something breaks at some point, you won't know which to trust - the rows in Table B, or the no_of_choices in Table A. A better solution is to do one of the following (depending on which table you are querying):
SELECT COUNT(no_of_choices) FROM B WHERE page_no = 1
or
SELECT A.*, COUNT(choice_no) AS choice_no FROM A LEFT JOIN B USING(page_no)
You get the same result, but now you have one record to go off of, so you won't have inconsistent data.
I've been asked if I can keep track of the changes to the records in a MySQL database. So when a field has been changed, the old vs new is available and the date this took place. Is there a feature or common technique to do this?
If so, I was thinking of doing something like this. Create a table called changes. It would contain the same fields as the master table but prefixed with old and new, but only for those fields which were actually changed and a TIMESTAMP for it. It would be indexed with an ID. This way, a SELECT report could be run to show the history of each record. Is this a good method? Thanks!
Here's a straightforward way to do this:
First, create a history table for each data table you want to track (example query below). This table will have an entry for each insert, update, and delete query performed on each row in the data table.
The structure of the history table will be the same as the data table it tracks except for three additional columns: a column to store the operation that occured (let's call it 'action'), the date and time of the operation, and a column to store a sequence number ('revision'), which increments per operation and is grouped by the primary key column of the data table.
To do this sequencing behavior a two column (composite) index is created on the primary key column and revision column. Note that you can only do sequencing in this fashion if the engine used by the history table is MyISAM (See 'MyISAM Notes' on this page)
The history table is fairly easy to create. In the ALTER TABLE query below (and in the trigger queries below that), replace 'primary_key_column' with the actual name of that column in your data table.
CREATE TABLE MyDB.data_history LIKE MyDB.data;
ALTER TABLE MyDB.data_history MODIFY COLUMN primary_key_column int(11) NOT NULL,
DROP PRIMARY KEY, ENGINE = MyISAM, ADD action VARCHAR(8) DEFAULT 'insert' FIRST,
ADD revision INT(6) NOT NULL AUTO_INCREMENT AFTER action,
ADD dt_datetime DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP AFTER revision,
ADD PRIMARY KEY (primary_key_column, revision);
And then you create the triggers:
DROP TRIGGER IF EXISTS MyDB.data__ai;
DROP TRIGGER IF EXISTS MyDB.data__au;
DROP TRIGGER IF EXISTS MyDB.data__bd;
CREATE TRIGGER MyDB.data__ai AFTER INSERT ON MyDB.data FOR EACH ROW
INSERT INTO MyDB.data_history SELECT 'insert', NULL, NOW(), d.*
FROM MyDB.data AS d WHERE d.primary_key_column = NEW.primary_key_column;
CREATE TRIGGER MyDB.data__au AFTER UPDATE ON MyDB.data FOR EACH ROW
INSERT INTO MyDB.data_history SELECT 'update', NULL, NOW(), d.*
FROM MyDB.data AS d WHERE d.primary_key_column = NEW.primary_key_column;
CREATE TRIGGER MyDB.data__bd BEFORE DELETE ON MyDB.data FOR EACH ROW
INSERT INTO MyDB.data_history SELECT 'delete', NULL, NOW(), d.*
FROM MyDB.data AS d WHERE d.primary_key_column = OLD.primary_key_column;
And you're done. Now, all the inserts, updates and deletes in 'MyDb.data' will be recorded in 'MyDb.data_history', giving you a history table like this (minus the contrived 'data_columns' column)
ID revision action data columns..
1 1 'insert' .... initial entry for row where ID = 1
1 2 'update' .... changes made to row where ID = 1
2 1 'insert' .... initial entry, ID = 2
3 1 'insert' .... initial entry, ID = 3
1 3 'update' .... more changes made to row where ID = 1
3 2 'update' .... changes made to row where ID = 3
2 2 'delete' .... deletion of row where ID = 2
To display the changes for a given column or columns from update to update, you'll need to join the history table to itself on the primary key and sequence columns. You could create a view for this purpose, for example:
CREATE VIEW data_history_changes AS
SELECT t2.dt_datetime, t2.action, t1.primary_key_column as 'row id',
IF(t1.a_column = t2.a_column, t1.a_column, CONCAT(t1.a_column, " to ", t2.a_column)) as a_column
FROM MyDB.data_history as t1 INNER join MyDB.data_history as t2 on t1.primary_key_column = t2.primary_key_column
WHERE (t1.revision = 1 AND t2.revision = 1) OR t2.revision = t1.revision+1
ORDER BY t1.primary_key_column ASC, t2.revision ASC
Edit:
Oh wow, people like my history table thing from 6 years ago :P
My implementation of it is still humming along, getting bigger and more unwieldy, I would assume. I wrote views and pretty nice UI to look at the history in this database, but I don't think it was ever used much. So it goes.
To address some comments in no particular order:
I did my own implementation in PHP that was a little more involved, and avoided some of the problems described in comments (having indexes transferred over, signifcantly. If you transfer over unique indexes to the history table, things will break. There are solutions for this in the comments). Following this post to the letter could be an adventure, depending on how established your database is.
If the relationship between the primary key and the revision column seems off it usually means the composite key is borked somehow. On a few rare occasions I had this happen and was at a loss to the cause.
I found this solution to be pretty performant, using triggers as it does. Also, MyISAM is fast at inserts, which is all the triggers do. You can improve this further with smart indexing (or lack of...). Inserting a single row into a MyISAM table with a primary key shouldn't be an operation you need to optimize, really, unless you have significant issues going on elsewhere. In the entire time I was running the MySQL database this history table implementation was on, it was never the cause of any of the (many) performance problems that came up.
if you're getting repeated inserts, check your software layer for INSERT IGNORE type queries. Hrmm, can't remember now, but I think there are issues with this scheme and transactions which ultimately fail after running multiple DML actions. Something to be aware of, at least.
It's important that the fields in the history table and the data table match up. Or, rather, that your data table doesn't have MORE columns than the history table. Otherwise, insert/update/del queries on the data table will fail, when the inserts to the history tables put columns in the query that don't exist (due to d.* in the trigger queries), and the trigger fails. t would be awesome if MySQL had something like schema-triggers, where you could alter the history table if columns were added to the data table. Does MySQL have that now? I do React these days :P
It's subtle.
If the business requirement is "I want to audit the changes to the data - who did what and when?", you can usually use audit tables (as per the trigger example Keethanjan posted). I'm not a huge fan of triggers, but it has the great benefit of being relatively painless to implement - your existing code doesn't need to know about the triggers and audit stuff.
If the business requirement is "show me what the state of the data was on a given date in the past", it means that the aspect of change over time has entered your solution. Whilst you can, just about, reconstruct the state of the database just by looking at audit tables, it's hard and error prone, and for any complicated database logic, it becomes unwieldy. For instance, if the business wants to know "find the addresses of the letters we should have sent to customers who had outstanding, unpaid invoices on the first day of the month", you likely have to trawl half a dozen audit tables.
Instead, you can bake the concept of change over time into your schema design (this is the second option Keethanjan suggests). This is a change to your application, definitely at the business logic and persistence level, so it's not trivial.
For example, if you have a table like this:
CUSTOMER
---------
CUSTOMER_ID PK
CUSTOMER_NAME
CUSTOMER_ADDRESS
and you wanted to keep track over time, you would amend it as follows:
CUSTOMER
------------
CUSTOMER_ID PK
CUSTOMER_VALID_FROM PK
CUSTOMER_VALID_UNTIL PK
CUSTOMER_STATUS
CUSTOMER_USER
CUSTOMER_NAME
CUSTOMER_ADDRESS
Every time you want to change a customer record, instead of updating the record, you set the VALID_UNTIL on the current record to NOW(), and insert a new record with a VALID_FROM (now) and a null VALID_UNTIL. You set the "CUSTOMER_USER" status to the login ID of the current user (if you need to keep that). If the customer needs to be deleted, you use the CUSTOMER_STATUS flag to indicate this - you may never delete records from this table.
That way, you can always find what the status of the customer table was for a given date - what was the address? Have they changed name? By joining to other tables with similar valid_from and valid_until dates, you can reconstruct the entire picture historically. To find the current status, you search for records with a null VALID_UNTIL date.
It's unwieldy (strictly speaking, you don't need the valid_from, but it makes the queries a little easier). It complicates your design and your database access. But it makes reconstructing the world a lot easier.
You could create triggers to solve this. Here is a tutorial to do so (archived link).
Setting constraints and rules in the database is better than writing
special code to handle the same task since it will prevent another
developer from writing a different query that bypasses all of the
special code and could leave your database with poor data integrity.
For a long time I was copying info to another table using a script
since MySQL didn’t support triggers at the time. I have now found this
trigger to be more effective at keeping track of everything.
This trigger will copy an old value to a history table if it is changed
when someone edits a row. Editor ID and last mod are stored in the
original table every time someone edits that row; the time corresponds
to when it was changed to its current form.
DROP TRIGGER IF EXISTS history_trigger $$
CREATE TRIGGER history_trigger
BEFORE UPDATE ON clients
FOR EACH ROW
BEGIN
IF OLD.first_name != NEW.first_name
THEN
INSERT INTO history_clients
(
client_id ,
col ,
value ,
user_id ,
edit_time
)
VALUES
(
NEW.client_id,
'first_name',
NEW.first_name,
NEW.editor_id,
NEW.last_mod
);
END IF;
IF OLD.last_name != NEW.last_name
THEN
INSERT INTO history_clients
(
client_id ,
col ,
value ,
user_id ,
edit_time
)
VALUES
(
NEW.client_id,
'last_name',
NEW.last_name,
NEW.editor_id,
NEW.last_mod
);
END IF;
END;
$$
Another solution would be to keep an Revision field and update this field on save. You could decide that the max is the newest revision, or that 0 is the most recent row. That's up to you.
Here is how we solved it
a Users table looked like this
Users
-------------------------------------------------
id | name | address | phone | email | created_on | updated_on
And the business requirement changed and we were in a need to check all previous addresses and phone numbers a user ever had.
new schema looks like this
Users (the data that won't change over time)
-------------
id | name
UserData (the data that can change over time and needs to be tracked)
-------------------------------------------------
id | id_user | revision | city | address | phone | email | created_on
1 | 1 | 0 | NY | lake st | 9809 | #long | 2015-10-24 10:24:20
2 | 1 | 2 | Tokyo| lake st | 9809 | #long | 2015-10-24 10:24:20
3 | 1 | 3 | Sdny | lake st | 9809 | #long | 2015-10-24 10:24:20
4 | 2 | 0 | Ankr | lake st | 9809 | #long | 2015-10-24 10:24:20
5 | 2 | 1 | Lond | lake st | 9809 | #long | 2015-10-24 10:24:20
To find the current address of any user, we search for UserData with revision DESC and LIMIT 1
To get the address of a user between a certain period of time
we can use created_on bewteen (date1 , date 2)
MariaDB supports System Versioning since 10.3 which is the standard SQL feature that does exactly what you want: it stores history of table records and provides access to it via SELECT queries. MariaDB is an open-development fork of MySQL. You can find more on its System Versioning via this link:
https://mariadb.com/kb/en/library/system-versioned-tables/
Why not simply use bin log files? If the replication is set on the Mysql server, and binlog file format is set to ROW, then all the changes could be captured.
A good python library called noplay can be used. More info here.
Just my 2 cents. I would create a solution which records exactly what changed, very similar to transient's solution.
My ChangesTable would simple be:
DateTime | WhoChanged | TableName | Action | ID |FieldName | OldValue
1) When an entire row is changed in the main table, lots of entries will go into this table, BUT that is very unlikely, so not a big problem (people are usually only changing one thing)
2) OldVaue (and NewValue if you want) have to be some sort of epic "anytype" since it could be any data, there might be a way to do this with RAW types or just using JSON strings to convert in and out.
Minimum data usage, stores everything you need and can be used for all tables at once. I'm researching this myself right now, but this might end up being the way I go.
For Create and Delete, just the row ID, no fields needed. On delete a flag on the main table (active?) would be good.
The direct way of doing this is to create triggers on tables. Set some conditions or mapping methods. When update or delete occurs, it will insert into 'change' table automatically.
But the biggest part is what if we got lots columns and lots of table. We have to type every column's name of every table. Obviously, It's waste of time.
To handle this more gorgeously, we can create some procedures or functions to retrieve name of columns.
We can also use 3rd-part tool simply to do this. Here, I write a java program
Mysql Tracker
In MariaDB 10.5+ this is as easy to setup as
CREATE TABLE t (x INT) WITH SYSTEM VERSIONING
PARTITION BY SYSTEM_TIME;
Past history can then be queried by doing
SELECT * FROM t FOR SYSTEM_TIME AS OF TIMESTAMP '2016-10-09 08:07:06';
There is currently no counterpart for this in MySQL.
See the documentation for more info. If you're on an older version of MariaDB, the documentation has an alternate syntax that has been available since MariaDB 10.3.4.