I'm wondering if I can do this without a trigger:
I want to have an INT column which is auto-updated to increment the integer value.
This is similar but not the same as ON UPDATE CURRENT_TIMESTAMP on a TIMESTAMP or DATETIME column.
I read in the docs it's not possible with ON UPDATE because I can only use literal values unless the column is a TIMESTAMP.
MySQL default and on update
MySQL timestamp initialization
Can I do something else instead of a trigger?
[UPDATE 1]
This example is what I'm trying to achieve:
CREATE TABLE myTable (
stuff VARCHAR (100),
special_col INT DEFAULT 123456789
);
INSERT INTO myTable (stuff) VALUES ('something or other');
SELECT * FROM myTable;
stuff | special_col
----------------------------------------------------
something or other | 123456789
UPDATE myTable SET stuff = 'new info';
SELECT * FROM myTable;
stuff | special_col
----------------------------------------------------
new info | 123456790
The value that the column increments by doesn't have to be 1. It just has to be higher than before.
[UPDATE 2]
I thought there might be some cool mySQL syntax I could use - it does so much non-ANSI SQL, I figured there might be a custom way of doing it.
Anyway, I have given up and I'm implementing a trigger:
CREATE TRIGGER trg_data_update BEFORE UPDATE ON myTable
FOR EACH ROW SET NEW.special_col = OLD.special_col + 1;
Add one to the special_col in the UPDATE as per docs
UPDATE myTable SET stuff = 'new info', special_col=special_col+1;
Related
Trying to
create trigger that is called on INSERT & sets originId = id (AUTO_INCREMENT),
I've used SQL suggested here in 1st block:
CREATE TRIGGER insert_example
BEFORE INSERT ON notes
FOR EACH ROW
SET NEW.originId = (
SELECT AUTO_INCREMENT
FROM information_schema.TABLES
WHERE TABLE_SCHEMA = DATABASE()
AND TABLE_NAME = 'notes'
);
Due to information_schema caching I have also set
information_schema_stats_expiry = 0
in my.cnf file. Now information gets updated almost instantly on every INSERT, as I've noticed..
But, performing "direct" INSERTs via console with ~2min intervals, I keep getting not updated AUTO_INCREMENT values in originId.
(They shoud be equal to id fields)
While explicit queries, fetching AUTO_) result in updated correct values.
Thus I suspect that the result of SELECT AUTO_INCREMENT... subquery gets somehow.. what? cached?
How can one get around this?
Thank you.
Edit 1
I intended to implement sort of VCS this way:
User creates new Note, app marks it as 'new' and performs an INSERT in MySQL table. It is the "origin" note.
Then user might edit this Note (completely) in UI, app will mark is as 'update' and INSERT it in MySQL table as a new row, again. But this time originId should be filled with an id of "origin" Note (by app logics). And so on.
This allows PARTITIONing by originId on SELECT, fetching only latest versions to UI.
initial Problem:
If originId of "origin" Note is NULL, MySQL 8 window function(s) in default (and only?) RESPECT_NULL mode perform(s) framing not as expected ("well, duh, it's all about your NULLs in grouping-by column").
supposed Solution:
Set originId of "origin" Notes to id on their initial and only INSERT, expecting 2 benefits:
Easily fetch "origin" Notes via originId = id,
perform correct PARTITION by originId.
resulting Problem:
id is AUTO_INCREMENT, so there's no way (known to me) of getting its new value (for the new row) on INSERT via backend (namely, PHP).
supposed Solution:
So, I was hoping to find some MySQL mechanism to solve this (avoiding manipulations with id field) and TRIGGERs seemed a right way...
Edit 2
I believed automated duplicating id AUTO_INCREMENT field (or any field) within MySQL to be extra fast & super easy, but it totally doesn't appear so now..
So, possibly, better way is to have vcsGroupId UNSIGNED INT field, responsible for "relating" Note's versions:
On create and "origin" INSERT - fill it with MAX(vcsGroupId) + 1,
On edit and "version" INSERT - fill it with "sibling"/"origin" vcsGroupId value (fetched with CTE),
On view and "normal" SELECT - perform framing with Window Function by PARTITION BY vcsGroupId, ORDER BY id or timestamp DESC, then just using 1st (or ascending order by & using last) row,
On view and "origin" SELECT - almost the same, but reversed..
It seems easier, doesn't it?
What you are doing is playing with fire. I don't know exactly what can go wrong with your trigger (beside that it doesn't work for you already), but I have a strong feeling that many things can and will go wrong. For example: What if you insert multiple rows in a single statement? I don't think, that the engine will update the information_schema for each row. And it's going to be even worse if you run an INSERT ... SELECT statement. So using the information_schema for this task is a very bad idea.
However - The first question is: Why do you need it at all? If you need to save the "origin ID", then you probably plan to update the id column. That is already a bad idea. And assuming you will find a way to solve your problem - What guarantees you, that the originId will not be changed outside the trigger?
However - the alternative is to keep the originId column blank on insert, and update it in an UPDATE trigger instead.
Assuming this is your table:
create table vcs_test(
id int auto_increment,
origin_id int null default null,
primary key (id)
);
Use the UPDATE trigger to save the origin ID, when it is changed for the first time:
delimiter //
create trigger vcs_test_before_update before update on vcs_test for each row begin
if new.id <> old.id then
set new.origin_id = coalesce(old.origin_id, old.id);
end if;
end;
delimiter //
Your SELECT query would then be something like this:
select *, coalesce(origin_id, id) as origin_id from vcs_test;
See demo on db-fiddle
You can even save the full id history with the following schema:
create table vcs_test(
id int auto_increment,
id_history text null default null,
primary key (id)
);
delimiter //
create trigger vcs_test_before_update before update on vcs_test for each row begin
if new.id <> old.id then
set new.id_history = concat_ws(',', old.id_history, old.id);
end if;
end;
delimiter //
The following test
insert into vcs_test (id) values (null), (null), (null);
update vcs_test set id = 5 where id = 2;
update vcs_test set id = 4 where id = 5;
select *, concat_ws(',', id_history, id) as full_id_history
from vcs_test;
will return
| id | id_history | full_id_history |
| --- | ---------- | --------------- |
| 1 | | 1 |
| 3 | | 3 |
| 4 | 2,5 | 2,5,4 |
View on DB Fiddle
I have a table in MYSQL database with two fields:
Id (auto increment field).
Post_Id.
When I insert a new record both fields should have the same value. So I should update post_id with Id value, and at the same time make sure that I update the field with the right value not with any other new inserted record value.
I tried this SQL statement but it was very slow and I was not sure that I select the right value:
set #auto_id := (SELECT AUTO_INCREMENT
FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_NAME='table_name'
AND TABLE_SCHEMA=DATABASE() );
update table_name set post_id= #auto_id where id=#auto_id ;
I don't have long experience with MySQL and I cannot change the table structure .
The approach you followed is not transaction safe as well.
The best option I can think about is to use trigger
Edit: According to #lagripe's mentionings
CREATE TRIGGER sometrigger
AFTER INSERT ON sometable
BEGIN
SET NEW.post_id := (SELECT id from sometable order by DESC limit 1) + 1 ; // you may need +1 here. I couldn't test it.
END
or you may consider to use LAST_INSERT_ID
insert into table_name values ( .... );
update table_name set post_id = LAST_INSERT_ID();
but why do you need two columns with the same id at first place?
if you really need why don't you use computed/generated columns?
CREATE TABLE Table1(
id DOUBLE,
post_id DOUBLE as (id)
);
you can use triggers :
CREATE TRIGGER UpdatePOST_id
BEFORE INSERT ON table_db
FOR EACH ROW
SET NEW.post_id := (select id from table_db order by id DESC LIMIT 1)+1 ;
from now on, whatever you insert your as a value in post_id column will be replaced with the id inserted automatically.
Test :
|id|post_id|
|20| 20 |
|21| 21 |
|22| 22 |
|23| 23 |
To drop the trigger :
DROP trigger UpdatePOST_id
I have an auto increment column ID, and for some situation I wanted the other column to be equal to the primary key + 1 value
ID | other
1 | 2
2 | 3
3 | 4
4 | 123 (some situation, it is not always plus 1)
How can I achieve this?
Here's what I have tried
INSERT INTO table (`ID`,`other`) VALUES ('',(SELECT MAX(ID)+1 FROM table))
But that returns an error
You can't specify target table 'table' for update in FROM clause
Try Below query:
ALTER TABLE dbo.table ADD
Column AS ([ID]+1)
GO
It will definitely work
Using a normal AUTO_INCREMENT column as id, I cannot think of a way to do this in MySQL. Triggers, which otherwise would have been an option, don't work well with AUTO_INCREMENT columns.
The only way I see is to do two commands for an INSERT;
INSERT INTO bop (value) VALUES ('These values should be 1 and 2');
UPDATE bop SET other = id+1 WHERE id = LAST_INSERT_ID();
An SQLfiddle to test with.
The closest I'm getting to what you're looking for is to generate sequences separately from AUTO_INCREMENT using a function, and use that instead to generate the table id;
DELIMITER //
CREATE TABLE bop (
id INT UNIQUE,
other INT,
value VARCHAR(64)
)//
CREATE TABLE bop_seq ( seq INT ) // -- Sequence table
INSERT INTO bop_seq VALUES (1) // -- Start value
CREATE FUNCTION bop_nextval() RETURNS int
BEGIN
SET #tmp = (SELECT seq FROM bop_seq FOR UPDATE);
UPDATE bop_seq SET seq = seq + 1;
RETURN #tmp;
END//
CREATE TRIGGER bop_auto BEFORE INSERT ON bop
FOR EACH ROW
SET NEW.id = bop_nextval(), NEW.other=NEW.id + 1;
//
That'd let you do inserts and have it autonumber like you want. The FOR UPDATE should keep the sequence transaction safe, but I've not load tested so you may want to do that.
Another SQLfiddle.
I solved this by updating 2 times the DB..
I wanted to do +1 from 19 till ..
UPDATE `table` SET `id`=`id`+101 WHERE id <= 19
UPDATE `table` SET `id`=`id`-100 WHERE id <= 119 AND id >= 101
I want to create a table with two column, one "created" and one "modified", with a timestamp type. I don't want them to have a default value. But when I create them, it automatically puts CURRENT_TIMESTAMP as default for at least one of them, and also on update.
How can I just have type timestamp with no default or on update value for any of my column?
Edit: I use MySQL
If you are using SQL Server, the timestamp type is used for row versioning, not for storing an actual date or time value. See MSDN.
You could create the columns with a datetime datatype and then set your created and modified values with triggers.
CREATE TRIGGER trg_SetDateCreated ON MyTable
FOR INSERT AS
UPDATE MyTable
SET created = CURRENT_TIMESTAMP
WHERE MyTable.id = (SELECT Id FROM Inserted);
GO
CREATE TRIGGER trg_SetDateModified ON MyTable
FOR UPDATE AS
UPDATE MyTable
SET modified = CURRENT_TIMESTAMP
WHERE MyTable.id = (SELECT Id FROM Inserted);
GO
This is my first time using mysql and I am tying to learn how to use trigger.
I have two tables: lpr_upload and lpr_traveltime.
my lpr_upload table look like this
+----------+---------+--------+-----------+
| date_time | site_id | lane_id| lpr_text |
+----------+---------+--------+-----------+
Whenever I add a new row into lpr_upload, i want to check if lpr_tex of the new row already exist or not in the table. If it already exist, i want to add a new row into lpr_traveltime with lpr_text and new datetime - old datetime. This is what i have so far:
insert into lpr_opt_travel_times(travel_time, lpr_text)
value (select TIMEDIFF (new.date_time, ...) , lpr_text) on duplicate key update ...
I dont know how to finish this. also, how would I get the date_time from the row that already exist in the table?
Try this:
declare ldt_date_time datetime;
select date_time into ldt_date_time from tbl where tbl.lpr_text = NEW.lpr_text and date_time <> NEW.date_time;
if ldt_date_time is not null then
insert into lpr_opt_travel_times(travel_time, lpr_text) values (ldt_date_time, NEW.lpr_text);
end if;
Declare a var with date time datatype as I assume the column date_time is datetime type.
Select from NEW magic table the value of date_time for the inserted lpr_text but whose date_time is different from the inserted one.
If the var is null meaning no assignment took place and the var retains the initial default value of NULL then insert the row in your other table using the var and the inserted lpr_text