update table column after insert new record using MySQL triggers

Question

Imagine I have a MySQL table (tbl_test) with these fields: id, title, priority.
id will be incremented automatically. I need to fill priority field with a value as same as id field after inserting.
As I'm new in using MySQL triggers, please tell me what I have to write for it. I did something , but I think it is not true:

CREATE TRIGGER 'test' AFTER INSERT ON `tbl_test`
BEGIN
   SET new.priority = new.id;
END

Thanks for your assistance.

Solution 1

The way you are trying to set value to a column is an update. Because you are doing it after insert operation is completed.

You actually need a before trigger.

And to assign the same new auto incremented value of primary key column of same table, you better get it from information_schema.tables.

Example:

delimiter //
drop trigger if exists bi_table_name //

create trigger bi_table_name before insert on table_name
for each row begin
  set @auto_id := ( SELECT AUTO_INCREMENT 
                    FROM INFORMATION_SCHEMA.TABLES
                    WHERE TABLE_NAME='table_name'
                      AND TABLE_SCHEMA=DATABASE() ); 
  set new.priority= @auto_id;
end;
//

delimiter ;

Note: Make sure that you don't have any pre-defined trigger with the same name and/or action. If have some, then drop them before creating the new.

Observations:
As per mysql documentation on last_insert_id(),

"if you insert multiple rows using a single INSERT statement, LAST_INSERT_ID() returns the value generated for the first inserted row only."

hence, depending on last_insert_id() and auto_increment field values in batch inserts seems not reliable.

Solution 2

I don't think you can do that. An AFTER INSERT trigger cannot modify the same table, neither by issuing an UPDATE nor by something like this:

DROP TRIGGER new_tbl_test;

DELIMITER $$

CREATE TRIGGER new_tbl_test 
AFTER INSERT ON tbl_test for each row
begin
UPDATE tbl_test SET priority = new.id WHERE id = new.id;
END $$

DELIMITER ;

It gives error like

ERROR 1442 (HY000): Can't update table 'tbl_test' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.

---

What you can do, is use a transaction:

Example : Table structure is like below

mysql> show create table tbl_test\G
*************************** 1. row ***************************
       Table: tbl_test
Create Table: CREATE TABLE `tbl_test` (
  `ID` int(11) NOT NULL AUTO_INCREMENT,
  `title` char(30) DEFAULT NULL,
  `priority` int(11) DEFAULT NULL,
  PRIMARY KEY (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

Transaction

START TRANSACTION ;
  INSERT INTO tbl_test (title)
    VALUES ('Dr');
  UPDATE tbl_test
    SET `priority` = id
    WHERE id = LAST_INSERT_ID();
COMMIT ;

Check data

mysql> SELECT * FROM tbl_test;
+----+-------+----------+
| ID | title | priority |
+----+-------+----------+
|  1 | Dr    |        1 |
+----+-------+----------+
1 row in set (0.00 sec)

Comments

[Ravinder Reddy]

Must be a ; missing after end of first set ... statement. Add it.

[Mohammad Saberi]

I would like to to it using Triggers

[Ravinder Reddy]

It says syntax error is near (or before) for each ... Can you please add your complete modified trigger definition to your post?

[Ravinder Reddy]

And, I doubt that you forgot to add create trigger ..., first line from my example, which is a must to define a trigger.

[Mohammad Saberi]

I'm working with Navicat for MySQL. So it will add create trigger ... itself. I have to write other codes only

[Khosro]

Does this triggers is thread-safe?I mean if two concurrent row inserted into table,it does not cause the same auto-increment value for two inserted record or inconsistent value for priority column?

[DJDave]

This works (nice job) but I needed "delimiter $$" before "create trigger" to get around the syntax errors (and "end;" becomes "end$$")

[Ravinder Reddy]

@user1280840: Answer I posted has no errors. And the delimiter need not be '$$' but can be anything like in my answer '//'. I used delimiter // before create trigger and closed it as end; //. Hence no meaning in saying 'but I needed ....$$....' ...

[Ellert van Koperen]

To avoid problems i usualy put the delimiter after the drop command. For the rest, this post put me on the right track, thank you Ravinder!

[Ravinder Reddy]

@cmcdragonkai: you mean batch inserts? what were your observations??

[CMCDragonkai]

The id is not updated for each record. It only works for the first record.

[Ravinder Reddy]

I am not sure how you verified it. but look into different example on each row here stackoverflow.com/a/23649301/767881

[marksiemers]

Based on my testing, @CMCDragonkai is correct, this does not work with batch processing. To test, add a unique constraint to the priority column, and do an insert with multiple sets of values. In my testing, the unique constraint stops the insert 100% of the time.

[Richard A Quadling]

Mysql will only update the AUTO_INCREMENT value after the bulk insert is done. The same if you attempt to use DEFAULT CURRENT_TIMESTAMP(6). If the column receiving the value has a unique constraint, you cannot use the current table. The best solution I've seen so far is to replicate the ROWNUMBER functionality from MSSQL. A separate table that is only an autoincrement column. You insert, get LAST_INSERT_ID(), delete row, return value. But even with this, I'm not 100% sure LAST_INSERT_ID will return the row I just inserted when we are parallel bulk inserting.

[Ravinder Reddy]

@RichardAQuadling: As per mysql documentation on last_insert_id(), "if you insert multiple rows using a single INSERT statement, LAST_INSERT_ID() returns the value generated for the first inserted row only."

[Ravinder Reddy]

hence, depending on last_insert_id() and auto_increment field values in batch inserts seems not reliable.