I am using mySQL
Is there any way to set limit of table record/row? I have table X and want to set limit of total records/rows on table, for example 2rows. So no one can insert third record in table. This table should not allow to insert third record.
I do not want to use Triggers.
You can do it this the user grants, so the user cant write into this table and you can create a separate User for administration this table.
seee Manual : https://mariadb.com/kb/en/mariadb/grant/
If you dont want to use triggers, you 'll have to check the number of rows inside your application (if any).
Create an AFTER INSERT trigger on the table. - it's the only way to do it.
create trigger TableLimit
on TableName
after insert
as
declare #countTableRows int
select #countTableRows = Count(*)
from TableName
if #countTableRows > 2
begin
rollback
end
go
Related
I'm running a wateranalyzer and want to save the sensor-data in a MariaDB.
I split the data into 2 tables: one for the automated part and one table which stores data I enter manually:
Tables:
I'm having a hard time (with just basic knowledge about databases) to figure out how I can "bind" ID and DateTime from one table to the other one, so if manual data is added, ID is incremented by 1 and the actual Date and Time is set in DateTime.
I bet I can do this somehow in PHPmyadmin?
thanks for your time!
using triger. this example for you.
DELIMITER //
CREATE TRIGGER contacts_after_insert
AFTER INSERT
ON contacts FOR EACH ROW
BEGIN
DECLARE vUser varchar(50);
-- Find username of person performing the INSERT into table
SELECT USER() INTO vUser;
-- Insert record into audit table
INSERT INTO contacts_audit
( contact_id,
deleted_date,
deleted_by)
VALUES
( NEW.contact_id,
SYSDATE(),
vUser );
END; //
DELIMITER ;
Is it anything more complex than having an ID in Wasser that matches the other ID? That is, first insert into Luft, then get the id and only then, INSERT INTO Wasser....
(A Trigger seems unnecessarily complicated.)
As Rick Suggested, you need to have an ID column in the second table that references ID in first table. Trigger is a better option if the process of getting the ID and inserting it along with other columns (pH, Redox...) into the second table is complicated.
Make ID in the second table as a foreign key to the ID in first table.
I have a mysql Innodb table 'classrooms_subjects' as
id|classroom_id|subject_id
classroom_id & subject_id are composite keys. Whenever i insert a row with classroom_id & subject_id, my id field is inserted as 0.
Now i want to create a trigger which will enter id field as last_inserted_id()+1.
Also I need to take care of multiple records inserted at a time. My trigger is like below:
CREATE TRIGGER `increment_id` AFTER INSERT ON `classrooms_subjects`
FOR EACH ROW BEGIN
UPDATE classrooms_subjects
SET classrooms_subjects.id = LAST_INSERT_ID() + 1 WHERE id=0;
END
when i am inserting a record I am getting the error as:
"Cant update table in trigger because it is already used by statement which invoked this trigger
For general info: using an update statement inside the trigger isn't right.
Better to use a before insert trigger and simply assign the value of your column using NEW.id
http://dev.mysql.com/doc/refman/5.7/en/trigger-syntax.html
A column named with OLD is read only. You can refer to it (if you have
the SELECT privilege), but not modify it. You can refer to a column
named with NEW if you have the SELECT privilege for it. In a BEFORE
trigger, you can also change its value with SET NEW.col_name = value
if you have the UPDATE privilege for it. This means you can use a
trigger to modify the values to be inserted into a new row or used to
update a row. (Such a SET statement has no effect in an AFTER
trigger because the row change will have already occurred.)
You should probably structure your table to make the auto_increment work properly. Better a solution that works when multiple sessions are inserting to the DB at once.
I have created a MySQL Trigger BEFORE INSERT on table name agent_mst as below
BEGIN
DECLARE max_id INT;
SET max_id=(SELECT MAX(agent_id_pk)+1 FROM `agent_mst`);
IF (max_id IS NULL) THEN
SET max_id=1;
END IF;
SET NEW.date_added=NOW(),
NEW.date_updated=NOW(),
NEW.agent_code = CONCAT('SDA', LPAD(max_id, 4,'0'));
END
So what it does is, every time we inset a record, it generates agent_code field value to something like SDA0001, SDA0002, SDA0003, ...
Now suppose I delete a record with code SDA0003 and insert new record, it will definitely generate the agent code as SDA0004. As it is taking the max_id and increasing it with 1. But here I want to get SDA0003 again. So that all agent_codes can stay in sequence. How to do that?
Thanks in advance.
you need to identify the first (smallest) missing id.
check out in this link, a nice way to do it in a select query:
Find mininum not used value in mysql table
To know next auto increment id try to run below query and check column "Auto_increment":
SHOW TABLE STATUS FROM DBName where name = 'tableName'
I am wondering if it is possible to perform a SQL query then update another table with the generated ID and continue through all of the rows?
I have this SQL query that works but what I need to do is after each row is added to cards to then update merged.cars_id with the last generated ID so they are linked. normally I would do this with PHP but ideally I would like to just do it with MySQL if possible.
MAIN QUERY
INSERT INTO cards (first_contact_date, card_type, property_id, user_id)
SELECT first_contact_date, 'P', property_id, user_id FROM merged
THEN I NEED WITH MATCHING ROWS (Roughly)
UPDATE merged SET merged.card_id = LAST_INSERT_ID (FROM ABOVE) into the matching record..
Is something like this possible and how do I do it?
I would recommend using MySQL triggers to do this
http://dev.mysql.com/doc/refman/5.0/en/create-trigger.html
A trigger is a function that will be executed AFTER or BEFORE the INSERT or DELETE or UPDATE is done over any record of your table.
In your case you need to do a AFTER INSERT on cards that just updates the merged table. Make sure its AFTER insert as you wont be able to access the new row's ID otherwise.
The code would look something like this, assuming the id field from the cards table its named "id"
delimiter |
CREATE TRIGGER updating_merged AFTER INSERT ON cards
FOR EACH ROW BEGIN
UPDATE merged SET card_id = NEW.id;
END;
|
delimiter ;
May I suggest Stored Procedures?
http://dev.mysql.com/doc/refman/5.0/en/create-procedure.html
--EDIT--
Ah yes, triggers. For this particular situation, Jimmy has the answer. I will leave this post for the sake of the link.
I would set up a trigger to do this. For mysql, read http://dev.mysql.com/doc/refman/5.0/en/triggers.html. This is what triggers are designed to handle.
Given the following table:
id | value
--------------
1 6
2 70
Is there a way to add a column that is automatically calculated based on another column in the same table? Like a VIEW, but part of the same table. As an example, calculated would be half of value. Calculated should be automatically updated when value changes, just like a VIEW would be.
The result would be:
id | value | calculated
-----------------------
1 6 3
2 70 35
Generated Column is one of the good approach for MySql version which is 5.7.6 and above.
There are two kinds of Generated Columns:
Virtual (default) - column will be calculated on the fly when a
record is read from a table
Stored - column will be calculated when a
new record is written/updated in the table
Both types can have NOT NULL restrictions, but only a stored Generated Column can be a part of an index.
For current case, we are going to use stored generated column. To implement I have considered that both of the values required for calculation are present in table
CREATE TABLE order_details (price DOUBLE, quantity INT, amount DOUBLE AS (price * quantity));
INSERT INTO order_details (price, quantity) VALUES(100,1),(300,4),(60,8);
amount will automatically pop up in table and you can access it directly, also please note that whenever you will update any of the columns, amount will also get updated.
If it is a selection, you can do it as:
SELECT id, value, (value/2) AS calculated FROM mytable
Else, you can also first alter the table to add the missing column and then do an UPDATE query to compute the values for the new column as:
UPDATE mytable SET calculated = value/2;
If it must be automatic, and your MySQL version allows it, you can try with triggers
MySQL 5.7 supports computed columns. They call it "Generated Columns" and the syntax is a little weird, but it supports the same options I see in other databases.
https://dev.mysql.com/doc/refman/5.7/en/create-table.html#create-table-generated-columns
#krtek's answer is in the right direction, but has a couple of issues.
The bad news is that using UPDATE in a trigger on the same table won't work. The good news is that it's not necessary; there is a NEW object that you can operate on before the table is even touched.
The trigger becomes:
CREATE TRIGGER halfcolumn_update BEFORE UPDATE ON my_table
FOR EACH ROW BEGIN
SET NEW.calculated = NEW.value/2;
END;
Note also that the BEGIN...END; syntax has to be parsed with a different delimiter in effect. The whole shebang becomes:
DELIMITER |
CREATE TRIGGER halfcolumn_insert BEFORE INSERT ON my_table
FOR EACH ROW BEGIN
SET NEW.calculated = NEW.value/2;
END;
|
CREATE TRIGGER halfcolumn_update BEFORE UPDATE ON my_table
FOR EACH ROW BEGIN
SET NEW.calculated = NEW.value/2;
END;
|
DELIMITER ;
You can use generated columns from MYSQL 5.7.
Example Usage:
ALTER TABLE tbl_test
ADD COLUMN calc_val INT
GENERATED ALWAYS AS (((`column1` - 1) * 16) + `column2`) STORED;
VIRTUAL / STORED
Virtual: calculated on the fly when a record is read from a table (default)
Stored: calculated when a new record is inserted/updated within the
table
If you want to add a column to your table which is automatically updated to half of some other column, you can do that with a trigger.
But I think the already proposed answer are a better way to do this.
Dry coded trigger :
CREATE TRIGGER halfcolumn_insert AFTER INSERT ON table
FOR EACH ROW BEGIN
UPDATE table SET calculated = value / 2 WHERE id = NEW.id;
END;
CREATE TRIGGER halfcolumn_update AFTER UPDATE ON table
FOR EACH ROW BEGIN
UPDATE table SET calculated = value / 2 WHERE id = NEW.id;
END;
I don't think you can make only one trigger, since the event we must respond to are different.
I hope this still helps someone as many people might get to this article. If you need a computed column, why not just expose your desired columns in a view ? Don't just save data or overload the performance with triggers... simply expose the data you need already formatted/calculated in a view.
Hope this helps...