I was wondering if there is a way to Auto Increment two columns in the same table.
I have database with user_id so every registred user would have his unique nr.
But I want to add a file_id so every file the user upload gets a unique number.
Any tips ?
Thanks
To ensure your database is normalized correctly, you should store file uploads in a separate table.
This means one user has the potential to have multiple files and allows the table to correctly increment the identity field of the file records.
You can read about database normalization on Wikipedia:
http://en.wikipedia.org/wiki/Database_normalization
No, only one identity column per table is allowed.
if you are using SQl Server follow this:
If your second field is static (you dont have to edit and it follows a sinmple logic to generate its value)
what you can do is create the second field as a calculated one, for example:
create table autoINC(
field1 int identity(1,1),
field2 as field1+1,
real_field varchar(50))
insert into autoINC values('test')
select * from autoINC
1 2 test
if you need to update your second field, then you need to use a trigger
I think you could use a trigger or similar solution depending on the database.
In MSSQL a trigger on update should work.
But this will not ensure unique file names unless the filename also includes the userid.
Is there a reason you need fileid to be sequential for each user?
If not, add a userid field to the file table and have a separate fileid identity column in that.
Related
I have two MySQL databases. I would like to data from one database to another. Both have the same structure and entries except that one database has same IDs for different items within the same tables. I don't want to replace the data from the old to the new database. If IDs are there, I would like the new database to skip it. If it's a duplication, I would like a new ID to be generated.
I'd like to use phpmyadmin for this but have no idea if this is even possible.
0.) Make backup of both tables
PHPMYADMIN will be sufficient for your request.
First you need to ensure there is no duplicating id's or primary keys.
Assuming two tables testtable1 and testtable2 have columns testtable_id, name
1.) firstly you would make query on second table
UPDATE testtable2 SET testtable2.testtable_id = testtable2.testtable_id + (SELECT MAX( testtable1.testtable_id ) FROM testtable1);
2.) Than, again in testtable2, there is tool Copy table to (database.table): under Operations menu, set DB name and testtable1 name (db name should be already set), select Data only radio button option and click Go. 3.) Now, you have all data from both tables in testtable1.
Edit. Firstly I thought it is matter of two tables in same database. But nevertheless you can use step two for rest of the tables too. Just set correct DB and table name in step two. Also, before that, set query so expecting ID to be higher than MAX ID of table you want to extend. You can hard code parenthesis part with exact number of MAX ID first DB corresponding table.
I am wondering how I could, using PHP and mysql, create a table with a unique name every time.
So example if i click submit, a table will be created that is named "1".
then if i do it again another table is added and it is named "2"
I searched around but could only find answers to how to auto_increment the columns inside the table so I hoped it would be the same code, I tried this:
mysql_query("CREATE TABLE INT NOT NULL AUTO_INCREMENT PRIMARY KEY(TestColumn CHAR(30))");
It did not work.
So how do you create an auto_incremented table ???
Create a simple file which store a serial number. Then when your script creates a table, you increment the counter in the file with one. Next time, you read the number and use that for the table name. Naturally, you could do this in a table or a flat file.
Just for knowing which tables exist, and what they are for, you'd best create one master table storing not just the latest, but all tables created.
I am lost as to why you would want to do this.. I see no good reason for wanting this.
Create a table of tables and store the number or the number name in that table. Then you can look up MAX number there.
First at all, this function don't exist in PHP or in MySQL. Or maybe I don't know it.
There is 2 solutions to your problem :
Solution number 1 :
As AlanChavez said, you can use this request :
CREATE TABLE IF NOT EXIST ....
But, if you have to create 1000000000 table (it's an example), it will not be optimized.
Solution number 2 :
You can create a table with a single row, where is stored the last name used for your table.
I don't know if it's really optimized, but I think it can work.
I will never recommend to name a database table just with a digit. To keep track of number of click / page-load you can use file, session or another table.
My db table looks like this pic. http://prntscr.com/22z1n
Recently I've created delete.php page. it works properly but when i deleted 21th user next registered user gets 24th id instead of 21.
Is it possible to put newly registered users info to first empty row? (In this situation 21th row)
In my registration form, newly registering user can write names of existing users, and be friends with them after registration. For this friendship i have another table that associates id of newly registered user and existing user.
For this purpose i'm using mysql_insert_id during registration to get id for new user. But after deletion of 21th row during nex registration process mysql_insert_id gave me number 21. but stored in 24th row. And put to associations table 21 for new user. I wanna solve this problem
When you use an autoincrement id column, the value that the next entry will be assigned will not be reduced by deleting an entry. That is not what an autoincrement column is used for. The database engine will always increment that number on a new insert and never decrement that number on a delete.
A MySQL auto_increment column maintains a number internally, and will always increment it, even after deletions. If you need to fill in an empty space, you have to handle it yourself in PHP, rather than use the auto_increment keyword in the table definition.
Rolling back to fill in empty row ids can cause all sorts of difficulty if you have foreign key relationships to maintain, and it really isn't advised.
The auto_increment can be reset using a SQL statement, but this is not advised because it will cause duplicate key errors.
-- Doing this will cause problems!
ALTER table AUTO_INCREMENT=12345;
EDIT
To enforce your foreign key relationships as described in the comments, you should add to your table definition:
FOREIGN KEY (friendid) REFERENCES registration_table (id) ON DELETE SET NULL;
Fill in the correct table and column names. Now, when a user is deleted from the registration, their friend association is nulled. If you need to reassociate with a different user, that has to be handled with PHP. mysql_insert_id() is no longer helpful.
If you need to find the highest numbered id still in the database after deletion to associate with friends, use the following.
SELECT MAX(id) FROM registration_table;
Auto increment is a sequence key that's tracked as part of the table. It does not go back when you delete a row.
Easily, no. What you can do (but I don't suggest doing) is making an SQL function to determine the lowest number that isn't currently occupied. Or you can create a table of IDs that were deleted, and get the smallest number from there. Or, and this is the best idea, ignore the gaps and realize the database is fine.
What you want to do is achievable by adding an extra column to your table called something like user_order. You can then write code to manage inserts and deletions so that this column is always sequential with no gaps.
This way you avoid the problems you could have messing around with an auto_increment column.
It's not a good practice to reset auto_increment value, but if you really need to do it, so you can:
ALTER TABLE mytable AUTO_INCREMENT = 1;
Run this query after every delete. Auto_increment value will not be set to 1, this will set the lowest possible value automatically.
what is the best way to uniquely store and image name in a DB, assuming two persons have the same image name, or the same user uploads the same image twice. How can i manage scenerios like this so that when i want perform operations like updating, deleting, i don't end up deleting both images or all. My DB is MySQL.
Edit - Ok, now for some reason i generated a unique time stamp for all the image everything work on localhost, but when i take it online it doesn't work, i cannot delete it, but it works offline well.
Thank you
#cybeormin
In any table I would recomend an ID column that that is Auto Increment and set to the Primary Field. That way all rows are unique despite a user have two images of the same name.
Put a refcount on the images. When you want to delete the image just decrement the refcount instead. When the refcount hits 0, you delete the row for real (or have a cron job delete it).
I would discard the original image name, give every image an unique id and store the original name in the db.
In your mySQL DB, create a table for image uploads, create an ID column set to AUTO_INCREMENT, INT (integer) and make it the PKEY (primary key).
This will mean your ID's are always automatically created and generated when new content is added, and are always unique regardless of whether the same image is added any number of times.
Additional fields can then trap the other image information (filename, 'given' name, filesize, filetype and a BLOB field for the image itself), you can then also even add a psuedo ID which you make up based on any combination of factors..should you see fit.
Associate an unique key (primary key) with each entry. Something like:
fileId | file_name | file
and set the fileId to be auto incrementing and a primary key. You can then on your "user" table (if you have something like that) simply reference the fileId as a foreign key.
userId | ... | fileId | ...
Then, if you need to delete the file, simply use the fileId to find the one you which to delete.
How can we re-use the deleted id from any MySQL-DB table?
If I want to rollback the deleted ID , can we do it anyhow?
It may be possible by finding the lowest unused ID and forcing it, but it's terribly bad practice, mainly because of referential integrity: It could be, for example, that relationships from other tables point to a deleted record, which would not be recognizable as "deleted" any more if IDs were reused.
Bottom line: Don't do it. It's a really bad idea.
Related reading: Using auto_increment in the mySQL manual
Re your update: Even if you have a legitimate reason to do this, I don't think there is an automatic way to re-use values in an auto_increment field. If at all, you would have to find the lowest unused value (maybe using a stored procedure or an external script) and force that as the ID (if that's even possible.).
You shouldn't do it.
Don't think of it as a number at all.
It is not a number. It's unique identifier. Think of this word - unique. No record should be identified with the same id.
1.
As per your explanation provided "#Pekka, I am tracking the INsert Update and delete query..." I assume you just some how want to put your old data back to the same ID.
In that case you may consider using a delete-flag column in your table.
If the delete-flag is set for some row, you shall consider program to consider it deleted. Further you may make it available by setting the delete-flat(false).
Similar way is to move whole row to some temporary table and you can bring it back when required with the same data and ID.
Prev. idea is better though.
2.
If this is not what you meant by your explanation; and you want to delete and still use all the values of ID(auto-generated); i have a few ideas you may implement:
- Create a table (IDSTORE) for storing Deleted IDs.
- Create a trigger activated on row delete which will note the ID and store it to the table.
- While inserting take minimum ID from IDSTORE and insert it with that value. If IDSTORE is empty you can pass NULL ID to generate Auto Incremented number.
Of course if you have references / relations (FK) implemented, you manually have to look after it, as your requirement is so.
Further Read:
http://www.databasejournal.com/features/mysql/article.php/10897_2201621_3/Deleting-Duplicate-Rows-in-a-MySQL-Database.htm
Here is the my case for mysql DB:
I had menu table and the menu id was being used in content table as a foreign key. But there was no direct relation between tables (bad table design, i know but the project was done by other developer and later my client approached me to handle it). So, one day my client realised that some of the contents are not showing up. I looked at the problem and found that one of the menu is deleted from menu table, but luckily the menu id exist in cotent table. I found the menu id from content table that was deleted and run the normal insert query for menu table with same menu id along with other fields. (Id is primary key) and it worked.
insert into tbl_menu(id, col1, col2, ...) values(12, val1, val2, ...)