Create Table Else Alter Table - php

Question

This seems to be a simple problem, but after a while of searching I can't figure out the answer.
I currently have a MySQL table in my local database used by a webapp, and them same table on a database in a remote server. Right now, I'm using the CREATE TABLE IF NOT EXISTS command through PHP to create the table on the databases:
CREATE TABLE IF NOT EXISTS users (
`id` int(10) NOT NULL AUTO_INCREMENT,
`username` varchar(18) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=2 ;
However, let's say I make a modification to the local database, adding a collumn, for example. It would be really annoying to have to go and change the remote database every time I change the local one. Is there an easier way to run code to create a table if it doesn't exist, and if it does exist, make sure it's structure matches that of the create table structure?
Here's an example, to make what I'm trying to convey a little clearer. Let's say on the local database I have a users table, and I decide that in my webapp I want to have another collumn, password. So I go to the local database and add a password collumn. Is there PHP/MySQL code I can run to check if the users table exists, and if it does, make sure it has a password collumn, and if not, add it?

Answer 1

What you are actually looking for are Migrations, e.g. you are looking for a Schema Management Tool that lets you manage your Database structure in versioned code diffs.
For instance, for your described scenario you would first create a script to create the table, e.g. 001_create_user_table.sql. Then you'd use the schema manager to connect and deploy these changes to your databases.
When you want to change or add something, you just write another script, for instance, 002_Add_Password_Column_To_User_Table.sql. Fill in just the code to do that change. Then run the schema manager again.
Typically, you tell the Schema Manager to go through all existing migrations files. On each run, the Schema manager will update a changelog table in the database, so when you run it, it will know which of your scripts it should apply.
The good thing is, you can add these migrations to your regular VCS, so you will always know which database schema you had at which version of your application. And you will have a proper changelog for them.

Answer 2

To directly answer your question you can create temporary procedures to detect field existence like using a query like this:
SHOW COLUMNS FROM table_name LIKE 'column_name';
However in the real world, database changes are general rolled into three scripts. A create script and two deltas one up and one down. Then the database is versioned so that you know at what state the database is in at any given time.

Answer 3

To specifically check for a password column you can use DESCRIBE:
$colExists = false;
$res = mysql_query('DESCRIBE `users`');
while ($row = mysql_fetch_assoc($res)) {
if ($row['Field'] == 'password') {
$colExists = true;
break;
}
}
if (!$colExists) {
// create column
}
However, you should check into replication or some other automated tool to see if they would be a better solution for you.

Answer 4

Follow these steps (you can easily implement this in PHP, I assumed that the name of the table is Foo)
1.) Run the following code:
desc Foo
2.) Based on the result of the first step you can make your create table command (and you should)
3.) Store your data from the existing table which will be replaced in a variable (Optional, you only need this if you can potentially use data from the old table)
4.) Modify the extracted rows from step 3.) so they will be compatible with your new definition (Optional, you only need this if you can potentially use data from the old table)
5.) Get the rows from your new Foo table
6.) Merge the results got in steps 4.) an 5.) (Optional, you only need this if you can potentially use data from the old table)
7.) Run a drop table for the old table
8.) Generate a replace into command to insert all your rows into the newly created Foo table (you can read more about this here)
After these steps, as a result, you will have the new version of the table. If your tables are too large, you can do a CREATE TABLE IF NOT EXISTS command and if that was not successful, run the alter command.
Also, you can make a library to do these steps and will use that in the future instead of solving the same problem several times.
EDIT:
You can connect the database using this function: mysql-connect (documentation here)
You can run a query using this function: mysql-query (documentation here)
Based on the first step you will get the field names (let's assume you store it in a variable called $bar) and you can use your result to generate your select command (connecting to the database where you have important data. It may be both):
$field_list = "1";
foreach ($bar as $key => $value)
$field_list.= ",".$bar[$key];
mysql_connect(/*connection data*/);
mysql_query("select ".$field_list." from Foo");
You can use your new resource to build up an insert command to insert all your important data after deletion recreation (about resources read more here, about how you can generate your insert you can read here, but I suggest that you should use replace into instead of insert which works like the insert, except that it replaces the row if it already exists, it's better here than an insert, read more here)
So, use mysql_connect and mysql_query, and the resource returned by the mysql_query function can be used for replace into later (I've linked now the URL's for everything you need, so I'm pretty sure you'll solve the problem.), apologies for being not specific enough before.