Databases are essential for a lot of applications. iOS apps are no exception. While there are multiple methods of storing local data in an iOS app, sometimes the data already exists in one of your databases.  Or sometimes you want your app to interact with another platform, and that data needs to be in the cloud. Having access to a MySQL database in iOS is an obvious conclusion, and what I'm going to help you get started today.

Solutions to this problem fall under two categories, both with myriad options. You can use a web service to access the database, or connect to it directly. It may sound like connecting to it directly is easier, but it can involve writing a lot of extra code that can get buggy fast. 

iOS apps, and Mac apps for that matter, are written primarily in Objective-C. Objective-C provides easy to use methods for parsing JSON and some not so easy to use methods for XML. A simple asynchronous JSON data fetch can look like this:

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
    NSError *error;
    NSURL *url = [NSURL URLWithString:@"http://www.yoursite.com/yourscript"];
    NSString *result = [NSString stringWithContentsOfURL:url
                                              encoding:NSASCIIStringEncoding
                                                 error:&error];
    NSLog(@"result: %@ \n Error: %@", result, error);
});

Then, still inside the dispatch_async block, you can use result to create an NSDictionary, which holds key value pairs, with your resulting data:

NSData *rawData = [result dataUsingEncoding:NSASCIIStringEncoding];
NSDictionary *data = [NSJSONSerialization JSONObjectWithData:rawData 
                                                                                 options: kNilOptions 
                                                                                    error:&error];

Then access your values:

NSString *valueOne = [data objectForKey @"your key"];

To use this style of data acquisition with XML things get a little more complicated. There are a number of XML parsers for iOS, here's a list of some and their pros and cons.   About.com's own Daniel Nations has an article here that deals with using Apple's built in parser.

You can also use the ASIHTTPRequest library and this JSON library to make things even easier, especially with sending data back up. Once you've put it in your project, you can: 

NSURL *url = [NSURL URLWithString:@"http://www.yoursite.com/yourscript"];
ASIFormDataRequest *request = [ASIFormDataRequest requestWithURL:url];
[request setPostValue@"105" forKey:@"some_key"];
[request setPostValue:username forKey:@"user"];
[request setFile:filePath forKey:@"some_file"];
[request setDelegate:self];
[request startAsynchronous];

Then implement the delegate methods:

-(void)requestFinished:(ASIHTTPRequest*)theRequest{
     NSDictionary *data = [[request responseString] JSONValue];
     NSString *valueOne = [data objectForKey @"your key"];
}

-(void)requestFailed:(ASIHTTPRequest*)theRequest{
     NSError *theError = [theRequest error];
     //do something with the error
}

Alternatively, trying to connect directly to a MySQL database is quite different. Keep in mind that Objective-C is a superset of C, so if you can do it in C, you can do it in Objective-C. One route to connecting directly to a MySQL database is including the C MySQL client library, and writing a lot of custom code. This takes time, and if you're doing this in an iOS app, you're probably wanting that data to be represented in objects, which means even more code.

So what's the best solution? There are a number of third party libraries that have done most of the leg work for you. MySQL Cocoa has been around for a while and will get the job done.

Creating a web service and using JSON is, to me, the obvious and easiest method for interacting with a database. It's also become the standard practice for doing so, so finding help online is a lot easier.