I hear about so many different types of databases and so many similar kinds of databases, which is the one I should start to learn about so that I get a good understanding of how databases work and how it can be used in programming?
If you use Firefox or a similar program that makes use of SQLite, that would be the best place to start -- you'll get experience working with a database populated with real data that is relevant to you, and one that has been designed for practical use instead of just a book example.
Copy ~/.mozilla/firefox/<profile_name>/*.sqlite into another directory and start playing with those files. It should be easy to come up with things you want to find out. Start with some basic single-table ones like 'what are my 10 most visited pages'. Then expand to use multiple tables ('what are my 10 most visited pages that I have not bookmarked').
Once you have an idea of how queries work, think about the database design and the decisions that led to it -- why did the developers choose to relate tables to each other that way? What kinds of queries are the indexes there to optimize for?
Once you've done this, rinse and repeat with a variety of programs so you can get multiple perspectives. http://www.sqlite.org/famous.html or google around for others.
SQLite is such a beautiful program. It doesn't do much, but what it does do, it does very well. If I need to analyze some data, say in a logfile, I'll often write a trivial bit of Python to load it into SQLite and do my work in SQL. For a few 10s of Gs of data, this works brilliantly. I'd go so far as to say it's the least-buggy program I use day to day, and that includes the OSs I run it on and "real" commercial databases.
Yep I like to think of sqlite as being to relational databases as memcache is to caches. Both are super simple and straightforward, with a minimum of moving parts, and yet does something really well. You can start with each, and then later evolve into something more featureful. But often you'll not need to!
Re: SQLIte, OReilly is coming out with a new book, Using SQLite, in August. It's not clear to me how much of an introduction it offers to SQL (versus SQLite specifics), but I'm guessing there's some introductory material in there as well.
Michael Owens's _The Definitive Guide to SQLite_ is good, too. It's a couple years old, but provides a good overview of both SQLite (standalone, its C API, and a couple wrappers) and the relational model. SQLite's biggest change since its publication is that it now has optional foreign key (http://www.sqlite.org/foreignkeys.html) support.
If you want a more academic treatment, I would recommend C. J. Date's _An Introduction to Database Systems_.
Also, it's been my experience that books which claim to cover relational databases in general, but which actually assume MySQL throughout, are usually pretty bad.
As pointed out by others here already, SQL may not be the optimal fit for every scenario at large scales but it will nearly always work at smaller scales, and I can't imagine that you're already trying to write the next amazon.com. So in other words, start with SQL.
There are three main free software SQL databases: MySQL, PostgreSQL, and SQLite. Don't use MySQL, it'll get you into a great deal of bad habits, is missing an ass-ton of useful features (the default engine doesn't even support transactions), and likes to corrupt data. It also has questionable licensing and is owned by Oracle (a commercial database vendor) which as you can imagine doesn't have a lot of incentive to improve it.
SQLite is a very robust database that is great for read-mostly sites (like blogs) and for embedding into applications (Firefox for example). Its primary limitation is that there can only be one writer at a time. It is very easy to pick up and get going with since there's no persistent daemon, you just point it at a file and start writing. I'd recommend starting here.
PostgreSQL is the more featureful database and supports hundreds of concurrent connections, locking, access control, etc. This is the real "workhorse" database you're likely to find in production at large sites that do make use of free-software RDBMS. Eventually you will need to use something more powerful than SQLite, and this is probably it.
I recommend checking out Apache Friends site download XAMPP. That will set up a fully functional web environment (Apache, PHP, MySQL) which will let you play a bit with database. It also comes packaged with PHPMyAdmin, which is great for learning how to create tables and the like.
This cannot be stressed enough, a good foundation in Relational concepts can save you hours of frustration in performance tuning. Too many times developers learn how to dump some stuff in a table and away they go. Understanding relational concepts is as important as understanding good programming concepts, it is not and should not be an afterthought for developers. As the parent implied once you have a grasp of this, you can weigh the pros and cons of the noSQL databases for particular projects.
A more specific recommendation: _An Introduction to Database Systems_ by C. J. Date.
Be warned that he's got an axe to grind - He's often critical of SQL because it doesn't live up to the mathematical elegance of the relational model. (I happen to agree with him.) He knows his stuff, though.
If you want the Cliffs notes of the textbook, read Date's Database in Depth: Relational Theory for Practitioners. You might want to read it a couple of times though.
The only thing you really need to understand about databases is that they store data for you, ie. they persist data. So once you get that then you start asking questions like "where" and "how".
The "where" is on hard drives. Don't let anyone tell you that a purely memory resident data store is a database, it's not. It's a cache. If you aren't writing to disk you are not a persistent database. Think memcached and redis. Although I would say the later is more of a database because you can configure it to write on every update.
The "how" is where all the devilish details are. How do you put data into and get data out of a database. How does the database store your data on disk. Well, there are two main schools of thought in the market place at the moment. Relational[0] and non-relational. Relational is the older more mature, more well supported, more well understood, more "standards" compliant of the two. Nowadays, non-relational is championed by the NoSQL[1] movement which eschews the relational model for a looser concept, less defined, less "compliant", less understood model. You will find many more and varied answers to "how" in the non-relational space precisely because the renaissance in non-relational is so fresh (the concept itself has been around a long time). Nevertheless, NoSQL has been attracting considerable attention over the last year or so in the open source community.
Every developer worth his salt will know or at least be familiar with the relational model and at least one or two of it's implementations. MySQL, PostgreSQL and SQLite are just three of the most popular open source RDBMS's (relational database management system)[2] around today. They all have varying degrees of support for the primary language used to speak to RDBMS's, SQL[3].
There are many open source, non-relational implementations in production today. HBase, Cassandra, Riak, Voldemort and MongoDB are all playing in the NoSQL field. Popular systems within NoSQL draw their lineage from either the Google approach (gfs[4], big table[5]) or the Amazon approach (dynamo[6]). Yet another is Neo4j, a graph database[7] which, at the moment, is lumped under the "NoSQL" moniker but is a separate beast entirely. Just like their relational cousins, the way NoSQL solutions do what they do and what they are best suited for vary.
As with most interesting discussions, which approach is best is left as an exercise for the reader. I'll just throw my two cents in for good measure. First I would say that you need to know SQL. Knowing SQL and not a specific system (like mysql or postgresql) will allow you to move in between relational systems and let you pick the right one for the job. After taking a look and familiarizing yourself with the SQL world, I would take a very serious look at what is going on with NoSQL. Best I can tell, there is a lot of mindshare and "cool" factor foisted upon this space right now because of the scale non-relational concepts can bring to enterprise users. Be sure, though, that rdbms is the predominant player in the database world. Every company large and small use some form of rdbms. Newer more forward looking companies that have large and very large scale needs to store and analyze vast quantities of unstructured or loosely structured data are looking more and more to non-relational NoSQL solutions.
I specifically avoid linking to particular implementations but rather link to broader concepts here. It is the concept that is more important than the implementation when learning about the big and confusing world of databases.
I want to be careful in how I phrase this, because I feel like this was a good effort, but I disagree with many parts of this description.
First, databases are not "just" about persistence, and I don't mean that in a pedantic way. The point of a database management system is to manage your data over time and allow you to make use of it. This includes things like stating constraints to ensure that the data in the database is always correct and providing methods for querying your data at varying levels of complexity.
Second, there are a number of in-memory databases. Oracle sells one. SQLite is often used as one. The fact that they are in-memory does not make them not databases.
Third, relational versus non-relational is not really an argument about "how the data is stored on disk" or about being "standards compliant." A database traditionally consists primarily of three things: a data model, a query language, and an implementation. A data model, like "everything is a table with column headers and rows" can be written to disk in many different ways and queried in many different ways. (It's an abstraction.) Indexing structures and data formats may be mostly the same even when the data model is different. The (current) "relational vs non-relational" debate, such as it is, is usually about whether it makes sense to change the data model and query language in order to ensure that the implementation is scalable on "cloud-like" platforms with specific data access needs. (That said, there are many different types of non-relational models and query languages designed for different purposes.)
Fourth, there isn't one SQL. There are in fact a number of standards, SQL-86, SQL-89, SQL-92, and so on. It makes sense to learn the standard first (at least up to 92 or so), rather than learning particular implementation specifics. Most databases implement SQL-92 or above, though there is somewhat wide variance in additional features like indexing structures and functions and in data modeling languages.
The only thing you really need to understand about databases is that they store data for you, ie. they persist data.
...is like saying "the only thing you really need to know about sex is that it is for procreation." :-)
Sorry - couldn't resist. Utilizing databases effectively is often an area that is overlooked or kind of an afterthought. In my experience, often issues of scale stem from poor database design.
I put this in a separate comment, but any of Tom Kyte's books are excellent for gaining an understanding of how to use databases effectively. Although they are Oracle focused, there is still a lot to be learned about practical database design and utilization from his books. He also has a great site called “Ask Tom” where he answers questions. I have learned a ton from simply reading questions and his responses. Again, all Oracle focused, but very valuable nonetheless.
The problem with the standards is that getting copies costs actual money. If one really wants to start learning about databases, it can be very difficult to decide what to pay for. Implementation documentation, at least, is generally free.
I completely agree. My intent was to suggest that one should learn to the standard first rather than any particular implementation, not that one should read any of the actual standards documents directly. (Yikes!) By analogy, if you want to learn C, read K&R to learn something approximating C89, rather than picking up a book on how to code to the specific dialect of C understood by GCC 4.5.
For what it is worth, I learned from Database Systems: The Complete Book.
DS:TCB is pretty explicit about which of the SQL it teaches is part of which standard. That said, I suspect that any general database book should do a reasonable job.
If you want to learn more than just how to use a database I recommend Ullman's and Garcia-Molina's "Database Systems: The Complete Book."[1] It has overview of classic topics such as relational algebra and calculus, practical topics such as using a SQL database, and the second half is devoted to an overview of how databases are actually implemented. Unfortunately I don't know of a book that will introduce you to things like column-stores and such, my knowledge comes from reading papers on them.
Relational databases are no longer fashionable, but they are backed by some great theory and they remain very important in commercial practice. Read http://philip.greenspun.com/sql/ for a quick introduction, but use http://www.postgresql.org/ rather than Oracle for the exercises/examples because it's much easier to install.
I trust you'll have no shortage of recommendations about CAP, column-based stores, and support for schema flexibility in other comments.
I think it depends also on what strengths you already have and how you are personally motivated to learn. For example, I have found java's h2 database to be easy to work with (and you never have to leave java). I don't think any db could be easier to get started with and have less administrative overhead than sqlite. If, to learn how to program computer games, one is advised to first write tetris, to learn programming with databases, you might write a message board.
I always thought that "Database Design for Mere Mortals: A Hands-On Guide to Relational Database Design" by Michael J. Hernandez was a great starting point. Sometimes it's nice to have a real book and you can't go wrong with a used one from Amazon for under 5 bucks.
I'm probably going to loose the few points I have left, but hey...
If you have access to a Windows machine my two cents says get a copy of SQL Server Express. In addition to that you will want a copy of the AdventureWorks database and the SQL Server Books Online.
SQL Server Express is a solid and free (as in beer) database engine. There are processor and file size limitations, but other than that it is pretty much the same as every other version they sell.
The Books Online (available online or as an installable download) has tutorial style information on every level of the system; database engine, reporting services etc.
The AdventureWorks database is a sample database that is used in the BOL and just about every other SQL Server tutorial I have seen. It is quite extensive and will save you from having to create your own data.
I'm sure somebody here will try and pick this apart, but this is how I learned. If it means anything, I have been employed as a Databse Analyst for the past four years.
If I had to start learning this today (2010), I'd really do a mix of sql (probably mysql) and non-sql (probably mongodb http://www.learnivore.com/search/mongodb).
Although specific to Oracle, any of Tom Kyte's books are invaluable when it comes to learning about databases and how they work.
If you do work with Oracle, Tom's "Ask Tom" site is excellent for learning tips and tricks, as well as for finding elegant solutions to tricky SQL problems. Again, it is oracle focused, but still very valuable. Link to Ask Tom:
There are a lot of great suggestions here for learning on your own. I'll just add that if your company/venture will support this learning progress, check out this workshop by Xavier Shay called Your Database Is Your Friend. He travels around the world putting on this workshop, and though I haven't been yet, I've heard good things.
1) load some data into a table (probably using sqlite) and play around with it using sql
2) read up on some database theory
3) check out a few popular options: sqlite, mongodb, couchdb and play around a bit more.
4) find a big dataset that interests you, load it into one or more of the databases you want to experiment with, and experiment away by trying to extract interesting data, etc. Notice how each one performs, etc.
I don't know what your level is, but I recently used this book to get started on the basics: Beginning Database Design: From Novice to Professional by Clare Churcher
Google, Wikipedia, get and study the source to some open source databases, etc. How does one start to learn about anything/everything now that we have Internet access? Twenty years ago you would go about something like this starting with a library or a university. Now we still have those things plus the WWW. Knock yourself out.
Copy ~/.mozilla/firefox/<profile_name>/*.sqlite into another directory and start playing with those files. It should be easy to come up with things you want to find out. Start with some basic single-table ones like 'what are my 10 most visited pages'. Then expand to use multiple tables ('what are my 10 most visited pages that I have not bookmarked').
Once you have an idea of how queries work, think about the database design and the decisions that led to it -- why did the developers choose to relate tables to each other that way? What kinds of queries are the indexes there to optimize for?
Once you've done this, rinse and repeat with a variety of programs so you can get multiple perspectives. http://www.sqlite.org/famous.html or google around for others.