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A given field in a given record can contain data, or not. If you don’t enter a value into a field in a particular record, you might think that the field was simply empty, but life isn’t that simple. Instead the field is said to contain a null value. If, for example, a field is supposed to contain the phone number of a friend but you don’t know the phone number, you don’t enter any data. The field is then said to contain a null value.

In the database column that I write for , I was discussing primary keys a while ago and offered the opinion that for a small car restoration business, the registration number (also known as a license number) of the cars in the workshop would make a good primary key. A reader questioned this opinion and the point is interesting because it illustrates the scope that a primary key must cover.

If you are a DBA (DataBase Administrator) you get lots of phone calls every day. I can’t tell you what your next call will be but I can tell you what it won’t be. You are quite definitely not about to receive one that starts “The database is running far, far too quickly. Can you slow it down a bit?” In other words, no matter how fast the database runs, people always want it to run faster.

Indexing is the first topic we’ll cover in the area of software optimization of database performance.

SQL is the language used to query a database and, in some respects, it’s a strange language. It’s declarative rather than procedural, which means that when you write a query you declare to the database engine what you want it to do, but you don’t tell it how to achieve that end result (that would be a procedural approach). For instance, if you want to query for matching values in two joined tables, the query contains nothing to indicate which table should be examined first.

Denormalization is the process of turning a normalized database into one where some or all of the tables are not in 3NF. Denormalization is not only perfectly acceptable, there are times when to build a fully normalized database would be absolutely the wrong decision.