Querying a Database
Queries are one of the things that make databases so powerful. A "query" refers to the action of retrieving data from your database. Usually, you will be selective with how much data you want returned. If you have a lot of data in your database, you probably don't want to see everything. More likely, you'll only want to see data that fits a certain criteria.
For example, you might only want to see how many individuals in your database live in a given city. Or you might only want to see which individuals have registered with your database within a given time period.
As with many other tasks, you can query a database either programatically or via a user interface.
Option 1: Programatically
The way to retrieve data from your database with SQL is to use the "SELECT" statement.
Using the SELECT statement, you can retrieve all records...
SELECT * FROM Individual
...or just some of the records:
SELECT * FROM Individual WHERE FirstName = 'Homer'
The 2nd query only returns records where the value in the "FirstName" column equals "Homer". Therefore, if only one individual in our database had the name "Homer", that person's record would be shown.
Something like this:
SQL is a powerful language and the above statement is very simple. You can use SQL to choose which columns you want to display, you could add further criteria, and you can even query multiple tables at the same time. If you're interested in learning more about SQL, be sure to check out our SQL tutorial after you've finished this one!
Option 2: User Interface
You might find the user interface easier to generate your queries, especially if they are complex.
Database management systems usually offer a "design view" for your queries. Design view enables you to pick and choose which columns you want to display and what criteria you'd like to use to filter the data.
Here's an example of design view in Microsoft Access:
When using design view, the database system actually uses SQL (behind the scenes) to generate the query.
- Adding Data
- Database Design