Definition of database query
A database query extracts data from a database and formats it in a readable form. A query must be written in the language the database requires—usually that language is SQL.
For example, when you want data from a database, you use a query to request the specific information you want. Perhaps you have an Employee table and you want to track sales performance numbers. You might query your database for the employee who recorded the highest sales in a given period.
The SQL SELECT Statement
A database query must follow the query format required by the database. The most common format is the Structured Query Language (SQL) standard query format used by many database management systems. SQL is a powerful language capable of advanced queries.
SQL uses a SELECT statement to select specific data.
Here's an excerpt from the database's Employees table:Excerpt from the Northwinds database Employees table
|Davolio||Nancy||Sales Representative||507 - 20th Ave. E.||Seattle||WA|
|Fuller||Andrew||908 W. Capital Way||Tacoma|
|Leverling||Janet||722 Moss Bay Blvd.||Kirkland|
To return an employee's name and title from the database, the SELECT statement would look something like this:
SELECT FirstName, LastName, Title FROM Employees;
This would return:
|Vice President, Sales|
To refine the results further, you might add a WHERE clause:
SELECT FirstName, LastName FROM Employees
This returns the FirstName and LastName of any Employee who is from Tacoma:
Note that SQL returns data in a row/column form that is similar to Microsoft Excel, making it easy to view and work with. Other query languages might return data as a graph or chart.
The Power of Queries
A database has the potential to reveal complex trends and activities, but this power is only harnessed through the use of the query. A complex database consists of multiple tables storing a myriad of data. A query allows you to filter it into a single table so you can more easily analyze it.