|Retrieving Data with SQL Queries|
|Part 1: Introducing the SELECT Statement|
The Structured Query Language offers database users a powerful and flexible data retrieval mechanism -- the SELECT statement. In this article, we'll take a look at the general form of the SELECT statement and compose a few sample database queries together. If this is your first foray into the world of the Structured Query Language, you may wish to review the article SQL Fundamentals before continuing. If you're looking to design a new database from scratch, the article Creating Databases and Tables in SQL should prove a good jumping-off point.
Now that you've brushed up on the basics, let's begin our exploration of the SELECT statement. As with previous SQL lessons, we'll continue to use statements that are compliant with the ANSI SQL standard. You may wish to consult the documentation for your DBMS to determine whether it supports advanced options that may enhance the efficiency and/or efficacy of your SQL code.
The general form of the SELECT statement appears below:
The first line of the statement tells the SQL processor that this command is a SELECT statement and that we wish to retrieve information from a database. The select_list allows us to specify the type of information we wish to retrieve. The FROM clause in the second line specifies the specific database table(s) involved and the WHERE clause gives us the capability to limit the results to those records that meet the specified condition(s). The final three clauses represent advanced features outside the scope of this article -- we'll explore them in future SQL lessons.
The easiest way to learn SQL is by example. With that in mind, let's begin looking at some database queries. Throughout this article, we'll use the employees table from the fictional XYZ Corporation human resources database to illustrate all of our queries. Here's the entire table:
Retreiving an Entire Table
XYZ Corporation's Director of Human Resources receives a monthly report providing salary and reporting information for each company employee. The generation of this report is an example of the SELECT statement's simplest form. It simply retrieves all of the information contained within a database table -- every column and every row. Here's the query that will accomplish this result:
Pretty straightforward, right? The asterisk (*) appearing in the select_list is a wildcard used to inform the database that we would like to retrieve information from all of the columns in the employees table identified in the FROM clause. We wanted to retrieve all of the information in the database, so it wasn't necessary to use a WHERE clause to restrict the rows selected from the table. Here's what our query results look like:
In the next section of this lesson, we'll look at some more powerful queries that allow you to restrict the information retrieved from the database. Read on!