Join Bill Weinman for an in-depth discussion in this video Counting rows, part of SQL Essential Training.
Sometimes you may want to know the number of matching results, but not necessarily use the results themselves. Here's how to do that. I'm going to select the world database again. And if we want to know the number of rows in this table, I just say SELECT, COUNT, parenthesis star parenthesis, that's the asterisk character, FROM Country. And it will count the rows, but it won't return all of the rows. It'll just return the count.
And here you see, we have the count is 239. COUNT is a standard function in SQL. You use it to count the number of results in a query. It's considered an aggregate function because it returns one value for a set of results. So, how about if we want to know just those countries that have a population over 100 million? We can put a WHERE clause in here, and we can say WHERE Population. And that's one of our columns in the table. And use a greater than sign and type in one million, 1, six zeros.
And I'll press Go. And you see there's 154 of those. And if I want 100 million, I just put in a couple more zeros and press Go. And there's only 10 of those. How many of those countries with more than 100 million are in Europe? I can add a clause to my WHERE. I can say, AND Continent equals, and put Europe in single quote marks, and press Go. And we see there's just one of those.
So there's many scenarios where the count of results is relevant, and the actual results are not. The COUNT function is one of the more commonly used functions in SQL.
- Understanding SQL terminology and syntax
- Creating new tables and records
- Inserting and updating data
- Writing basic SQL queries
- Sorting and filtering
- Accessing related tables with JOIN
- Working with strings
- Finding the numeric type of a value
- Using aggregate functions and transactions
- Updating a table with triggers
- Creating views
Skill Level Beginner
Q: For Mac OS X: When I try to start the Apache Web Server from the XAMPP control panel, it doesn't start, and when I open "localhost" in my web browser, I see a white screen that says "It Works!" instead of the XAMPP page.
sudo apachectl stop
Q: I'm on a Mac, and I get an error in SID that says "attempt to write a read only database." How can I fix this?
A: This usually means that the database folder does not have sufficient permissions for writing by the web user. This can happen if you create the SQL folder new, rather than copying it from the Exercise Files. Here's how to fix this:
- Open a Finder window and Navigate to /Applications/XAMPP/htdocs/SQL
- Control-click on the SQL folder and select "Get Info" from the context menu.
- Under "Sharing and Permissions" (you may need to open the disclosure triangle), in the "everyone" row, select "Read & Write."Then you can close the Info window.
- Now repeat the process for the three *.db files inside the folder.