Join Bill Weinman for an in-depth discussion in this video How aggregates work, part of SQL Essential Training.
…Aggregate data is information derived from more than one row at a time.…SQL has powerful features for dealing with aggregate data.…For this lesson, we're going to use the world.db database.…And, if you've been following along, you've seen queries like this before.…So the result of this query shows us that there are 239 rows in the Country table.…The Count function is an aggregate function.…It returns a single value from a query that spans many rows of data.…Now, what if you want to know how many…countries are represented in each region within this table?…This is where you start seeing the power of aggregate functions in SQL.…
So, we'll start by just splitting this up into separate lines,…so that we can do some more stuff with it easily.…And we're going to select Region and Count with the asterisk from Country.…And, we're going to say Group By Region.…So now you see a result where it has a count for each region.…And that's because of the Group By clause.…The GROUP BY clause groups results before calling the aggregate function.…
- 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.
1. SQL Quick Start
Understanding joins2m 23s
Dates and times1m 59s
10. Subselects and Views
11. A Simple CRUD Application
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