Join Bill Weinman for an in-depth discussion in this video Using aggregate functions, part of SQL Essential Training (2014).
…SQL provides a number of functions that operate on aggregate values.…For this lesson, we're going to use the world.db database.…And you've seen the count function before, looks like this.…And this tells us that there are 239 rows in the country table.…The count function with an asterisk for its parameter is actually…a special case for counting all the rows in a table.…If you provide a column name for count, it…only counts the non null values in that column.…
So take out the asterisk and we'll put in a column name.…And now we see that there are 232 instead of 239.…The difference is, that with a column name,…it only counts the non-null values in that column.…So this is telling us that there are 232 values in the population column.…The average function returns the average value for a column, average numeric value.…So this gives us the average value for the…population column for rows in which there is a value.…Remember, it's not counting the rows where there's a null value in population.…
You can, of course, use group by with any 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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