Join Bill Weinman for an in-depth discussion in this video Aggregating DISTINCT values, part of SQL Essential Training.
…For circumstances where you want your aggregate functions to operate on distinct…values, SQL allows the distinct key word to be used in aggregate functions.…Let's see how this works.…We're going to be using the world.db database for this exercise.…We're going to start with this query here:…select count of Head of State from country.…And, you'll see, there's 238 rows where a head of state is not null.…
So, this counts the non-null values in the head of state column of the country table.…Now, if you look at this column, you'll…notice there there are a number of duplicate values.…You simply select head of state from country, like this and we'll…order it by Head of State so we can see the duplicates.…And we see, there's Beatrix three times, we have Elizabeth the…Second a number of times, and you'll notice that this is a rather old database.…It has George W Bush, somewhere down here its got Jacques Chirac.…
Database is out of date.…But it serves our purpose.…And you'll notice of course, that there are a number of duplicates.…
- 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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