Join Bill Weinman for an in-depth discussion in this video Selecting rows, part of SQL Essential Training.
A given table may have a lot of rows and often you will be interested in only some of them. So here's some quick tips on how to select just the rows you're interested in. Keep in mind I'll go over this in a lot more detail later in the course. So here I have SID open in my browser, and I'm going to select the world.db database. And we'll start by just selecting a bunch of rows from the country tables. Select and I just want the name and continent columns, and region, and we'll get it from, the country tables.
It's just like that select Name, Continent, Region from Country and I'll press the Go button. And we got 239 rows and they look like this. We have name continent and region. And so that's a lot of rows. Maybe we don't want all of those rows. Maybe we just want the ones that are in Europe. So I can add a where clause and say where continent equals Europe, like that, and put the Europe in single quote marks, which are the quote marks used in SQL.
And when I press go, now we just get 46 rows and there's the 46 rows, and those are all the countries in Europe. The where clause uses a condition to determine which rows to return, the equal sign is the equivalence condition. You'll see lessons later in the course on where and also like and in clauses for other ways to select rows. You may also limit the number of rows with the limit clause. And I can just add that after the where clause and I can say limit and give it a number, I'll say five, and this will limit the results to just five rows.
So it'll just give me the first five rows in whatever collation order is being used. And in this case, we don't have a collation order. But this limit can be handy for a lot of uses, in particular it's often used where you need to display a limited amount of data per page and then select other pages. So the where and limit clauses are common ways to select specific rows from a table. And we'll learn more about this later in the course. In the next movie we'll look at how to select specific columns.
- 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.