Join Bill Weinman for an in-depth discussion in this video Understanding NULL, part of SQL Essential Training.
…NULL is a special state for result with no value.…This is necessary because it is often important to distinguish between…a zero value or an empty string, where a NULL value result.…For this lesson we're going to be using the test.dv database and…in the last two movies, we've been working with a table called test.…That we created for learning about inserting…and deleting rows and it looks like this.…Notice the word NULL in the result in the a column.…NULL is a special state in SQL, it's not really a value, it's the lack of value.…
So you cannot test for it as you would a…normal value with an equals sign because it has no value.…So if I were to do this,…you see I get an empty result.…Because there is no row where a is equal to NULL…because NULL has no value to satisfy the equal to expression.…That's because NULL is not a value, it is the lack of a value.…To test for NULL you need to use a special condition called IS NULL like this.…Now, we get a result.…You can see the row with the NULL in the a column.…
The opposite of the IS NULL condition is the IS NOT NULL condition, like this.…
- 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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