NULL VALUES

Key moments

Introduction to NULL — The lesson introduces SQL null values as a potentially mysterious topic.

Defining NULL — A null value is defined as a field with no value, like an empty box or blank space.

NULL vs Zero or Space — It is emphasized that NULL is distinct from zero or a space, representing 'I don't know what goes here'.

How NULL is created — If an optional field is skipped during insertion or update, SQL saves the value as null.

Special Comparison Needed — Standard comparison operators cannot be used to find null values, requiring special treatment.

Using IS NULL — The IS NULL operator is used in the WHERE clause to find all records where a value is missing.

Using IS NOT NULL — The IS NOT NULL operator is used to find records where the specified field contains a value.

Summary and Reminder — The lesson concludes by reminding the learner to always use IS NULL and IS NOT NULL for handling missing data.

Bottom line: You will define what a SQL NULL value represents and distinguish it from zero or an empty string. You will learn the specific operators required to accurately locate or exclude NULL values in any SQL query.

This lesson on NULL VALUES is hands-on and example-driven. You will define what a SQL NULL value represents and distinguish it from zero or an empty string. You will learn the specific operators required to accurately locate or exclude NULL values in any SQL query.

What You'll Be Able To Do

Define the meaning of a NULL value in a database field.
Differentiate between NULL, zero, and an empty string (space).
Write a WHERE clause using the IS NULL operator.
Construct a query to exclude records containing missing data.
Explain why standard comparison operators fail with NULL values.

Topics Covered in NULL VALUES

Introduction to NULL (0:00 - 0:15) — The lesson introduces SQL null values as a potentially mysterious topic.
Defining NULL (0:15 - 0:35) — A null value is defined as a field with no value, like an empty box or blank space.
NULL vs Zero or Space (0:35 - 0:55) — It is emphasized that NULL is distinct from zero or a space, representing 'I don't know what goes here'.
How NULL is created (0:55 - 1:10) — If an optional field is skipped during insertion or update, SQL saves the value as null.
Special Comparison Needed (1:10 - 1:30) — Standard comparison operators cannot be used to find null values, requiring special treatment.
Using IS NULL (1:30 - 1:50) — The IS NULL operator is used in the WHERE clause to find all records where a value is missing.
Using IS NOT NULL (1:50 - 2:10) — The IS NOT NULL operator is used to find records where the specified field contains a value.
Summary and Reminder (2:10 - 2:30) — The lesson concludes by reminding the learner to always use IS NULL and IS NOT NULL for handling missing data.

SQL Cheat Sheet

NULL — Represents a field that currently holds no value
IS NULL — Checks if a specific field contains a missing value
```
SELECT * FROM customers WHERE address IS NULL;
```
IS NOT NULL — Checks if a specific field contains any value
```
SELECT * FROM customers WHERE address IS NOT NULL;
```
WHERE <condition> — Filters records based on the specified criteria
```
SELECT * FROM customers WHERE address IS NULL;
```

Comparison Table

NULL	Zero (0)	Space (' ')
Represents 'no value' or 'unknown'	Represents a numerical quantity	Represents an empty string value
Requires IS NULL/IS NOT NULL	Uses standard operators (=, <, >)	Uses standard operators (=, LIKE)
Applicable to all data types	Only applicable to numeric types	Only applicable to string types

Common Pitfalls

Mistake: Using = or != to check for missing data. Avoid: Always use the specific IS NULL or IS NOT NULL operators.
Mistake: Assuming NULL is the same as an empty string or zero. Avoid: Remember NULL means "unknown" or "no value," not a quantity.
Mistake: Forgetting to use a WHERE clause when filtering for NULLs. Avoid: Place IS NULL or IS NOT NULL directly after the WHERE keyword.

FAQs

Can I use = to check if a value is NOT NULL? No. Standard comparison operators fail because NULL is not a value; it's a state. You must use IS NOT NULL.
If a field is optional, does it automatically become NULL if I skip it? Yes, if the field is not defined as NOT NULL, SQL will save the record with a NULL value in that field.
Why does NULL need special operators? NULL is treated as an unknown state, meaning it cannot logically equal or not equal any other value, including another NULL.

Hey there, future SQL masters. Now we're looking into something that can seem a bit mysterious at first. SQL null values. But don't worry, we'll break it down step by step. Let's get started. So what exactly is a null value? Well, in SQL, a null value is simply a field with no value. It's like leaving a blank space in your table. Think of it as an empty box in a spreadsheet. There's nothing inside. But here's the thing. Null isn't the same as zero or a space. It's like saying, "I don't know what goes here yet, so I'll leave it empty for now." So, if a field in your table is optional, you can insert or update a record without filling it in. When that happens, SQL saves it as null. It's important to remember that null means no value, not zero or space. How do we find these mysterious null values in our data? You might think you could use the usual comparison operators like equals or less than. But nope. Null needs special treatment. Instead, we use the IS NULL and IS NOT NULL operators. Let's take a look at the syntax. This is how you check if a field is null. You use IS NULL in your where clause to find all the records where the value is missing. For example, if we want to find all customers who didn't provide an address, we'd run a query like this. This would return only the customers with a null value in the address field. And if you want to find records where the field has a value, use IS NOT NULL. Simple, right? Let's see that in action. And boom, you get a list of all customers who filled in their address easy-peasy. Remember, when dealing with SQL null values, always use IS NULL and IS NOT NULL. It's your secret tool to finding those missing or filled fields. Thanks for watching. Like and subscribe to get notified about our latest videos. For more coding tips and tricks, keep it locked right here on w3schools.com. Don't forget to try out what you've learned in our interactive editor. Happy coding.

Frequently asked questions

Can I use `=` to check if a value is NOT NULL?

No. Standard comparison operators fail because NULL is not a value; it's a state. You must use IS NOT NULL.

If a field is optional, does it automatically become NULL if I skip it?

Yes, if the field is not defined as NOT NULL, SQL will save the record with a NULL value in that field.

Why does NULL need special operators?

NULL is treated as an unknown state, meaning it cannot logically equal or not equal any other value, including another NULL.

Ready to continue?