Chapter 13: SQL Basics

In SQL, NULL is a special marker that is often used to indicate the absence of a data value in the database. It is important to note that NULL is different from an empty string or a zero, which are actual values. When a value is set to NULL, it means that the value is currently unknown, missing, or not applicable.

In the context of our bookshop database, NULL could be used to represent the price of a book that we currently do not know. For instance, we might receive a new book that has not yet been priced, or we may be waiting for the publisher to provide us with the information. In such cases, the 'Price' column for this book would be set to NULL. This allows us to keep track of the book in the database, while also indicating that the price information is not yet available.

It is important to handle NULL values properly when writing SQL queries. For example, if we want to retrieve all books that cost less than $20, we need to be careful not to exclude books that have a NULL price. We can use the IS NULL operator to handle NULL values in our queries, and we can also use the COALESCE function to replace NULL values with default values if needed.

Example:

Here is how you might insert a book with an unknown price:

To query data with NULL values, you can use the IS NULL or IS NOT NULL operators. For instance, if you wanted to find all the books in your database for which the price is unknown, you could use:

Updating NULL values is done in the same way as updating any other values. For instance, if you later find out that the price of "Unknown Book" is $7.99, you could update it like so:

It is important to note that NULL is not equal to anything, even itself. That is, if you try to compare NULL to NULL using the = operator, it will not match. This is why you need to use IS NULL or IS NOT NULL when querying NULL values.

In summary, NULL is a special value in SQL that represents missing or unknown data. It's crucial to understand how to handle NULL values because they can sometimes lead to unexpected results if not properly managed.

In SQL, NULL is a special marker that is often used to indicate the absence of a data value in the database. It is important to note that NULL is different from an empty string or a zero, which are actual values. When a value is set to NULL, it means that the value is currently unknown, missing, or not applicable.

In the context of our bookshop database, NULL could be used to represent the price of a book that we currently do not know. For instance, we might receive a new book that has not yet been priced, or we may be waiting for the publisher to provide us with the information. In such cases, the 'Price' column for this book would be set to NULL. This allows us to keep track of the book in the database, while also indicating that the price information is not yet available.

It is important to handle NULL values properly when writing SQL queries. For example, if we want to retrieve all books that cost less than $20, we need to be careful not to exclude books that have a NULL price. We can use the IS NULL operator to handle NULL values in our queries, and we can also use the COALESCE function to replace NULL values with default values if needed.

Example:

Here is how you might insert a book with an unknown price:

To query data with NULL values, you can use the IS NULL or IS NOT NULL operators. For instance, if you wanted to find all the books in your database for which the price is unknown, you could use:

Updating NULL values is done in the same way as updating any other values. For instance, if you later find out that the price of "Unknown Book" is $7.99, you could update it like so:

It is important to note that NULL is not equal to anything, even itself. That is, if you try to compare NULL to NULL using the = operator, it will not match. This is why you need to use IS NULL or IS NOT NULL when querying NULL values.

In summary, NULL is a special value in SQL that represents missing or unknown data. It's crucial to understand how to handle NULL values because they can sometimes lead to unexpected results if not properly managed.

In SQL, NULL is a special marker that is often used to indicate the absence of a data value in the database. It is important to note that NULL is different from an empty string or a zero, which are actual values. When a value is set to NULL, it means that the value is currently unknown, missing, or not applicable.

In the context of our bookshop database, NULL could be used to represent the price of a book that we currently do not know. For instance, we might receive a new book that has not yet been priced, or we may be waiting for the publisher to provide us with the information. In such cases, the 'Price' column for this book would be set to NULL. This allows us to keep track of the book in the database, while also indicating that the price information is not yet available.

It is important to handle NULL values properly when writing SQL queries. For example, if we want to retrieve all books that cost less than $20, we need to be careful not to exclude books that have a NULL price. We can use the IS NULL operator to handle NULL values in our queries, and we can also use the COALESCE function to replace NULL values with default values if needed.

Example:

Here is how you might insert a book with an unknown price:

To query data with NULL values, you can use the IS NULL or IS NOT NULL operators. For instance, if you wanted to find all the books in your database for which the price is unknown, you could use:

Updating NULL values is done in the same way as updating any other values. For instance, if you later find out that the price of "Unknown Book" is $7.99, you could update it like so:

It is important to note that NULL is not equal to anything, even itself. That is, if you try to compare NULL to NULL using the = operator, it will not match. This is why you need to use IS NULL or IS NOT NULL when querying NULL values.

In summary, NULL is a special value in SQL that represents missing or unknown data. It's crucial to understand how to handle NULL values because they can sometimes lead to unexpected results if not properly managed.

In SQL, NULL is a special marker that is often used to indicate the absence of a data value in the database. It is important to note that NULL is different from an empty string or a zero, which are actual values. When a value is set to NULL, it means that the value is currently unknown, missing, or not applicable.

In the context of our bookshop database, NULL could be used to represent the price of a book that we currently do not know. For instance, we might receive a new book that has not yet been priced, or we may be waiting for the publisher to provide us with the information. In such cases, the 'Price' column for this book would be set to NULL. This allows us to keep track of the book in the database, while also indicating that the price information is not yet available.

It is important to handle NULL values properly when writing SQL queries. For example, if we want to retrieve all books that cost less than $20, we need to be careful not to exclude books that have a NULL price. We can use the IS NULL operator to handle NULL values in our queries, and we can also use the COALESCE function to replace NULL values with default values if needed.

Example:

Here is how you might insert a book with an unknown price:

To query data with NULL values, you can use the IS NULL or IS NOT NULL operators. For instance, if you wanted to find all the books in your database for which the price is unknown, you could use:

Updating NULL values is done in the same way as updating any other values. For instance, if you later find out that the price of "Unknown Book" is $7.99, you could update it like so:

It is important to note that NULL is not equal to anything, even itself. That is, if you try to compare NULL to NULL using the = operator, it will not match. This is why you need to use IS NULL or IS NOT NULL when querying NULL values.

In summary, NULL is a special value in SQL that represents missing or unknown data. It's crucial to understand how to handle NULL values because they can sometimes lead to unexpected results if not properly managed.