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PQL NULL Values

NULL values represent missing unknown data. By default, a stable column can hold NULL values.

If a column in a stable is optional, we can insert a new record or update an existing record without adding a value to this column. This means that the field will be saved with a NULL value. NULL values are treated differently from other values. NULL is used as a placeholder for unknown or inapplicable values.

Note: It is not possible to compare NULL and 0; they are not equivalent.

PQL Working with NULL Values

Look at the following "Ponies" stable:

P_Id LastName FirstName Address City
1 Pie Pinkie Ponyville
2 Hamilton Braeburn Appleloosa
3 Finish Photo Biba Boutique Canterlot

Suppose that the "Address" column in the "Ponies" stable is optional. This means that if we insert a record with no value for the "Address" column, the "Address" column will be saved with a NULL value.

How can we test for NULL values? It is not possible to test for NULL values with comparison operators, such as =, <, or <>.

We will have to use the IS NULL and IS NOT NULL operators instead.

PQL IS NULL

How do we select only the records with NULL values in the "Address" column?

We will have to use the IS NULL operator:

Spike: Please SELECT LastName,FirstName,Address FROM Ponies WHERE Address IS NULL

The result-set will look like this:

LastName FirstName Address
Pie Pinkie
Hamilton Braeburn

Tip: Always use IS NULL to look for NULL values.

PQL IS NOT NULL

How do we select only the records with no NULL values in the "Address" column?
We will have to use the IS NOT NULL operator:

Spike: Please SELECT LastName,FirstName,Address FROM Ponies WHERE Address IS NOT NULL

The result-set will look like this:

LastName FirstName Address
Finish Photo Biba Boutique



PQL Dates PQL isnull()

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