SQL: Using NULL values vs. default values

A NULL value in databases is a system value that takes up one byte of storage and indicates that a value is not present as opposed to a space or zero or any other default value. The field in a database containing the NULL value means that the content of this cell is unknown at the time of looking at it. A column that allows NULL values also allows rows to be inserted with no values at all in that column. There are several pros and cons of using NULL values as opposed to default values:

Pros

NULL value does not have the data type, therefore can be inserted to any data structure and any database column. Default values, on the other hand, need to have their data type specified and a default value in one column might look the same in another column, but it might be of a different type.

NULL is often used in schemas where a value is optional. It is a convenient method for omitting data entry for unknown fields without having to implement additional rules, like storing negative values in an integer field to represent omitted data.

Since the NULL value takes up only 1 bit of memory space, they may be useful when optimising the database. Using those values is much more efficient than default values, e.g. character’s 8 bits and integer’s 16bits.

While your system requirements may change over time and the default value types with them, NULL value is always NULL so there is no need to update the type of data.

Assigning Not Null to table schemas can also help with table validation, in a sense that the column with Not Null criteria will require a value to be inserted. Default values do not have these capabilities.

Cons

NULL values are easily confused with empty character strings, which return a blank value to the user when selected. In this sense, default values are less confusing and are the safer option, unless the default value is set to the empty string.

If NULL values are allowed in the database, they may cause the designer some extra time and work as they can make the database logic more complicated, especially when there are a lot of comparisons to null values in place.

Source: Pro and cons

I don't know why you're even trying to compare these to cases. null means that some column is empty/has no value, while default value gives a column some value when we don't set it directly in query.

Maybe some example will be better explanation. Let's say we've member table. Each member has an ID and username. Optional he might has an e-mail address (but he doesn't have to). Also each member has a postCount column (which is increased every time user write a post). So e-mail column can have a null value (because e-mail is optional), while postCount column is NOT NULL but has default value 0 (because when we create a new member he doesn't have any posts).

Null values are not ... values!

Null means 'has no value' ... beside the database aspect, one important dimension of non valued variables or fields is that it is not possible to use '=' (or '>', '<'), when comparing variables.

Writting something like (VB):

if myFirstValue = mySecondValue

will not return either True or False if one or both of the variables are non-valued. You will have to use a 'turnaround' such as:

if (isnull(myFirstValue) and isNull(mySecondValue)) or myFirstValue = mySecondValue

The 'usual' code used in such circumstances is

if Nz(myFirstValue) = Nz(mySecondValue, defaultValue)

Is not strictly correct, as non-valued variables will be considered as 'equal' to the 'defaultValue' value (usually Zero-length string).

In spite of this unpleasant behaviour, never never never turn on your default values to zero-length string (or '0's) without a valuable reason, and easing value comparison in code is not a valuable reason.