In SQL, NULL is a special value that represents the absence of a value
A NULL value indicates that a data point does not exist or is unknown
A NULL value is not the same as an empty string or a zero value. An empty string or zero is a value, whereas NULL is the absence of a value.
A NULL value can be assigned to any data type, like numeric, string, date,etc.
When a column is defined as NULLable, it means that it can contain NULL values.
To test for NULL values, you can use the IS NULL or IS NOT NULL operators
When using aggregate functions like SUM, COUNT, AVG, MIN, and MAX, NULL values are usually ignored.
NULL values can be problematic when using joins, because a join condition that includes a NULL value will always return unknown.
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