In SQL a null value is undefined or inapplicable or the field has no value assigned or defined. It is not equal to zero or space. Zero is a number and space is a character. Null is undefined value. Any rows of any data type can contain a null unless the column defined has primary key or is set not null ( which means the column cannot contain null values).
If a table field is null you see a value like null when you run a query which fetches data from that table.
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