stackoverflow.com › questions › 21987177 › what-is-null-in-sql

In simple worlds you can say that Null is not a data value, but a marker for an unknown value.

So any mathematical operations performed on NULL will result in NULL. For example,

10 + NULL = NULL

Similarly if you do string concatenation with string you get:-

'String ' || NULL || 'Concatenation'   -- Result is NULL

So you can say that Null means either "not applicable" or "don't know": it is not the same as zero (0) or any other default value, but more importantly, null is treated quite differently from other values in SQL, because it literally has no value.

An example to explain what it means when we say that NULL means UNKNOWN VALUE:

StudentName TestResult
X             78
A             89
B             67
C             NULL

So you can see that the student C got NULL marks in the test. So what does that mean?

Now one can say that the student does not sit in the test or it may be that the student's data is not avaialable. But it definitely does not mean that the student got 0(as if you assign 0 to the student then it would mean that the student appeared in the test and got zero) marks in the test. All we can say that the data for the student is

UNKNOWN or NULL

Answer from Rahul Tripathi on Stack Overflow

W3Schools

w3schools.com › sql › sql_null_values.asp

SQL NULL Values - IS NULL and IS NOT NULL

String Functions: Asc Chr Concat ... Plan SQL Bootcamp SQL Certificate SQL Training ... A field with a NULL value is a field with no value....

Hightouch

hightouch.com › sql-dictionary › sql-is-null

SQL IS NULL - Syntax, Use Cases, and Examples | Hightouch

December 29, 2023 - The SQL IS NULL operator is used to filter rows where a specified column's value is NULL. A NULL value in a database represents the absence of data, and the IS NULL operator is used to identify and select rows that contain NULL values in a ...

Discussions

What is NULL in SQL? - Stack Overflow

I am confuse about what is the value of NULL in SQL. NULL I think is empty. So will it contain any garbage value or unknown value or like 0 in integer or blank in character? More on stackoverflow.com

stackoverflow.com

What is a null in sql? is it the same as a empty cell?

NULL, in simplest terms, is the absence of data. Not to be confused with '', an empty string. If you are familiar with other languages, you could think of it as similar to undefined, in that the column has a type defined, but the specific cell has no data, and is more of a place holder for future data. When filtering for NULL, you will need to do things like table.col is null or table.col is not null In your Excel example, it depends on how you import the data, and what the definition of the target table is. The target table may have rules built into it to default the data for certain columns, or reject NULL values from other columns. You could have Null, an empty string, zero, weird dates.. etc. More on reddit.com

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IS NULL vs =NULL - SQLTeam.com Forums

How can the below return different things · NULL is an unknown value and therefore a NULL value can never be equal to another value, even if that other value is NULL. NULL = NULL is unknown and cannot be true - eliminating that row from the results · Please the see following links for the ... More on forums.sqlteam.com

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Need some knowledge on NULL and NOT NULL

a null is used for multiple purposes -- for example, not known, not applicable, etc. theoreticians take great joy in debating which uses are valid, and whether they can be substituted by some non-null placeholder suffice to say, a null is used when you don't know what value should go there if you don't want that situation to arise, just make the column NOT NULL and you'll never be able to not insert an actual value More on reddit.com

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