NULL is used to represent "no value" and allow that to be distinguished between 1 and 0, true or false, or an empty string versus a string with content. It's similar to nil in Ruby, null in JavaScript or NULL in PHP.

If you define a column as NOT NULL then it won't allow an INSERT without a value for that column being specified. If you have a DEFAULT then this will be applied automatically. If you use an ORM it may fill it in for you with a safe, minimal default.

Columns that can be NULL require an almost insignificant amount of additional storage per row, one bit, to hold the NULL or NOT NULL flag.

Remember that NULL in MySQL is unusual in that it is not greater than, less than, or equal to any other value. This is why IS NULL and IS NOT NULL are required for logical comparisons.

Answer from tadman on Stack Overflow
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Stack Overflow
stackoverflow.com › questions › 12323001 › what-is-the-meaning-of-null-and-not-null-in-mysql-database
What is the meaning of NULL and NOT NULL in MySQL database - Stack Overflow
Top answer
1 of 4
3

NULL is used to represent "no value" and allow that to be distinguished between 1 and 0, true or false, or an empty string versus a string with content. It's similar to nil in Ruby, null in JavaScript or NULL in PHP.

If you define a column as NOT NULL then it won't allow an INSERT without a value for that column being specified. If you have a DEFAULT then this will be applied automatically. If you use an ORM it may fill it in for you with a safe, minimal default.

Columns that can be NULL require an almost insignificant amount of additional storage per row, one bit, to hold the NULL or NOT NULL flag.

Remember that NULL in MySQL is unusual in that it is not greater than, less than, or equal to any other value. This is why IS NULL and IS NOT NULL are required for logical comparisons.

2 of 4
1

The checkbox probably sets the default value to null. There may also be a box that specifies whether or not the field accepts null values. Null basically means Empty or Nothing (in VB).

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W3Schools
w3schools.com › sql › sql_null_values.asp
SQL NULL Values - IS NULL and IS NOT NULL
A NULL value represents an unknown, missing, or inapplicable data in a database field. It is not a value itself, but a placeholder to indicate the absence of data.
Discussions
sql - NULL vs NOT NULL - Stack Overflow
An INT NOT NULL can not be empty in any way (unless you mean that it has the value 0) 2012-06-07T14:33:38.273Z+00:00 ... NULL examples: for INT we have 0; for CHAR or VARCHAR we have "", what is an empty string but is still a value. @ypercubeᵀᴹ, is that right? 2018-03-08T13:18:23.47Z+00:00 ... Just to clarify your pointÖ Empty strings are not being set to NULL in any database ... 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
🌐 r/learnSQL
10
13
October 5, 2022
What are pros and cons of using NULLS vs NOTNULLs in database?
Pros of using NULL: When your column can be NULL, NULL is a perfect fit. Pros of using NOTNULL: When your column cannot be NULL, NOTNULL is a perfect fit. Cons of using NULL: When your column cannot be NULL, NOTNULL would be a better fit. Cons of using NOTNULL: When your column can be NULL, NULL would be a better fit. More on reddit.com
🌐 r/SQL
11
6
March 15, 2022
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
🌐 r/SQL
33
14
December 22, 2021
Videos
06:54
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SQL Tutorial #17 - SQL IS NULL and IS NOT NULL | SQL NULL Values ...
May 12, 2022
03:06
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SQL NOT NULL Constraint | SQL Server Tutorial for Beginners - YouTube
February 6, 2022
05:58
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SQL NOT NULL CONSTRAINT - YouTube
October 23, 2020
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Reddit
reddit.com › r/sql › need some knowledge on null and not null
r/SQL on Reddit: Need some knowledge on NULL and NOT NULL
December 22, 2021 -

  • Where and why exactly a null is used?

  • What is exactly null and not null? To my understanding Not null we use when its mandatory to insert some value in that field, also when we give check constraint so by default the column will be not null right?

  • By adding new column through alter method default values are null, so how would I be able to insert values in it and is it right to give not null constraint to that new column while adding through alter method, basically when null and when not null to be used?...

god this is so confusing please help me, ik im asking alot but im really confused

Top answer
1 of 5
8
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
2 of 5
3
Thoughts in this can vary but I'll give my input. NULL should be used where it is known that there can definitely be no value. If a person's record has an optional telephone number when signing up to a website, and they do not complete a number upon their submission, then a NULL is a valid value. However, if we are utilising something like "effective from" and "effective to" of a row of data, then effective to should never be NULL, we always determine that a row is either currently effective, or is historic and superceded. As a result, the "active" row should not hold a NULL in effective to, even though it is not known. It should be a future, default date, and the column should be set with a NOT NULL constraint.
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LearnSQL.com
learnsql.com › blog › not-null-constraint-sql
What Is a NOT NULL Constraint in SQL? | LearnSQL.com
In this article, we will cover the NOT NULL constraint, which is used to avoid having NULL values in a column. Remember, a NULL value in a column represents the absence of a defined value.

NULL is used to represent "no value" and allow that to be distinguished between 1 and 0, true or false, or an empty string versus a string with content. It's similar to nil in Ruby, null in JavaScript or NULL in PHP.

If you define a column as NOT NULL then it won't allow an INSERT without a value for that column being specified. If you have a DEFAULT then this will be applied automatically. If you use an ORM it may fill it in for you with a safe, minimal default.

Columns that can be NULL require an almost insignificant amount of additional storage per row, one bit, to hold the NULL or NOT NULL flag.

Remember that NULL in MySQL is unusual in that it is not greater than, less than, or equal to any other value. This is why IS NULL and IS NOT NULL are required for logical comparisons.

Answer from tadman on Stack Overflow
🌐
Programiz
programiz.com › sql › not-null
SQL NOT NULL Constraint (With Examples)
In SQL, the NOT NULL constraint in a column means that the column cannot store NULL values. In this tutorial, you will learn about the SQL NOT NULL constraint with the help of examples.
🌐
Wikipedia
en.wikipedia.org › wiki › Null_(SQL)
Null (SQL) - Wikipedia
1 day ago - In a database table, the column reporting this answer would start with no value (marked by null), and it would not be updated with the value zero until it is ascertained that Adam owns no books. In SQL, null is a marker, not a value. This usage ...
HistoryNull propagationComparisons with NULL and the three-valued logic (3VL)Analysis of SQL Null missing-value semanticsCheck constraints and foreign keysOuter joinsAggregate functionsWhen two nulls are equal: grouping, sorting, and some set operationsEffect on index operationNull-handling functionsData typing of Null and UnknownBOOLEAN data typeControversyFurther reading
Find elsewhere
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Scaler
scaler.com › home › topics › sql not null constraint
What is SQL NOT NULL Constraint? - Scaler Topics
March 3, 2024 - Consider a table called "Cities" that contains information about numerous cities.This table has columns for city ID, name, population, and country. ⁤To apply the NOT NULL in sql, some fields must always contain a value.
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W3Schools
w3schools.com › sql › sql_ref_not_null.asp
SQL NOT NULL
The NOT NULL constraint enforces a column to not accept NULL values, which means that you cannot insert or update a record without adding a value to this field.
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GeeksforGeeks
geeksforgeeks.org › sql › sql-not-null-constraint
SQL NOT NULL Constraint - GeeksforGeeks
February 10, 2026 - All rows insert successfully because EmpID has valid (non-NULL) values. NOT NULL constraint is satisfied, so the database accepts the rows.
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Hightouch
hightouch.com › sql-dictionary › sql-not-null
SQL NOT NULL - Syntax, Use Cases, and Examples | Hightouch
December 29, 2023 - A NOT NULL constraint in SQL is a database constraint that ensures a column must always contain a value. It prohibits the insertion of NULL values, making it mandatory for every row to have a non-null value in the specified column.
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Stack Overflow
stackoverflow.com › questions › 10933639 › null-vs-not-null
sql - NULL vs NOT NULL - Stack Overflow
Top answer
1 of 5
12

If you need to represent unknown data in a column, you make it nullable. If you will always have data in the column, it's better to make it not nullable, as

  1. Dealing with nulls can be annoying and counterintuitive
  2. It saves a bit of space
  3. On some database systems, null values are not indexed.
2 of 5
3

When a field is set to NOT NULL, it cannot be empty. Which means you have to specify a value for that field when inserting a record.

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SQL Shack
sqlshack.com › commonly-used-sql-server-constraints-not-null-unique-primary-key
Constraints in SQL Server: SQL NOT NULL, UNIQUE and SQL PRIMARY KEY
June 16, 2021 - A NOT NULL constraint in SQL is used to prevent inserting NULL values into the specified column, considering it as a not accepted value for that column. This means that you should provide a valid SQL NOT NULL value to that column in the INSERT ...
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W3Schools
w3schools.com › sql › sql_notnull.asp
SQL NOT NULL Constraint
The NOT NULL constraint enforces a column to NOT accept NULL values. This enforces a field to always contain a value, which means that you cannot insert a new record, or update a record without adding a value to this field.
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Alma Better
almabetter.com › bytes › tutorials › sql › not-null-in-sql
NOT NULL in SQL
June 22, 2023 - Not null is a constraint used in SQL that ensures a value is not empty. This is important for businesses and organizations that rely on their databases for day-to-day operations.
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SQLServerCentral
sqlservercentral.com › home › articles › database design follies: null vs. not null
Database Design Follies: NULL vs. NOT NULL – SQLServerCentral
February 10, 2016 - In addition, the NOT NULL ensures that anyone manually manipulating data in the table cannot inadvertently place a NULL in this column and break your application. In the above example, Address_2 allows NULL values because many people don’t have a second line to their address and would want to leave the field blank. Optional foreign keys, such as ContractID should also be NULL, as an account with no contract cannot populate that column with any meaningful data.
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CockroachDB Docs
cockroachlabs.com › docs › stable › not-null
NOT NULL constraint
The NOT NULL constraint specifies the column may not contain NULL values.
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Hightouch
hightouch.com › sql-dictionary › sql-is-not-null
SQL IS NOT NULL - Syntax, Use Cases, and Examples | Hightouch
December 29, 2023 - The SQL IS NOT NULL operator is used to filter rows in a database table where a specified column's value is not NULL. It is the opposite of the IS NULL operator.
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Sololearn
sololearn.com › en › discuss › 1326682 › what-is-the-difference-between-null-and-not-null
What is the difference between null and not null | Sololearn: Learn to code for FREE!
June 5, 2018 - NULL means that database columns can hold NULL values . NOT NULL means a column to NOT accept NULL values. ... Null in other way means zero or empty value. Thus, Null means you can leave that field empty.
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DB Vis
dbvis.com › thetable › a-guide-to-the-postgres-not-null-constraint
A Guide to the Postgres Not Null Constraint
September 25, 2024 - The end goal is to prevent the ... which signifies a lack of information, the NOT NULL constraint mandates that a column must have a non-NULL value....
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Navicat
navicat.com › en › company › aboutus › blog › 1312-the-null-value-and-its-purpose-in-relational-database-systems
The NULL Value and its Purpose in Relational Database Systems
March 3, 2020 - This is what is commonly referred to as a "null pointer". In a database, zero is a value which has meaning, so the value NULL became is a special marker to mean that no value exists.
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