NULL

01 · What is it?

NULL means that a value is missing, unknown, or not applicable.

It is not the same thing as zero, an empty string, or the word “NULL”. It represents the absence of a usable value.

Simple definition: NULL says, “There is no known value here.”

Think of NULL as a blank answer on a form when the answer is not known or does not apply.

Value	Meaning
NULL	No usable value is known or needed.
0	The known number is zero.
''	The known text is empty.
'NULL'	The text literally says “NULL”.

NULLAnswer not known
or not applicable

NULL lets a database record that a field does not currently have a usable value.

It keeps “unknown” different from a guessed value.

It can show that a field does not apply to this record.

SQL treats NULL as unknown, so ordinary equality checks do not match it.

NULL appears in real data whenever information can arrive later, be optional, or not apply.

A user has not added a phone number or middle name.

An order has no shipped date because it has not shipped yet.

A sensor or imported record did not provide a reading.

SELECT order_id, shipped_at
FROM orders
WHERE shipped_at IS NULL;

Use IS NULL to test for NULL. Writing shipped_at = NULL does not work because NULL is not an ordinary value to compare with equals.

Input data Validation Database column NULL or known value Query result

A column definition can allow or reject NULL, and queries must use NULL-aware conditions when they need missing records.

A missing, unknown, or not-applicable value.

A known numeric value whose amount is 0.

A known text value containing no characters.

NULL ≠ 0 ≠ ''. They carry different meanings and can behave differently in queries.

ENGLISH TERM	中文	SIMPLE ENGLISH	中文小白理解
NULL	空值 / 未知值	A marker for a missing, unknown, or not-applicable value.	表示数据缺失、未知或不适用的特殊标记。
Missing Value	缺失值	A value that is not available in the data.	数据中目前没有提供的值。
Unknown Value	未知值	A value that may exist but is not known yet.	可能存在，但现在还不知道具体是什么的值。
IS NULL	判断空值	An SQL condition used to find NULL values.	SQL 中用来查找 NULL 的判断条件。
IS NOT NULL	判断非空值	An SQL condition used to find values that are not NULL.	SQL 中用来查找不是 NULL 的值的判断条件。
NOT NULL	非空约束	A rule that requires a column to have a value.	要求某一列必须有值，不能保存 NULL 的规则。
Empty String	空字符串	Text with zero characters, written as ''.	长度为零的文字，不等于 NULL。
Default Value	默认值	A value used automatically when no value is supplied.	用户没有提供值时，系统自动填入的值。