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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11NULL means a value is missing, unknown, or not applicable; 0 is a real numeric value; and '' is text with zero characters in databases that preserve it. They are not interchangeable. One important exception: Oracle Database 18c treats a zero-length character value as NULL, though Oracle warns applications not to rely on that behavior continuing.
What each value means
NULL: no known value
NULL is used when a value is absent, unknown, or not meaningful. It is not a number or a text value. For example, a contact’s phone number could be NULL when the number is not known.
'': text with no characters
An empty string is a character value whose length is zero. In MySQL, PostgreSQL, and SQL Server, it is distinguishable from NULL. It might represent a known text value that is blank—for example, a person known to have no phone number, if that is how an application defines the field.
0: a numeric value
Zero is a real number. If a measured quantity is actually zero, store zero rather than NULL. A missing measurement and a measured zero communicate different things.
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How database behavior differs
| Database documentation | Empty string vs. NULL |
Zero vs. NULL |
Null handling |
|---|---|---|---|
| MySQL 26.7 | Distinct | Distinct | Use IS NULL; = NULL does not find null rows. MySQL: Problems with NULL Values |
| Oracle Database 18c | A zero-length character value is currently treated as NULL; Oracle says this may change. |
Not equivalent | Use IS NULL or IS NOT NULL. Oracle: Nulls |
| SQL Server (documentation labeled SQL Server 17) | Distinct | Distinct | Use IS NULL or IS NOT NULL. Microsoft Learn: NULL and UNKNOWN |
| PostgreSQL 17 | Distinct | Distinct | Use IS NULL; for null-aware equality, use IS NOT DISTINCT FROM. PostgreSQL: Comparison Functions and Operators |
Oracle’s behavior is a portability concern: an empty-string check that distinguishes blank text from missing data in one database may not do so in Oracle 18c. Check the target database and version rather than assuming every SQL engine treats '' alike.
How to test for these values
For a column named phone, these predicates express different checks in databases that preserve empty strings:
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-- Missing or unknown phone number
SELECT * FROM contacts WHERE phone IS NULL;
-- Known text value with zero characters
SELECT * FROM contacts WHERE phone = '';
-- This does not find NULL rows
SELECT * FROM contacts WHERE phone = NULL;Use IS NULL and IS NOT NULL to test for nullness. Ordinary comparisons such as = NULL do not evaluate to true for null values. The empty-string predicate is not a reliable way to distinguish blank text from NULL in Oracle Database 18c, because Oracle treats a zero-length character value as null.
Why comparisons with NULL behave differently
SQL conditions can evaluate to TRUE, FALSE, or UNKNOWN. A comparison involving NULL generally produces UNKNOWN, not a definite true or false. A WHERE clause keeps rows only when its condition is true, so a predicate like phone = NULL filters out null rows.
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UNKNOWN can also affect compound conditions: it is not simply another spelling of FALSE when SQL evaluates further logical operations. SQL Server and PostgreSQL document this three-valued behavior; see SQL Server’s NULL and UNKNOWN reference and PostgreSQL’s logical operators reference.
Choose the value that matches the data
- Use
NULLwhen the value is unknown, missing, or not applicable. - Use
''when the value is known to be text containing no characters, and the database preserves empty strings distinctly. - Use
0when the numeric value is genuinely zero.
The intended meaning belongs to the application’s data model. MySQL’s documentation illustrates this with a phone number: NULL can mean the number is not known, while '' can mean the person is known to have no phone. That is an example, not a universal interpretation; define what each value means for your own field.
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Null-aware equality in PostgreSQL
When comparing values in PostgreSQL and treating two nulls as equal for the comparison, use IS NOT DISTINCT FROM. It returns true when both operands are null and otherwise behaves like equality for non-null operands. Confirm the equivalent syntax for any other database engine you use.
Finally, whether an inserted NULL is stored as null can depend on column defaults, constraints, and database settings. MySQL documents special cases for some column types and settings, including conditional TIMESTAMP behavior, so check the target column and configuration.
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