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NOT NULL prevents a column from storing SQL NULL; it does not validate every other value. Empty text, zero, or a placeholder such as 'unknown' is still non-null. To enforce the rules your data needs, pair presence requirements with constraints such as CHECK, UNIQUE, or FOREIGN KEY.
What NOT NULL actually guarantees
NOT NULL addresses one specific condition: the column must not contain SQL NULL. The PostgreSQL 18 documentation describes it as requiring that a column “must not assume the null value” and notes that, in PostgreSQL, explicit NOT NULL is more efficient than the equivalent CHECK (column_name IS NOT NULL) (PostgreSQL 18: Constraints).
It does not judge whether a non-null value is meaningful, correctly formatted, or within an acceptable range. For example, '', 0, and 'unknown' are not SQL NULL. Whether any of them is invalid depends on the column’s actual requirements.
Does NOT NULL reject empty strings?
No. An empty string is a value, not SQL NULL. MySQL’s documentation explicitly treats NULL and the empty string as distinct values (MySQL 8.4: Problems with NULL Values). So a NOT NULL text column can still contain ''. It can also accept whitespace-only text unless another rule rejects it.
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Do not assume that a placeholder such as 'N/A' means the database value is missing: it is ordinary text. Decide whether the domain permits it, then encode that rule where appropriate.
Why a CHECK constraint can still allow NULL
SQL conditions can evaluate to TRUE, FALSE, or UNKNOWN. Comparisons involving NULL commonly produce UNKNOWN, not FALSE. PostgreSQL considers a CHECK satisfied when its expression is true or null; MySQL 8.4 accepts TRUE or UNKNOWN and rejects FALSE. SQL Server likewise documents that a NULL can make a check expression UNKNOWN, avoiding an error (PostgreSQL 18: Constraints; MySQL 8.4: CHECK Constraints; Microsoft SQL Server: Unique constraints and check constraints).
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That is why CHECK (price > 0) alone does not require a price to be present. If the price must both exist and be positive, declare both requirements: price NOT NULL and CHECK (price > 0).
Choose a constraint that matches the rule
| Requirement | Typical mechanism | What to account for |
|---|---|---|
| A value must be supplied | NOT NULL |
Rejects SQL NULL, not arbitrary non-null content. |
| A value must satisfy a condition on its row | CHECK |
Handle NULL/UNKNOWN; combine with NOT NULL when presence is required. |
| A value must not duplicate another row’s value | UNIQUE |
Details of NULL handling can vary by database. |
| A value must refer to an existing row | FOREIGN KEY |
A nullable referencing column may also need NOT NULL if the relationship is mandatory. |
CHECK is suited to row-value conditions; a foreign key is the relational mechanism for requiring a reference to a value in another table. In PostgreSQL, a CHECK is intended to enforce conditions on the row being inserted or updated, not cross-row or cross-table guarantees: later changes elsewhere could invalidate such a condition (PostgreSQL 18: Constraints; PostgreSQL 18: Check Constraints; Microsoft SQL Server: Unique constraints and check constraints).
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Example: require a non-empty name and a positive price
This PostgreSQL-style example separates presence from row-local validity:
CREATE TABLE products (
product_id integer PRIMARY KEY,
name text NOT NULL CHECK (length(name) > 0),
price numeric NOT NULL CHECK (price > 0)
);Here, NOT NULL disallows missing values, while the checks reject an empty name and a non-positive price. The name expression does not reject whitespace-only text. If that is invalid too, the rule must explicitly test for it. Function behavior, type coercion, empty-string handling, and collation can differ by engine, so verify the syntax and semantics against the database and version you deploy rather than treating this example as universal.
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Check engine, version, and configuration
- PostgreSQL 18: explicit
NOT NULLis documented as more efficient than an equivalent null-check constraint, and aCHECKpasses when its expression is true or null (PostgreSQL 18: Constraints). - MySQL 8.4: a
CHECKsucceeds onTRUEorUNKNOWN, and fails onFALSE(MySQL 8.4: CHECK Constraints). - SQL Server: a
CHECKrejects a false condition, whileNULLcan make that condition unknown (Microsoft SQL Server: Unique constraints and check constraints). - MySQL 8.0: strict SQL mode affects handling of invalid data. The manual says disabling strict mode can permit coercion and does not recommend that forgiving behavior. Check the deployed server’s SQL mode when invalid-looking input seems to be accepted (MySQL 8.0: SQL Modes).
These examples do not form a complete compatibility matrix. For a production schema, confirm the engine, version, and active configuration, then test the constraint with both NULL and representative invalid non-null values.
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