NOT NULL requires a column to contain a value; CHECK requires a row to satisfy a condition. A check such as CHECK (price > 0) does not necessarily make price required: in PostgreSQL 17, a check passes when its expression is true or null. Use both constraints when a value must be present and must meet a rule.
What each constraint validates
| Constraint | What it requires | What it does not do by itself |
|---|---|---|
NOT NULL |
The column must not contain SQL NULL. |
It does not restrict which non-null values are allowed. |
CHECK |
A condition evaluated against the row must not be false; the rule can restrict a value or compare columns in the same row. | It does not necessarily require every referenced column to be non-null. |
For example, NOT NULL on price rejects a row with no price, but permits zero or a negative value unless another rule forbids them. CHECK (price > 0) rejects a non-null price that is zero or negative, but may accept a null price because SQL comparisons involving null produce an unknown result rather than true.
Why a CHECK can allow NULL
SQL distinguishes true, false, and unknown. A comparison such as price > 0 is unknown when price is NULL. In PostgreSQL 17, a check constraint is satisfied when its expression is true or null; it fails when the expression is false. MySQL 8.4 likewise documents that a check condition must evaluate to TRUE or UNKNOWN, including for null values. This means a check is not, by itself, a dependable presence requirement in those documented versions.
For null tests, use IS NULL or IS NOT NULL, rather than ordinary equality comparisons. MySQL’s documentation explains that comparisons involving NULL do not ordinarily evaluate to true.
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When to use each constraint
Use NOT NULL for a required field
Choose NOT NULL when the rule is simply that a field must be supplied. It rejects an insert or update that leaves that column null, without dictating which non-null values are valid.
Use CHECK for allowed values or row relationships
Choose CHECK when the value must meet a condition, such as a positive price, or when two columns in the same row must satisfy a relationship. PostgreSQL’s constraints documentation illustrates a table-level check comparing price and discounted_price.
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Use both when presence and validity are required
If a price must exist and must be positive, express both requirements:
CREATE TABLE products (
name text NOT NULL,
price numeric NOT NULL CHECK (price > 0)
);The NOT NULL condition prevents a missing price; the check rejects a non-null price that is zero or below.
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PostgreSQL’s NOT NULL alternative
In PostgreSQL, NOT NULL is functionally equivalent to CHECK (column_name IS NOT NULL), but PostgreSQL says the explicit NOT NULL constraint is more efficient. Prefer the dedicated constraint when the rule is column presence.
Limits of CHECK constraints
A check is intended to validate the row being inserted or updated, not to enforce arbitrary relationships across a table or database. PostgreSQL assumes check conditions are immutable and does not support checks that depend on data outside the row being checked. Do not use a check as a substitute for a foreign key, a uniqueness constraint, or a cross-row aggregate rule; choose the constraint or mechanism designed for that invariant.
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Check behavior depends on the database and version
Constraint semantics and syntax are database-specific. PostgreSQL 17 documents checks as passing on true or null; MySQL 8.4 documents acceptance of TRUE or UNKNOWN and includes an enforcement option in its syntax. These references do not establish behavior for every historical release or every SQL engine. SQLite’s CREATE TABLE reference documents both constraint types, but the cited material does not establish a complete cross-engine comparison of NULL handling or enforcement details. Confirm the target engine and version before relying on a particular behavior.
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- PostgreSQL 17: Constraints
- MySQL 8.4: CHECK Constraints
- SQLite: CREATE TABLE
- MySQL 8.4: Problems with NULL Values
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