To test that a database column rejects NULL, attempt an insert and an update that set a required field to SQL NULL, then assert that both writes fail. For an optional field, set it to NULL in both operations and assert that they succeed, assuming no other constraint or trigger rejects them. Run these tests against the database engine and version used in production. NOT NULL rejects SQL NULL; it does not reject an empty string.
What to test
A required field is one whose column is declared NOT NULL. The core test is to prove that a valid value is accepted and SQL NULL is rejected. An optional, nullable field should accept NULL. Test inserts and updates separately: both can write a value to a column, and the constraint must hold for each operation.
| Field | Insert | Update | Expected result |
|---|---|---|---|
Required (NOT NULL) |
Insert a valid value, then attempt NULL. |
Set a valid value to NULL. |
Valid insert succeeds; writes of NULL fail. |
| Optional (nullable) | Insert NULL. |
Set the value to NULL. |
Both writes succeed, unless another rule or trigger rejects them. |
| Text with a blank-value policy | Insert ''. |
Set the value to ''. |
Assert the separate policy; NOT NULL alone permits an empty string. |
PostgreSQL documents NOT NULL as a column constraint, and SQLite documents constraint checking for both inserts and updates. See the PostgreSQL 16 constraints documentation and SQLite CREATE TABLE documentation.
Set up an isolated test table
Use a disposable test database and the target engine’s native schema syntax. This example uses standard SQL types and expresses the two field behaviors directly:
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CREATE TABLE field_test (
id INTEGER PRIMARY KEY,
required_value TEXT NOT NULL,
optional_value TEXT
);Use a non-key field to test nullability independently. In PostgreSQL, a primary key already imposes not-null behavior, so testing the key alone would not demonstrate that a separate required-field rule works. See the PostgreSQL 18 constraints documentation.
Exercise inserts and updates
Run the successful cases and expected failures as distinct assertions. The following statements show the intended outcomes; the comments are not SQL assertions, so your test framework must check each result.
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-- Required value supplied: should succeed.
INSERT INTO field_test (id, required_value, optional_value)
VALUES (1, 'present', NULL);
-- Required value explicitly NULL: should fail.
INSERT INTO field_test (id, required_value, optional_value)
VALUES (2, NULL, 'optional');
-- Optional value NULL: should succeed.
INSERT INTO field_test (id, required_value, optional_value)
VALUES (3, 'present', NULL);
-- Updating a required value to NULL: should fail.
UPDATE field_test SET required_value = NULL WHERE id = 1;
-- Updating an optional value to NULL: should succeed.
UPDATE field_test SET optional_value = NULL WHERE id = 1;- Assert valid writes succeed. Insert a non-null value into the required column, and insert or update the optional column with
NULL. - Assert required-column violations fail. Attempt an insert with explicit
NULL, then separately update an existing row to set the required column toNULL. Check for the database’s constraint-violation result or exception rather than accepting any unrelated failure. - Keep cases isolated. One expected failure should not prevent later assertions from running. If a failed statement occurs inside a transaction, recover or roll back according to the database driver’s rules before continuing.
- Test omission if the application relies on it. Omitting a required column is not identical to explicitly supplying
NULL: defaults and SQL mode can affect the outcome. State the schema and engine configuration for this test; explicitNULLis the clearest null-rejection case.
Do not confuse NULL with an empty string
SQL NULL and '' are different values. The MySQL Reference Manual notes: “Both statements insert a value into the phone column, but the first inserts a NULL value and the second inserts an empty string.” A NOT NULL column can therefore contain an empty string. If the application considers blank text missing, test that rule separately. To find SQL nulls in MySQL, use IS NULL, not = NULL. See MySQL: Problems with NULL Values.
Why a CHECK constraint is not always enough
A check such as CHECK (value <> '') does not necessarily reject NULL. In PostgreSQL, a CHECK constraint is satisfied when its expression evaluates to true or to NULL; a comparison involving a null operand commonly evaluates to NULL. Use NOT NULL to require a non-null value, and add a separate check or application validation for any rule that forbids blank strings. See the PostgreSQL 16 constraints documentation.
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Use the production engine and version
Constraint syntax, error reporting, transaction recovery, and schema migration behavior depend on the database engine and version. Run the tests against the same engine and version as production, or make any differences explicit in the test configuration. A unit test against a substitute engine may not establish that the production database enforces the constraint in the same way.
If your test accompanies a SQLite schema migration, check the SQLite library version before choosing the migration syntax. SQLite 3.53.0, released on 2026-04-09, added ALTER TABLE ... ALTER COLUMN ... SET NOT NULL. Earlier versions require another migration approach; the SQLite documentation describes table reconstruction for schema changes such as adding a NOT NULL constraint. See SQLite ALTER TABLE.
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