Informix Error -974: Cannot drop not null constraint on the serial column.

Reasons / Common Causes

-974 is a well-defined restriction on SERIAL, SERIAL8, and BIGSERIAL columns: their implicit NOT NULL constraint can be disabled, but it can't be dropped outright while the column itself still exists, per the official guidance.

1. A statement attempting to drop the NOT NULL constraint on a SERIAL/SERIAL8/ BIGSERIAL column, per the official guidance — the direct cause.
2. Not realizing serial-type columns treat NOT NULL differently from ordinary columns — per the official guidance, the constraint can only be disabled, never dropped, until the column itself is dropped.

Solutions / Resolution

1. Disable the constraint instead of dropping it, per the official guidance, if the goal is to stop enforcing it while keeping the column.
2. Drop the column itself if the NOT NULL constraint genuinely needs to go away entirely — per the official guidance, dropping the constraint alone isn't supported for these types.

Examples

The blocked drop

CREATE TABLE orders (order_id SERIAL, ...);
ALTER TABLE orders MODIFY (order_id SERIAL DROP NOT NULL);
-- -974: Cannot drop not null constraint on the serial column.

Disabling instead

ALTER TABLE orders MODIFY CONSTRAINT (...) DISABLED;
-- keeps the column and constraint definition, stops enforcement

Diagnostic Checks

1. Confirm the column's declared type is SERIAL, SERIAL8, or BIGSERIAL — this restriction is specific to those types, not ordinary NOT NULL columns.
2. Decide whether disabling (keep the column) or dropping the column (remove it entirely) actually matches the intended schema change before proceeding.

Related Errors / Related Topics

• -975 — "Invalid object and object mode combination." A related family of restrictions on which mode/operation combinations are valid for a given object type.

Serial-type NOT NULL constraints can be disabled but not dropped independently — drop the column itself if the constraint truly needs to go away.