Informix Error -515: Constraint constraint-name has already been dropped.

Reasons / Common Causes

-515 is specifically about dropping the same constraint twice within a single ALTER TABLE statement — not about a constraint that was dropped in some earlier, separate statement.

1. A single ALTER TABLE naming the same constraint in more than one DROP CONSTRAINT clause — per the official guidance, the direct, only cause.
2. A constraint referenced by two different names that turn out to be the same constraint — for example, an explicit name and a system-generated name both pointing at the same underlying constraint, both listed for dropping in the same statement.
3. Generated DDL that lists constraints to drop from a stale or duplicated source list, producing a repeated DROP CONSTRAINT entry in one statement.

Solutions / Resolution

1. Remove the duplicate DROP CONSTRAINT clause from the ALTER TABLE statement, keeping only one instance per constraint name.
2. Confirm two differently-named clauses don't actually reference the same constraint, by checking sysconstraints before building the statement:

   SELECT constrname, constrid FROM sysconstraints WHERE tabid =
     (SELECT tabid FROM systables WHERE tabname = 'orders');
   

3. Deduplicate any generated list of constraints to drop before assembling the ALTER TABLE statement, if the list is built programmatically.

Examples

The disallowed duplicate

ALTER TABLE orders
  DROP CONSTRAINT ck_orders_status,
  DROP CONSTRAINT ck_orders_status;
-- -515: ck_orders_status named twice in the same statement

Corrected

ALTER TABLE orders
  DROP CONSTRAINT ck_orders_status;

Diagnostic Checks

1. Scan the ALTER TABLE statement's DROP CONSTRAINT clauses for repeated names before running it.
2. Query sysconstraints to resolve constraint names to IDs when two clauses use different names, to rule out both pointing at the same underlying constraint.

Related Errors / Related Topics

No closely related error codes are cross-referenced for -515 in this set yet.

Scoped to a single statement: the same constraint named twice in one ALTER TABLE's drop clauses, not a constraint dropped by an earlier, separate statement.