SQLCODE -624: Table already has a primary key

What it means

A table can have only one primary key. It can have several unique constraints, but each must cover a different set of columns. When ALTER TABLE ADD PRIMARY KEY runs against a table that already has a primary key, or ADD UNIQUE repeats the columns and periods of an existing constraint, Db2 rejects it with -624. The table is unchanged. The message names the table.

-624 comes up mostly when DDL is run twice or when a migration script doesn't know what the table already has. It doesn't tell you whether the existing key uses the columns you wanted. Check the catalog before you change anything.

Common causes

• A migration script adds a primary key that the CREATE TABLE already defined.
• An ALTER script is rerun after it succeeded the first time.
• Someone tries to change the primary key by adding a new one instead of dropping the old one first.
• ADD UNIQUE repeats the columns of an existing unique constraint.
• A modeling or schema compare tool generates an ADD PRIMARY KEY because its copy of the model doesn't match what's in the catalog.

How to find the problem

1. List the table's keys with SELECT CONSTNAME, TYPE FROM SYSIBM.SYSTABCONST WHERE TBCREATOR = 'PROD' AND TBNAME = 'CUSTOMER'. TYPE P is the primary key and U a unique constraint.
2. Get each constraint's columns from SYSIBM.SYSKEYCOLUSE, or the primary key columns from SYSIBM.SYSCOLUMNS where KEYSEQ is greater than zero.
3. Compare them with the key in your ALTER statement.
4. If a script was rerun, read the output of the earlier run. The ALTER often succeeded the first time, and the second run is the one that failed.

How to fix it

• If the existing key is the one you wanted, skip the ALTER.
• To change the primary key, drop the old one with ALTER TABLE ... DROP PRIMARY KEY, then add the new one. Dropping a primary key also drops the foreign keys in other tables that reference it, so plan to re-create those.
• Make migration scripts check SYSIBM.SYSTABCONST before adding keys.
• If a tool generated the ALTER, refresh its copy of the model from the catalog before you generate the next change script.

What it looks like

DSNT408I SQLCODE = -624, ERROR:  TABLE PROD.CUSTOMER ALREADY HAS A PRIMARY KEY OR UNIQUE CONSTRAINT WITH SPECIFIED COLUMNS AND PERIODS
DSNT418I SQLSTATE   = 42889 SQLSTATE RETURN CODE

Technical reviewer: Albert Joseph, WND Software.

Last reviewed 2026-10-18 by the WND Software team. Written in our own words. For the full reference, check IBM’s Db2 for z/OS documentation for your release.