# SQLCODE -603: Duplicate rows block a unique index

Source: https://wndsoftware.com/mainframe-errors/sqlcode-603

### SQLCODE -603: Duplicate rows block a unique index

SQLCODE -603 (SQLSTATE 23515) means CREATE UNIQUE INDEX failed because the table already holds rows with the same values in the index columns. Db2 can't build a unique index over duplicates.


#### Other names for this code

Also written as `SQL -603`, `SQLCODE=-603`, `SQLCODE = -603`, `db2 -603`, `sqlstate 23515`, `SQL0603N`, `unique index cannot be created because the table contains rows which are duplicates`.


### What it means

A unique index allows each key value only once. When you create one on a table that already has data, Db2 reads the rows to build the index. If two rows share the same key, it stops, rejects the CREATE with -603 and no index is created.

The duplicates got in because nothing enforced uniqueness before. The table had no unique index on those columns, or a different one.

Nulls count. In a plain unique index, two rows with null in the key columns count as duplicates. If you need many nulls but unique non-null values, Db2 for z/OS offers `CREATE UNIQUE WHERE NOT NULL INDEX`.

On Db2 releases with temporal tables, the message also mentions periods. A unique index that includes BUSINESS_TIME WITHOUT OVERLAPS fails the same way when rows for the same key have overlapping business time periods.


### Common causes

- Data was loaded or copied into the table before the unique index existed.
- The table is getting a primary key or unique constraint late in its life and the data was never checked.
- Several rows have null in the key columns, and the index is a plain unique index.
- Batch reruns inserted the same rows twice while no unique index stopped them.
- For a temporal table, business time periods overlap for the same key.


### How to find the problem

- List the duplicate keys with `SELECT CUST_NO, COUNT(*) FROM PROD.CUSTOMER GROUP BY CUST_NO HAVING COUNT(*) > 1`, using all the index columns in the GROUP BY.
- Count rows with nulls in the key columns separately, since they also count as duplicates.
- Select the full duplicate rows and compare them. Exact copies point at a double load. Rows that differ point at a key that isn't really unique.


### How to fix it

- If the duplicates are mistakes, delete or merge them, then create the index again.
- If the duplicates are valid data, the columns aren't a unique key. Create a non-unique index instead, or add a column that makes the key unique.
- If only nulls repeat, use CREATE UNIQUE WHERE NOT NULL INDEX.
- After the index exists, Db2 enforces it, and later duplicates fail with -803 at INSERT or UPDATE time.


### What it looks like


```text
DSNT408I SQLCODE = -603, ERROR:  A UNIQUE INDEX CANNOT BE CREATED BECAUSE THE TABLE CONTAINS ROWS WHICH ARE DUPLICATES WITH RESPECT TO THE VALUES OF THE IDENTIFIED COLUMNS AND PERIODS
DSNT418I SQLSTATE   = 23515 SQLSTATE RETURN CODE
```


### IBM documentation

[IBM documentation: -603](https://www.ibm.com/docs/en/db2-for-zos/13.0.0?topic=codes-603). Use the documentation for your installed release.

[Report a correction to WND Software](https://wndsoftware.com/contact#message).

Technical reviewer: [Albert Joseph](https://wndsoftware.com/authors/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.


### Related codes

[SQLCODE -803 Duplicate key Read →](https://wndsoftware.com/mainframe-errors/sqlcode-803)
[SQLCODE -540 Table definition incomplete, missing index Read →](https://wndsoftware.com/mainframe-errors/sqlcode-540)
[SQLCODE -542 Key column allows nulls Read →](https://wndsoftware.com/mainframe-errors/sqlcode-542)
[SQLCODE -811 More than one row returned Read →](https://wndsoftware.com/mainframe-errors/sqlcode-811)
[SQLCODE -601 Object name already exists Read →](https://wndsoftware.com/mainframe-errors/sqlcode-601)

### Questions


#### What is the difference between -603 and -803?

-603 happens when you create a unique index on data that already has duplicates. -803 happens when an index already exists and an INSERT or UPDATE would add a duplicate.


#### Do nulls count as duplicates?

In a plain unique index, yes. CREATE UNIQUE WHERE NOT NULL INDEX lets many rows have null in the key.


#### Is -603 the same on Db2 for LUW?

Yes. Db2 for LUW reports SQL0603N with SQLSTATE 23515.

[Db2 SQLCODEs](https://wndsoftware.com/mainframe-errors/db2-sqlcodes) · [SQLCODE -601](https://wndsoftware.com/mainframe-errors/sqlcode-601) · [SQLCODE -604](https://wndsoftware.com/mainframe-errors/sqlcode-604)

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