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Stable set of rows?

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My students are into the upload lab and they’ve encountered the wonderful ORA-30926 error from a MERGE statement. An example of the MERGE for an upload is in this earlier blog post.

This is the wonderful error message, which doesn’t seem to have meaning for many. The key is the non-deterministic where clauses phrase. That phrase means that the the query inside the USING clause returns a non-unique set of rows. The returned rows can’t be matched uniquely against the target table in the ON clause. The ON clause is where the MERGE statment matches the source query’s rows against the target table’s rows. The best join condition in a MERGE statement is one between a surrogate primary and foreign key column.

Error: ORA-30926
Text:  Unable TO GET a stable SET OF ROWS IN the SOURCE TABLES.
Cause:  A stable SET OF ROWS could NOT be got because OF LARGE dml 
        activity OR a non-deterministic WHERE clause. 
Action: Remove any non-deterministic WHERE clauses AND reissue the dml.

The problem is very much like when you write what you think is a single-row subquery but find out it’s actually a multiple-row subquery when it return an ORA-01422 error in Oracle.

As a rule, I’ve found that using the DISTINCT operator in the source SELECT statement fixes it most of the time because the join isn’t returning a unique set of rows. Although, the better solution requires that you identify how to gain a unique result set. Alternatively, you need to re-examine the logic of your WHERE clause. It also happens when the SELECT clause returns date-time data types like SYSDATE instead of date data types. A date-time can be converted by using the TRUNCate function like this:

 (SELECT   ...
  ,        TRUNC(SYSDATE) AS creation_date
  FROM     ...) SOURCE
ON (target.primary_key_column = SOURCE.primary_key_column)
UPDATE SET last_updated_by = SOURCE.last_updated_by
,          last_update_date = SOURCE.last_update_date
( column_list_of_values );

Hope this helps my students and others … 😉

Written by maclochlainn

March 5th, 2010 at 1:43 am

Posted in Oracle,Oracle XE,sql