How to Avoid Infinite Loops When Using Correlated Subqueries for UPDATE Statements in Oracle?

Update with Join Query in Oracle: Handling Correlated Updates

In Oracle, performing an update query with a correlated subquery can lead to unexpected behavior or errors if the subquery does not return a single row. Consider the following query:

UPDATE table1 t1 SET (t1.col, t1.Output) = (
  SELECT t2.col, t3.Output + t2.col
  FROM tabl2 t3 
  LEFT JOIN table1 t2 ON t3.Join_Key = t2.Join_Key
  WHERE t2.col is not NULL);

Issue:

The provided query executes indefinitely because the subquery may return multiple rows for each row in table1. In this scenario, Oracle attempts to update each row in table1 multiple times, resulting in an endless loop.

Resolution:

To resolve this issue, it is necessary to ensure that the subquery returns a single row for each row in table1 being updated. One approach is to add a condition that relates rows in table1 to rows in the subquery:

UPDATE table1 t1 SET (t1.col,t1.Output) = (
  SELECT t2.col, t3.Output + t2.col
  FROM tabl2 t3 
  LEFT JOIN table1 t2 ON t3.Join_Key = t2.Join_Key
  WHERE t2.col is not NULL
    AND t1.some_key = t2.some_key);

In this modified query, the additional condition, t1.some_key = t2.some_key, ensures that only matching rows are included in the subquery, resulting in a single row for each table1 row.

Additionally, it's worth considering whether the query intends to update all rows in table1 or only a subset of them. If the goal is to update only specific rows based on the subquery results, additional filtering criteria can be added to the UPDATE clause.

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