Combining Parallel Rows in SQL

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When working with multiple tables and requiring the combination of parallel rows, a common challenge arises. Unlike Cartesian products, which combine all possible combinations of rows from two or more tables, we want to join only the parallel rows from each table to create a new table. In this article, we will explore how to achieve this in SQL, using examples and explanations to illustrate the process.

Understanding Row Numbers

The first step in combining parallel rows is to assign row numbers to each row within each table. This can be done using the ROW_NUMBER() function, which assigns a unique number to each row within a result set based on the specified column (in this case, LONGID for Table1 and id for Table2). The resulting rows are ordered by the assigned column.

Example: Assigning Row Numbers

-- Create sample tables
CREATE TABLE table1 (
  LongId NUMBER PRIMARY KEY
);

CREATE TABLE table2 (
  Id NUMBER PRIMARY KEY
);

INSERT INTO table1 (LongId) VALUES (100);
INSERT INTO table1 (LongId) VALUES (200);
INSERT INTO table1 (LongId) VALUES (300);

INSERT INTO table2 (Id) VALUES (1);
INSERT INTO table2 (Id) VALUES (2);
INSERT INTO table2 (Id) VALUES (3);

Combining Parallel Rows with ROW_NUMBER()

To combine the parallel rows, we use a subquery to assign row numbers and another subquery to select only the rows with matching row_number() values. We then join these two result sets together using the USING clause.

SQL Query

SELECT id, longid 
FROM (
  SELECT t.*, 
         row_number() over( order by LONGID ) as XXX
  FROM table1 t
) 
JOIN (
  SELECT t.*, 
        row_number() over( order by id) as XXX
  FROM table2 t
)
USING ( xxx )

How it Works

1. The first subquery assigns a unique row_number() to each row within Table1, ordered by the LONGID column.
2. The second subquery assigns a unique row_number() to each row within Table2, ordered by the id column.
3. The two result sets are joined together using the USING clause, which combines rows with matching row_number() values from both tables.
4. The final result set contains only the parallel rows combined.

Example Output

The resulting table will have the same number of columns as the original tables and only include the parallel rows combined:

| ID | LONGID |
|----|--------|
|  1 |    100 |
|  2 |    200 |
|  3 |    300 |

Additional Considerations

While this approach works for combining parallel rows, there are a few additional considerations to keep in mind:

• Data type compatibility: Make sure that the data types of the columns being compared (e.g., LONGID and id) match. If they don’t, you may need to cast or convert one or both of them.
• Null values: If either table contains null values, ensure that the comparison operator (=) can handle nulls correctly in your database management system (DBMS). Some DBMSes require special handling for null values.
• Indexing: Consider creating indexes on the columns being used for row numbering to improve performance. This is especially important if the tables are large and you expect frequent queries.

Conclusion

Combining parallel rows in SQL can be achieved using the ROW_NUMBER() function, which assigns unique numbers to each row within a result set based on a specified column. By joining two subqueries with matching row_number() values, we can combine only the parallel rows from each table into a new result set. This approach requires careful consideration of data type compatibility, null values, and indexing to ensure optimal performance. With this technique, you can efficiently create a new table containing only the parallel rows combined.

Last modified on 2024-06-25