Understanding the Problem Statement

The problem at hand revolves around a table of employees, each identified by their position numbers and a field called position_sequence that assigns an employee’s positions as either 1 or 2. The task is to write a SQL query that finds rows where there is only one position for an employee but the position_sequence is marked as 2 instead of 1.

Background Information

To approach this problem, we need to understand how the row_number() function works in SQL, particularly when it comes to partitioning and ordering. The row_number() function assigns a unique number to each row within a partition of a result set, ordered by the column specified in the OVER clause.

Correct Data Example

The correct data example illustrates an employee with two positions, where one is assigned sequence 1 and the other is assigned sequence 2. This is the expected behavior as per the problem statement.

+---------+--------+----------+
| Employee | Position| Sequence |
+=========+========+==========+
| bob      | builder | 1       |
| bob      | cleaner | 2       |
+---------+--------+----------+

Incorrect Data Example

The incorrect data example, on the other hand, presents an employee with only one position but marks it as sequence 2 instead of 1. This is the case that we need to identify.

+---------+--------+----------+
| Employee | Position| Sequence |
+=========+========+==========+
| sally   | builder | 2       |
+---------+--------+----------+

Using NOT EXISTS

One way to approach this problem is by using the NOT EXISTS clause in SQL. This allows us to identify rows where there does not exist a matching row with certain conditions.

The NOT EXISTS Clause

The NOT EXISTS clause is used to test whether or not there exists at least one row within a subquery that satisfies the given condition. If no such row exists, then it returns true for the outer query.

SELECT t.*
FROM t
WHERE sequence = 2 AND
      NOT EXISTS (SELECT 1 FROM t t2 WHERE t2.employee = t.employee AND t2.sequence = 1);

This SQL statement is used to identify rows where sequence equals 2 and does not have a matching row with employee equal to the current row’s employee and sequence equal to 1.

How It Works

• The query selects all columns (t.*) from table t.
• It filters for rows where sequence equals 2.
• For each of these rows, it checks if there is a row in t (which is the same as the outer query’s table) that has an employee equal to the current row’s employee and sequence equal to 1.
• If such a row exists, then the NOT EXISTS clause will evaluate to false, meaning the condition was not met. The row with sequence = 2 will be returned by the original query.
• However, if no such row exists (i.e., there is no employee with sequence 1 and equal to current employee’s employee), then the NOT EXISTS clause evaluates to true, and the rows with sequence = 2 are not returned.

Using Row Number() Function

Another way to solve this problem involves using the ROW_NUMBER() function. This allows us to generate an imputed sequence that would have been present if all employees had two positions. We can then identify which of these imputed sequences are missing compared to what is actually present in the table.

Generating Imputed Sequences

We use the ROW_NUMBER() function with the OVER clause to assign a unique number to each row within a partition of our result set, ordered by the column specified. This will give us an imputed sequence for each employee that would have been present if all employees had two positions.

SELECT t.*
FROM (
  SELECT t.*,
         ROW_NUMBER() OVER (PARTITION BY employee ORDER BY sequence) as imputed_sequence
  FROM t
) t
WHERE imputed_sequence <> sequence;

How It Works

• The query selects all columns (t.*) from table t within a subquery.
• Within this subquery, the ROW_NUMBER() function is used to generate an imputed sequence for each employee that would have been present if all employees had two positions. This is achieved by partitioning the result set by the employee column and ordering it by the sequence column.
• The outer query then selects all columns (t.*) from the subquery’s table where the imputed_sequence does not equal the sequence.
• This effectively identifies rows where there are only one position for an employee but the sequence is marked as 2 instead of 1.

Conclusion

Both methods presented in this article provide ways to identify rows with an imbalanced number of positions and sequences. The choice between using the NOT EXISTS clause or the ROW_NUMBER() function depends on personal preference, familiarity with the SQL syntax, and the context of the problem at hand.

Last modified on 2025-02-04