layout: post title: 1661 Average Time of Process per Machine description: SQL Query Explanation tag: SQL —

1661 Average Time of Process per Machine

Understanding the Problem

This problem requires calculating the average processing time per machine, where each machine may have multiple processes with distinct process_id.

Key Observations

• Although this problem is categorized under JOIN, I opted to use subqueries instead.

• The first level of grouping is based on machine_id, and the second level is process_id. Thus, the SQL must contain:

  GROUP BY machine_id, process_id
  

  This ensures that we correctly distinguish different processes within each machine.

Approach

1. Create a temporary table (table_1) to structure process start and end times:
   • We extract start and end timestamps for each process_id using CASE WHEN:

   SELECT machine_id, process_id,
          MAX(CASE WHEN activity_type = 'start' THEN timestamp END) AS start,
          MAX(CASE WHEN activity_type = 'end' THEN timestamp END) AS end
   FROM Activity
   GROUP BY machine_id, process_id
   

   • This ensures that for each process_id, we only have one row per process per machine.
2. Calculate the average processing time per machine:
   • Using table_1, we compute the average time taken per process:

   SELECT machine_id,
          ROUND(AVG(end - start), 3) AS processing_time
   FROM table_1
   GROUP BY machine_id
   

   • The difference (end - start) gives the duration of each process.
   • Using AVG() ensures we get the average processing time per machine.
   • ROUND(..., 3) is used to match the expected precision of three decimal places.

Conclusion

By structuring the query this way, we effectively:

• Extract relevant start and end times for each process within a machine.
• Compute the correct processing time while ensuring each machine’s processes are considered separately.
• Utilize subqueries efficiently instead of relying on explicit JOIN operations.

This method ensures an optimized and structured approach to solving the problem.