Finding the First Row for Each ID-Grade Combination in MySQL

In this article, we will explore how to find the first row for each ID-Grade combination in MySQL, given a set of data that includes timestamps and grades. We will examine the concept of window functions, partitioning, and joining tables to achieve this goal.

Understanding the Problem

We are presented with two tables: MyTable1 and MyTable2. The first table contains student information with IDs, names, timestamps, test numbers, and grades. The second table contains average grades and standard deviations for each ID.

Our objective is to select the entire row from MyTable1 where the grade was X standard deviations away from the corresponding average in MyTable2. We need to find the first instance of this occurrence for each unique ID-Grade combination.

Background Information

To tackle this problem, we must understand some fundamental concepts in MySQL:

• Window Functions: These are functions that can operate on a set of rows within a result set that is defined by a specific query. Window functions are used to perform calculations across a set of data.

• **Partitioning:** This refers to dividing the result set into partitions based on certain conditions, such as a column value. In MySQL 8.0 and later versions, we can use partitioning with window functions.
  

• Joins: These are used to combine rows from two or more tables based on related data.

The Solution

We will break down our solution into several steps:

Step 1: Using Window Functions with Partitioning (MySQL 8.0 and later versions)

SELECT t1.*, 
       ROUND((t1.Grade - t2.Avg) / t2.StDev, 2) AS StDevsAway 
FROM (
    SELECT t1.*, 
           ROW_NUMBER() OVER(PARTITION BY t1.ID, t1.Grade ORDER BY t1.Timestamp) RN
    FROM MyTable1 t1
) t1
JOIN MyTable2 t2 ON t1.ID = t2.ID
WHERE RN = 1;

In this solution:

• We use the ROW_NUMBER() window function to assign a unique number to each row within each partition (ID-Grade combination).
• The rows are ordered by their timestamps.
• We then join MyTable2 with our result set on the ID column.
• Finally, we select only the rows where the row number is 1, which represents the first instance of the grade being X standard deviations away.

Step 2: Using Window Functions without Partitioning (earlier versions)

SELECT t1.*, 
       ROUND((t1.Grade - t2.Avg) / t2.StDev, 2) AS StDevsAway 
FROM MyTable1 t1
JOIN MyTable2 t2 ON t1.ID = t2.ID
WHERE t1.Timestamp = (
    SELECT MIN(t11.Timestamp) FROM MyTable1 t11 WHERE t11.ID = t1.ID AND t11.Grade = t1.Grade
)

In this solution:

• We join MyTable1 with MyTable2 on the ID column.
• We then use a subquery to find the minimum timestamp for each ID-Grade combination.
• Finally, we select only the rows where the timestamp matches the minimum timestamp.

Step 3: Using Joining Tables (alternative approach)

SELECT t1.*, 
       ROUND((t1.Grade - t2.Avg) / t2.StDev, 2) AS StDevsAway 
FROM (
    SELECT t1.*, 
           ROW_NUMBER() OVER(PARTITION BY t1.ID, t1.Grade ORDER BY t1.Timestamp) RN
    FROM MyTable1 t1
) t1
JOIN MyTable2 t2 ON t1.ID = t2.ID
WHERE RN = 1;

This solution is similar to the previous one but uses an alternative approach by joining MyTable2 with our result set.

Conclusion

In this article, we explored how to find the first row for each ID-Grade combination in MySQL. We examined three approaches: using window functions with partitioning (MySQL 8.0 and later versions), using window functions without partitioning (earlier versions), and joining tables (alternative approach).

Last modified on 2024-03-21