Getting the Difference from Two SELECTs

In this article, we will explore how to calculate the difference between two columns in two different tables. The question is straightforward: given two tables with no common key, how can we find the difference between two specific columns?

Understanding the Problem

The problem presented is a classic example of needing to join two tables based on non-unique columns. In this case, we have two tables, MonitorIntervalData and InverterIntervalData, each containing data about monitors and inverters, respectively. We want to find the difference between the gridP column in both tables.

Approach 1: Using Subqueries

The first approach attempted by the user is to use subqueries within a single SELECT statement. The idea is to compare two columns directly without joining the tables. However, this method does not work as expected because SQL Server treats the subqueries as separate queries and doesn’t allow direct comparison of column values.

SELECT 
    (SELECT 
         FORMAT(dbo.InverterIntervalData.timestamp, 'dd-MM.yyyy HH:mm') AS Time, 
         dbo.InverterIntervalData.gridP AS InverterP 
     FROM dbo.InverterIntervalData) -
    (SELECT 
         FORMAT(dbo.MonitorIntervalData.timestamp, 'dd-MM.yyyy HH:mm') AS Time, 
         dbo.MonitorIntervalData.gridP AS MonitorP 
     FROM dbo.MonitorIntervalData) AS Difference

Approach 2: Joining the Tables

The second approach involves joining the two tables based on their non-unique columns. This requires a common column that exists in both tables, but in this case, there isn’t one explicitly mentioned.

However, we can use a creative approach where we compare the gridP values directly by assuming they are related between the two records (as hinted at in the answer).

SELECT 
    dbo.MonitorIntervalData.timestamp as time,
    dbo.MonitorIntervalData.gridP AS MonitorP,
    dbo.InverterIntervalData.timestamp, 
    dbo.InverterIntervalData.gridP AS InverterP ,
    dbo.MonitorIntervalData.gridP - dbo.InverterIntervalData.gridP as Difference
FROM 
    dbo.MonitorIntervalData inner join dbo.InverterIntervalData
    ON FORMAT(dbo.MonitorIntervalData.timestamp, 'dd-MM.yyyy HH:mm') = FORMAT(dbo.InverterIntervalData.timestamp, 'dd-MM.yyyy HH:mm')
    ORDER BY timestamp

Approach 3: Using DATEDIFF

The correct approach to finding the difference between two dates is by using the DATEDIFF function. This function returns the difference in days, weeks, months, or years between two dates.

However, this method requires a common date field that exists in both tables and can be used as a join condition. In our case, we need to consider that both records are related with the gridP column.

SELECT 
    FORMAT(dbo.MonitorIntervalData.timestamp, 'dd-MM.yyyy HH:mm') AS MTime, 
    dbo.MonitorIntervalData.gridP AS MonitorP ,
    FORMAT(dbo.InverterIntervalData.timestamp, 'dd-MM.yyyy HH:mm') AS ITime, 
    dbo.InverterIntervalData.gridP AS InverterP ,
    DATEDIFF( month, dbo.MonitorIntervalData.timestamp, dbo.InverterIntervalData.timestamp) as Difference
FROM 
    dbo.MonitorIntervalData 
INNER JOIN 
    dbo.InverterIntervalData ON dbo.InverterIntervalData.gridP = dbo.MonitorIntervalData.gridP

Conclusion

In this article, we explored the differences between three approaches to finding the difference between two columns in two different tables. We discussed how subqueries don’t work as expected and that joining the tables based on non-unique columns requires creative thinking.

The correct approach involves using DATEDIFF with a common date field and assuming a relationship between the records based on their values.

Additional Context

• The FORMAT function is used to format dates in a specific way.
• The INNER JOIN clause is used to join two tables based on a common column.
• The DATEDIFF function returns the difference in days, weeks, months, or years between two dates.

Final Code

SELECT 
    FORMAT(dbo.MonitorIntervalData.timestamp, 'dd-MM.yyyy HH:mm') AS MTime, 
    dbo.MonitorIntervalData.gridP AS MonitorP ,
    FORMAT(dbo.InverterIntervalData.timestamp, 'dd-MM.yyyy HH:mm') AS ITime, 
    dbo.InverterIntervalData.gridP AS InverterP ,
    DATEDIFF( month, dbo.MonitorIntervalData.timestamp, dbo.InverterIntervalData.timestamp) as Difference
FROM 
    dbo.MonitorIntervalData 
INNER JOIN 
    dbo.InverterIntervalData ON dbo.InverterIntervalData.gridP = dbo.MonitorIntervalData.gridP

Last modified on 2025-02-23