Understanding the Problem and Query Optimization

In this article, we’ll explore a SQL query that aims to sum the values of two columns (NumInstalled and NumPresent) in each row from two tables joined on a common column. We’ll delve into the provided query, its output, and the expected results, as well as discuss potential optimizations.

The Current Query

The given SQL query is:

SELECT vUI.ArticleID, 
    ISNULL(vUCS.NumInstalled,0)+ISNULL(vUCS.NumPresent,0) AS NumInstalled,
    vUCS.NumPending
FROM v_Update_DeploymentSummary_Live vUCS 
INNER JOIN v_UpdateInfo vUI 
    ON vUCS.CI_ID=vUI.CI_ID
WHERE vUCS.CollectionID='RA00686' 

Output and Expected Results

The current output is:

The expected output, with the sum of NumInstalled and NumPresent, is:

Optimizing the Query

To improve performance, we can create an index on the CollectionID column in the v_Update_DeploymentSummary_Live table.

CREATE NONCLUSTERED INDEX NCX_v_Update_DeploymentSummary_Live_Indx1 ON v_Update_DeploymentSummary_Live(CollectionID)

However, this would not solve the problem of joining two tables. To do so, we’ll need to use a more complex query or denormalize the data.

Using CASE Statements

One possible solution is to use a CASE statement in the query:

SELECT vUI.ArticleID, 
    SUM(CASE WHEN vUCS.NumInstalled > 0 THEN vUCS.NumInstalled ELSE 0 END) + 
    SUM(CASE WHEN vUCS.NumPresent > 0 THEN vUCS.NumPresent ELSE 0 END) AS NumInstalled,
    vUCS.NumPending
FROM v_Update_DeploymentSummary_Live vUCS 
INNER JOIN v_UpdateInfo vUI 
    ON vUCS.CI_ID=vUI.CI_ID
WHERE vUCS.CollectionID='RA00686' 

This query will calculate the sum of NumInstalled and NumPresent separately for each row, effectively “adding” the values if they are greater than 0.

Understanding the Query

Let’s break down the query:

• The outer SELECT statement selects the columns we want to display (ArticleID, NumInstalled, and NumPending).
• The innermost part of the SELECT statement uses a CASE statement to check if the value in the NumInstalled or NumPresent column is greater than 0.
• If it is, the corresponding value is returned; otherwise, 0 is returned.
• The SUM() function then adds up these values for each row.

Indexing

To improve performance, we can create an index on the CollectionID column:

CREATE NONCLUSTERED INDEX NCX_v_Update_DeploymentSummary_Live_Indx1 ON v_Update_DeploymentSummary_Live(CollectionID)

This will help speed up the query by allowing SQL Server to quickly locate rows that match the specified CollectionID.

Conclusion

In this article, we explored a SQL query that aimed to sum the values of two columns (NumInstalled and NumPresent) in each row from two tables joined on a common column. We discussed potential optimizations, including creating an index on the CollectionID column.

We also explored alternative solutions using CASE statements to effectively “add” the values if they are greater than 0.

By applying these techniques, we can improve the performance and efficiency of our SQL queries, ensuring that our data is accurately summed and displayed in a timely manner.

Last modified on 2024-10-13