How Can I Perform Complex Calculations Using Nested Calculated Columns in SQL Views?

Using Calculated Columns for Complex Computations in Views

In SQL, calculated columns are a valuable tool for extending data models within views. They allow you to derive new values from existing columns, unlocking possibilities for more sophisticated analysis and presentation. However, what happens when you need to use a calculated column itself as part of a subsequent calculation within the same view?

Consider a common scenario where you have a table with the following columns:

• ColumnA (Number)
• ColumnB (Number)
• ColumnC (Number)

In one of your views, you've created a calculated column called calccolumn1 by simply adding ColumnA and ColumnB. Now, you wish to use calccolumn1 to compute another column, calccolumn2, by dividing calccolumn1 by ColumnC.

Initial Query

Initially, you might attempt to write the query as follows:

Select  
    ColumnA,
    ColumnB,
    ColumnA + ColumnB As calccolumn1
    calccolumn1 / ColumnC as calccolumn2

Unfortunately, this will not work because calccolumn2 cannot reference the previously calculated calccolumn1 directly.

Nested Query Approach

One solution to this problem is to use a nested query:

Select
    ColumnA,
    ColumnB,
    calccolumn1,
    calccolumn1 / ColumnC as calccolumn2
From (
    Select
        ColumnA,
        ColumnB,
        ColumnC,
        ColumnA + ColumnB As calccolumn1
    from t42
);

This method creates a subquery that calculates calccolumn1. The main query can then use calccolumn1 from the subquery in its calculations.

Repeated Calculation Approach

Another option, if feasible, is to simply repeat the calculation for calccolumn1 in the main query:

Select
    ColumnA,
    ColumnB,
    ColumnA + ColumnB As calccolumn1,
    (ColumnA + ColumnB) / ColumnC As calccolumn2
from t42; 

This approach avoids the complexity of a nested query, especially when the calculation for calccolumn1 is relatively simple.

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