So far, we’ve learned how to retrieve data from a table using SELECT. At this point, it might look like SQL can only return the exact data that is already stored in the
MSMuhammad SufiyanSoftware Engineer · Jul 26
Backend Engineering HubT-
So far, we’ve learned how to retrieve data from a table using SELECT.
At this point, it might look like SQL can only return the exact data that is already stored in the database.
But SQL is much more powerful than that.
With SQL, we can calculate new values on the fly while fetching data.
These are called calculated columns.
Why Do We Need Calculated Columns?
Let’s look at our cities table again.
We already store:
population
area
From this data, we can calculate a very useful metric:
Population Density
(population ÷ area)
This tells us how many people live per square kilometer in a city.
Instead of manually calculating this for each city, SQL can do it for us.
Calculating Values in a SELECT Query
Let’s write a query that:
selects the city name
calculates population density
select
name,
population / area
from cities;
What’s happening here?
population / area is a calculation
PostgreSQL performs this calculation for every row
The result is shown as a new column in the output
This new column does not exist in the table.
It is created temporarily for this query only.
Temporary or “Imaginary” Columns
You can think of calculated columns as:
temporary
virtual
created only at query time
They:
are not stored in the database
exist only in the query result
Using Other Math Operators
We are not limited to division only.
PostgreSQL supports many math operations, such as:
+ addition
- subtraction
* multiplication
/ division
Example: Adding Two Columns
select
name,
population + area
from cities;
This query adds population and area for each city.
Example: Multiplication (Integer Limit)
select
name,
population * area
from cities;
This will result in an error.
Why?
Because:
PostgreSQL integer values have a limit (~2.14 billion)
multiplying large numbers exceeds that limit
This is a good reminder that:
data types matter, even in calculations.
Column Name Problem (?column?)
When we run this query:
select
name,
population / area
from cities;
PostgreSQL shows the calculated column name as:
?column?
This happens because:
PostgreSQL knows we calculated something
but it doesn’t know what to call it
This name is not very helpful.
Renaming a Calculated Column (Using AS)
To give our calculated column a meaningful name, we use AS.
select
name,
population / area as population_density
from cities;
Now the output column is clearly named:
population_density
Much better 👍
Column Aliases Are Flexible
We can name the column anything we want.
select
name,
population / area as density
from cities;
Both queries work the same — only the column name changes.
Key Points to Remember
SQL can calculate values while fetching data
Calculated columns are temporary
Math operators work directly on column values
Use AS to rename calculated columns
Data type limits still apply during calculations
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Why Do We Need Calculated Columns?
Calculating Values in a