SQL Basics : Calculated Fields

Objective


To learn about calculated fields.

Calculated Fields


Calculated fields are useful to calculate total, averages or other calculations based on table data. The calculated field does not exist in the tables, it is created on demand within a SELECT statement. Why do we need to let the database do the calculation instead of doing it in your client code? Because it is faster to perform the calculations on the database server.

Concatenating Fields


Let's display the name of all the users by concatenating the firstname and lastname.

sqlite> select first_name || ' ' || last_name from users;
first_name || ' ' || last_name
------------------------------
bugs bunny                    
daffy duck                    
porky pig                     
pluto dog                     
pluto pet                     
tasmanian devil               
tweety bird                   
elmer fudd                    
speedy gonzales               
yosemite sam                  
sylvester

Here we are using the two pipes || operator to concatenate firstname and lastname. You can change the name of the resulting header to name by using the AS keyword. The new header 'name' is an alias for the concatenated field.

sqlite> select first_name || ' ' || last_name as name from users;
name      
----------
bugs bunny
daffy duck
porky pig 
pluto dog 
pluto pet 
tasmanian 
tweety bir
elmer fudd
speedy gon
yosemite s
sylvester

The last name is getting cut-off. Let's fix that by specifying the width for the columns.

sqlite> .width 20 20
sqlite> select first_name || ' ' || last_name as name from users;
name                
--------------------
bugs bunny          
daffy duck          
porky pig           
pluto dog           
pluto pet           
tasmanian devil     
tweety bird         
elmer fudd          
speedy gonzales     
yosemite sam        
sylvester           

Performing Calculations


Let's create a line_items table.

sqlite> CREATE TABLE line_items(
   ...> id int primary key not null,
   ...> product_id int not null,
   ...> quantity int,
   ...> price real,
   ...> order_number int);
sqlite> .tables
line_items  products    users

Populate line_times with some sample data.

sqlite> INSERT INTO line_items(id, product_id, quantity, price, order_number)
   ...> VALUES(1, 1, 4, 10, 1);
sqlite> INSERT INTO line_items(id, product_id, quantity, price, order_number)
   ...> VALUES(2, 3, 10, 10, 1);
sqlite> INSERT INTO line_items(id, product_id, quantity, price, order_number)
   ...> VALUES(3, 2, 1, 40, 1);
sqlite> INSERT INTO line_items(id, product_id, quantity, price, order_number)
   ...> VALUES(4, 4, 15, 30, 1);

We now have four rows in line_items table.

sqlite> select * from line_items;
id                    product_id            quantity    price       order_number
--------------------  --------------------  ----------  ----------  ------------
1                     1                     4           10.0        1           
3                     2                     1           40.0        1           
2                     3                     10          10.0        1           
4                     4                     15          30.0        1

Let's calculate lineitemprice for each of the line_items :

sqlite> select product_id, quantity, price, quantity * price AS line_item_price from line_items;
product_id            quantity              price       line_item_price
--------------------  --------------------  ----------  ---------------
1                     4                     10.0        40.0           
2                     1                     40.0        40.0           
3                     10                    10.0        100.0          
4                     15                    30.0        450.0

The last column shows the lineitemprice for each row in the line_items table. You can add, subtract, multiply and divide to perform any mathematical calculations.

Summary


In this article you learned how to use calculated fields and why it is used. You also learned about the aliases for the calculated fields.


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