How do I add two column values in a table with CQL? - cassandra

Question

I am needing to add two values together to create a third value with CQL. Is there any way to do this? My table has the columns number_of_x and number_of_y and I am trying to create total. I did an update on the table with a set command as follows:
UPDATE my_table
SET total = number_of_x + number_of_y ;
When I run that I get the message back saying:
no viable alternative at input ';'.

Answer 1

Per docs. assignment is one of:
column_name = value
set_or_list_item = set_or_list_item ( + | - ) ...
map_name = map_name ( + | - ) ...
map_name = map_name ( + | - ) { map_key : map_value, ... }
column_name [ term ] = value
counter_column_name = counter_column_name ( + | - ) integer
And you cannot mix counter and non counter columns in the same table so what you are describing is impossible in a single statement. But you can do a read before write:
CREATE TABLE my_table ( total int, x int, y int, key text PRIMARY KEY )
INSERT INTO my_table (key, x, y) VALUES ('CUST_1', 1, 1);
SELECT * FROM my_table WHERE key = 'CUST_1';
key | total | x | y
--------+-------+---+---
CUST_1 | null | 1 | 1
UPDATE my_table SET total = 2 WHERE key = 'CUST_1' IF x = 1 AND y = 1;
[applied]
-----------
True
SELECT * FROM my_table WHERE key = 'CUST_1';
key | total | x | y
--------+-------+---+---
CUST_1 | 2 | 1 | 1
The IF clause will handle concurrency issues if x or y was updated since the SELECT. You can than retry again if applied is False.
My recommendation however in this scenario is for your application to just read both x and y, then do addition locally as it will perform MUCH better.
If you really want C* to do the addition for you, there is a sum aggregate function in 2.2+ but it will require updating your schema a little:
CREATE TABLE table_for_aggregate (key text, type text, value int, PRIMARY KEY (key, type));
INSERT INTO table_for_aggregate (key, type, value) VALUES ('CUST_1', 'X', 1);
INSERT INTO table_for_aggregate (key, type, value) VALUES ('CUST_1', 'Y', 1);
SELECT sum(value) from table_for_aggregate WHERE key = 'CUST_1';
system.sum(value)
-------------------
2