Question

I’m using SQL to perform a basic calculation and I can’t figure out the correct syntax. I have a table where I want to add a new column, and to populate the values of that column using a combination of the existing values. Here is the code that I am illustrating the problem with.

-- create a table
CREATE TABLE test (
x numeric(10,3),
y numeric(10,3)
);

-- add some sample values
INSERT INTO test (x,y) VALUES( 7,3 );
INSERT INTO test (x,y) VALUES( 8,4 );

-- add a new column
ALTER TABLE test ADD testcalc numeric(10,3);

-- values in new column (testcalc) using the sum of values from x and y
INSERT INTO 
    test (testcalc) 
SELECT 
    t.x + t.y
FROM
    test as t;

This produces the following table:

I understand that the values are being inserted as new rows, but how can I add them as values to my column so that the table is structured like this?

x | y | testcalc
7 | 3 | 10
8 | 4 | 12

Answer 1

You need to use UPDATE instead of INSERT in such a way in the last part of the query:

UPDATE test SET
testcalc = x + y

SELECT * FROM test