I want to find the avg of the total of two columns. I want to count the total of col1 and the total of col2 then find the average(how many different rows they are in).
I have managed to come up with a solution in the this sqlfiddle (also see below) is this the best way? I initially thought I would need to use the avg function but couldn't work it out using this.
CREATE TABLE test (
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
uid INT,
col1 INT,
col2 INT
) DEFAULT CHARACTER SET utf8 ENGINE=InnoDB;
INSERT INTO test (id, uid, col1, col2) VALUES
(1,5,8,12),
(2,1,2,3),
(3,1,2,33),
(4,5,25,50),
(5,5,22,3);
(
SELECT ((sum(col1) + sum(col2))/count(*))
FROM test
WHERE uid=5
)
By definition, AVG(col1) = SUM(col1)/COUNT(*)
and AVG(col2) = SUM(col2)/COUNT(*)
, therefore (SUM(col1)+SUM(col2))/COUNT(*)
= AVG(col1) + AVG(col2)
.
Also, the commutativity of addition gives us (SUM(col1)+SUM(col2))/COUNT(*) = SUM(col1+col2)/COUNT(*)
and hence AVG(col1+col2)
.