Find the average of two combined columns in sql

Question

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
    )

Solution 1

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).

Solution 2

To use the avg function,

SELECT avg(col1 + col2)
FROM test
WHERE uid=5;

SQLFIDDLE DEMO

Solution 3

SELECT avg(col1 + col2) as avgtotal

FROM test
WHERE uid=5

Solution 4

i got my answer here , so i will add this note which may help others:

1.avg(col1+col2) as avg_col1_plus_col2,
2.avg(col1) + avg(col2) as avg_col1_plus_avg_col2,
3.avg(col1+col2)/2 as avgTotal1, 
4.avg(col1)/2+avg(col1)/2 as avgTotal2

sentence 1 is equal to sentence 2 as eggyal explained,grammar is ok but logically its not the result that we want, so we need to divide the average by columns numbers as in sentence 3 and 4.

View more solutions

Comments

[luckystars]

Why use group by? The uid is primary key.

[luckystars]

So SELECT avg(col1) + avg(col2) FROM test WHERE uid=5 should work.

[Orangecrush]

My bad. Was trying something else and forgot to take that off. Edited answer now.

[Ben]

This is so close, and it's a great approach, but it's incorrect. The sum of two averages is not the average of two sums. The sum of two averages divided by 2 is the average of two sums. consider this pseudocode: avg(1,2,3) + avg(1,2,3) == 4, but avg(1,2,3,3,2,1) == 2 == avg(avg(1,2,3),avg(1,2,3)) == (avg(1,2,3) + avg(1,2,3)) / 2

[eggyal]

@Ben: none of your cases that equal 2 appear in my answer.

[DependencyHell]

missing closing parenthesis