MySQL Division From Same Column with Different Keys - mysql

Question

I have a table with 5 fields
id = this is autoincrementing field
form_id = this field is the form being filled out
entry_id = this is the identifier of the entry
meta_key = this is the identifier of the specific form fields
meta_value = this is the value entered into the form field
If I do a:
SELECT SUM(meta_value) FROM tbl WHERE meta_key = 55;
I get the sum of all the people who like apples.
If I do a:
SELECT SUM(meta_value) FROM tbl WHERE meta_key = 75;
I get the sum of all the people total.
I am trying to write one query that will give me the % of people who like apple so in essence ((RESULT of Q1) / (RESULT of Q2)) * 100
So far I've looked into self joins and subqueries but I'm essentially stuck now because all the tutorials on subqueries focus on the WHERE clause.

Answer 1

Simplest thing I can think of:
SELECT
(SELECT SUM(meta_value) FROM tbl WHERE meta_key = 55) /
(SELECT SUM(meta_value) FROM tbl WHERE meta_key = 75) * 100
as percentage;
Would that work for you?

Answer 2

You can try this query, it avoid any possibility of division by zero, just in case total = 0.
SELECT
CASE
WHEN b.sum_b IS NULL OR b.sum_b = 0 THEN 0
ELSE (a.sum_a / b.sum_b) * 100
END as percent
FROM (
SELECT SUM(meta_value) as sum_a FROM tbl WHERE meta_key = 55
) a,
(
SELECT SUM(meta_value) as sum_b FROM tbl WHERE meta_key = 75
) b

Answer 3

You can use conditional aggregation
SELECT (SUM(CASE WHEN meta_key = 55 THEN meta_value ELSE 0 END) /
SUM(CASE WHEN meta_key = 75 THEN meta_value ELSE 0 END)) * 100 -- be careful you may got divide by zero error
FROM tbl
WHERE meta_key IN (55, 75);
However, this would produce integer division result. So, you can include 1.0 before division if you want exact result.

Answer 4

To avoid division by zero, I would simply recommend:
SELECT (SUM(CASE WHEN meta_key = 55 THEN meta_value ELSE 0 END) /
SUM(CASE WHEN meta_key = 75 THEN meta_value END)
) * 100
FROM tbl
WHERE meta_key IN (55, 75);
Note that the second CASE has no ELSE. If there are no matches, then this returns NULL, avoiding the divide-by-zero.
That said, MySQL does not return an error for divide-by-zero. It simply returns NULL. But that is highly database-specific behavior. I think every other database returns an error and you should be conscious of such potential issues.