Question

I have three tables like this

CREATE TABLE  `money`.`categories` (
  `ID` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `Title` text NOT NULL,
  PRIMARY KEY (`ID`)
)

CREATE TABLE  `money`.`expenditure` (
  `ID` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `Purpose` text NOT NULL,
  `Amount` bigint(20) unsigned NOT NULL,
  `Date` datetime NOT NULL,
  `CategoryID` bigint(20) unsigned NOT NULL,
  PRIMARY KEY (`ID`),
  KEY `FK_expenditure_1` (`CategoryID`),
  CONSTRAINT `FK_expenditure_1` FOREIGN KEY (`CategoryID`) REFERENCES `categories` (`ID`)
)

CREATE TABLE  `money`.`income` (
  `ID` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `Source` text NOT NULL,
  `Amount` bigint(20) unsigned NOT NULL,
  `Date` datetime NOT NULL,
  PRIMARY KEY (`ID`)
)

It is actually not complicated at all. Very simple.

I have three queries to find

1. the sum of income for a particular month:
   select sum(amount) tot_inc from money.income ic where month(ic.date) = 02;

2. sum of expenditure for a particular month:
   select sum(amount) tot_exp from money.expenditure ex where month(ex.date) = 02;

3. sum of expenditure for a particular month in a particular category:
   select sum(amount) tot_exp from money.expenditure ex where month(ex.date) = 02 and ex.categoryid = 3;

I have two questions:

1. How can I find out the percentage of total income that has gone away as total expenditure for a particular month. Basically I want to find out (100*tot_exp/tot_inc) from queries 1 and 2. but how can I write it in query?

2. Building on top of that, I want to find out how much percentage each category of expenditure contributed to ina particular month. I understand it is a combination of group by and aggregate and/or math functions but I can’t even begin to visualize how the query should be written. How can I get these numbers?

Answer 1

SELECT 
(100*(SELECT SUM(amount) tot_inc FROM money.income ic WHERE month(ic.date) = "02")/SUM(amount)) AS "Result"
FROM expenditure e 
WHERE MONTH(ex.date) = "02"
GROUP BY CategoryID
HAVING result > 10

I think it would be something like this