Compare data using the year in a query - mysql

Question

I have a data and they are recorded by each year, I am trying to compare two years( the past year and the current year) data within one mysql query
Below are my tables
Cost Items
| cid | items |
| 1 | A |
| 2 | B |
Cost
| cid | amount | year |
| 1 | 10 | 1 |
| 1 | 20 | 2 |
| 1 | 30 | 1 |
This is the result I am expecting when i want to compare the year 1 and year 2. Year 1 is the past year and year 2 is the current year
Results
items | pastCost | currentCost |
A | 10 | 20 |
A | 30 | 0 |
However the below query is what i used by gives a strange answer.
SELECT
IFNULL(ps.`amount`, '0') as pastCost
IFNULL(cs.`amount`, '0') as currentCost
FROM
`Cost Items` b
LEFT JOIN
`Cost` ps
ON
b.cID=ps.cID
AND
ps.Year = 1
LEFT JOIN
`Cost` cu
ON
b.cID=cu.cID
AND
cu.Year =2
This is the result i get from my query
items | pastCost | currentCost |
A | 10 | 20 |
A | 30 | 20 |
Please what am i doing wrong? Thanks for helping.

Answer 1

I'm missing something about your query; the SQL text shown can't produce that result.
There is no source for the items column in the SELECT list, and there is no table aliased as cs. (Looks like the expression in the SELECT list would need to be cu.amount
Aside from that, the results being returned look exactly like what we'd expect. Each row returned from year=2 is being matched with each row returned from year=1. If there were three rows for year=1 and two rows for year=2, we'd get six rows back... each row for year=1 "matched" with each row for year=2.
If (cid, year) tuple was UNIQUE in Cost, then this query would return a result similar to what you expect.
SELECT b.items
, IFNULL(ps.amount, '0') AS pastCost
, IFNULL(cu.amount, '0') AS currentCost
FROM `Cost Items` b
LEFT
JOIN `Cost` ps
ON ps.cid = b.cid
AND ps.Year = 1
LEFT
JOIN `Cost` cu
ON cu.cid = b.cid
AND cu.Year = 2
Since (cid, year) is not unique, you need some additional column to "match" a single row for year=1 with a single row for year=2.
Without some other column in the table, we could use an inline view to generate a value. I can illustrate how we can make MySQL return a resultset like the one you show, one way that could be done, but I don't think this is really the solution to whatever problem you are trying to solve:
SELECT b.items
, IFNULL(MAX(IF(a.year=1,a.amount,NULL)),0) AS pastCost
, IFNULL(MAX(IF(a.year=2,a.amount,NULL)),0) AS currentCost
FROM `Cost Items` b
LEFT
JOIN ( SELECT #rn := IF(c.cid=#p_cid AND c.year=#p_year,#rn+1,1) AS `rn`
, #p_cid := c.cid AS `cid`
, #p_year := c.year AS `year`
, c.amount
FROM (SELECT #p_cid := NULL, #p_year := NULL, #rn := 0) i
JOIN `Cost` c
ON c.year IN (1,2)
ORDER BY c.cid, c.year, c.amount
) a
ON a.cid = b.cid
GROUP
BY b.cid
, a.rn
A query something like that would return a resultset that looks like the one you are expecting. But again, I strongly suspect that this is not really the resultset you are really looking for.
EDIT
OP leaves comment with vaguely nebulous report of observed behavior: "the above solution doesnt work"
Well then, let's check it out... create a SQL Fiddle with some tables so we can test the query...
SQL Fiddle here http://sqlfiddle.com/#!9/e3d7e/1
create table `Cost Items` (cid int unsigned, items varchar(5));
insert into `Cost Items` (cid, items) values (1,'A'),(2,'B');
create table `Cost` (cid int unsigned, amount int, year int);
insert into `Cost` (cid, amount, year) VALUES (1,10,1),(1,20,2),(1,30,1);
And when we run the query, we get a syntax error. There's closing paren missing in the expressions in the SELECT list, easy enough to fix.
SELECT b.items
, IFNULL(MAX(IF(a.year=1,a.amount,NULL)),0) AS pastCost
, IFNULL(MAX(IF(a.year=2,a.amount,NULL)),0) AS currentCost
FROM `Cost Items` b
LEFT
JOIN ( SELECT #rn := IF(c.cid=#p_cid AND c.year=#p_year,#rn+1,1) AS `rn`
, #p_cid := c.cid AS `cid`
, #p_year := c.year AS `year`
, c.amount
FROM (SELECT #p_cid := NULL, #p_year := NULL, #rn := 0) i
JOIN `Cost` c
ON c.year IN (1,2)
ORDER BY c.cid, c.year, c.amount
) a
ON a.cid = b.cid
GROUP
BY b.cid
, a.rn
Returns:
items pastCost currentCost
------ -------- -----------
A 10 20
A 30 0
B 0 0