mysql how to join a table and do some math

Question

I have these tables, rolls and rollsout. I would like to perform a join and do some math .

rolls

      |size|
      ------
      |3x3| 
      |4x4| 
      |3x3| 
      |3x3| 

rollsout

      |type|
      ------
      |3x3| 

Expected output after SUBTRACTing, JOINing ==>

      |size|Remaining(table1 - table2)|
      --------------------------------
      |3x3|   2                       |                 
      |4x4|   1                       |               

My code:

SELECT tarpaulin.size, COUNT( * ) , tarpaulinout.Ww, tarpaulinout.dc 
FROM tarpaulin LEFT JOIN (SELECT size AS Ww, COUNT( * ) AS dc FROM tarpaulinout 
GROUP BY size) AS tarpaulinout ON tarpaulin.size = tarpaulinout.Ww 
GROUP BY tarpaulin.size

But the O/P for the above code is

      |size|count(*)|size |dc    |
      ----------------------------
      |3x3|3        | 3x3 |1     |
      |4x4|1        | NILL| NILL |

i can get this close, kindly instruct me on how to achieve my desired output. Thanks in advance.

Answer 1

You can just UNION the two sets together, then take a SUM across both, ie

select size, sum(counter) remaining
from
(
    select size, 1 counter
    from rolls
    union all
    select type, -1 counter
    from rollsout
) x
group by size;

Answer 2

You need to group the first table and second table separately on size to get the count of records and then perform a LEFT JOIN on the size attribute to get all the records from tarpaulin table and the matching ones from tarpaulinout table, and do the subtraction, as below.

SELECT a.size, a.rcount - ISNULL(b.rcount, 0)
FROM
    (SELECT size, count(*) FROM tarpaulin GROUP BY size) as a
    LEFT JOIN (SELECT size, count(*) FROM tarpaulinout GROUP BY size) as b
    ON a.size = b.size

Answer 3

Select x.Size,(x.Cnt1 - Isnull(y.Cnt2, 0)) AS Remaining
From
(Select r.Size,count(r.Size) Cnt1
From rolls r
Group by r.Size)x

Left Outer join
(Select ro.Type,Count(ro.Type) Cnt2
From rollsout ro
Group by ro.Type)y

On x.Size = y.Type

Answer 4

SELECT a.size, a.Rcount - b.Acount FROM (SELECT size, COUNT( * ) AS Rcount FROM tarpaulin GROUP BY size ) AS a LEFT JOIN ( SELECT size, COUNT( * ) AS Acount FROM tarpaulinout GROUP BY size ) AS b ON a.size = b.size 

对Vikdor代码的一点修改对我有用，感谢快速输入的人:)