如何联接两个表并基于不同的列在Mysql中汇总它们的列

Question

I am joining two tables by the regNo column. I want to add the points of Table1.points and Table2.points where the regNo matches and just incase it doesn't match I also want it be included with its points in the list as shown in the image bellow

I have read through the existing problems but not finding the solution to this eg How can I sum columns across multiple tables in MySQL?

(
    SELECT `Ex`.regNo,(`In`.`points`+`Ex`.`points`) AS 'Points' 
    FROM Table1`In` 
    LEFT JOIN Table2`Ex` ON `In`.`regNo` = `In`.`regNo`
)
UNION
(
    SELECT`Ex`.regNo,(`In`.`points`+`Ex`.`points`) AS 'Points' 
    FROM Table1`In` 
    RIGHT JOIN Table2`Ex` ON `In`.`regNo` = `In`.`regNo`
);

I want it to give the list arranged as per unique (DISTINCT) regNo

Answer 1

You need UNION followed by GRoUP BY :

SELECT regNo, SUM(points) AS total
FROM (
    SELECT regNo, points
    FROM Table1

    UNION ALL

    SELECT regNo, points
    FROM Table2
) AS u
GROUP BY regNo

Answer 2

You are looking for a FULL JOIN between both tables.

SELECT 
    COALESCE(t1.id, t2.id) id
    COALESCE(t1.regNo, t2.regNo) regNo
    COALESCE(t1.points, 0) +  COALESCE(t2.points 0) points
FROM
    table1 t1
    FULL JOIN table2 t2 on t1.regNo = t2.regNo

NB : you did not specify what you expect to be done to generate the new id so by default the above query will display the table1.id if available, else table2.id .

If you would better generate a new, auto-incremented field, then :

SET @i=0;
SELECT 
    @i:=@i+1 id
    COALESCE(t1.regNo, t2.regNo) regNo
    COALESCE(t1.points, 0) +  COALESCE(t2.points 0) points
FROM
    table1 t1
    FULL JOIN table2 t2 on t1.regNo = t2.regNo

Answer 3

Please check this. You need to use full outer join and null replacement before aggregation

select 
   COALESCE(table1.regno, table2.regno) regno,
   sum(COALESCE(table1.points,0)) + sum(COALESCE(table2.points,0)) points
from Table1 
full outer join Table2 
  on table1.regno = table2.regno
group by 
  COALESCE(table1.regno, table2.regno)