MySQL 8.0

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select version();


version()


8.0.36


CREATE TABLE Table1 (`key` varchar(20), `num` int);

INSERT INTO Table1 VALUES
   ('Katie',23),
   ('Jack',41),
   ('June',43);

Records: 3  Duplicates: 0  Warnings: 0


CREATE TABLE Table2 (`key` varchar(20), `num` int);

INSERT INTO Table2 VALUES
   ('Paty',20),
   ('Jack',21),
   ('June',4);

Records: 3  Duplicates: 0  Warnings: 0


-- result from the UNION ALL is:
SELECT `key`, Num
FROM Table1
UNION ALL
SELECT `key`, Num
FROM Table2


key
Num


Katie
23

Jack
41

June
43

Paty
20

Jack
21

June
4


-- Groupin by `key`:
SELECT
     `key`,
     SUM(Num) as Num
FROM (
     SELECT `key`, Num
     FROM Table1
     UNION ALL
     SELECT `key`, Num
     FROM Table2
     ) x
GROUP BY `key`
ORDER BY `key`


key
Num


Jack
62

June
47

Katie
23

Paty
20