MySQL 8.0

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CREATE TABLE tab1 (
  `ID` INTEGER,
  `Value_ID` INTEGER,
  `date` VARCHAR(10)
);

INSERT INTO tab1
  (`ID`, `Value_ID`, `date`)
VALUES
  ('1', '1', '01-02-2020'),
  ('2', '2', '02-02-2020'),
  ('3', '3', '04-06-2020'),
  ('4', '2', '01-03-2019');
CREATE TABLE tab2 (
  `ID` INTEGER,
  `Value_ID` INTEGER,
  `date` VARCHAR(10)
);

INSERT INTO tab2
  (`ID`, `Value_ID`, `date`)
VALUES
  ('1', '3', '01-03-2018'),
  ('2', '4', '07-02-2019'),
  ('3', '5', '10-02-2020'),
  ('4', '4', '09-02-2020');
SELECT COALESCE(t2.`Value_ID`,t1.`Value_ID`) as `Value_ID`,IF (IFNULL(t1.`date`,'31-12-9999') <= t2.`date`,t1.`date`,t2.`date`)
FROM (SELECT `Value_ID`, MIN(`date`) as `date` FROM tab1 GROUP BY `Value_ID`) t1 
right JOIN (SELECT `Value_ID`, MIN(`date`) as `date` FROM tab2 GROUP BY `Value_ID`) t2 
ON t1.`Value_ID` = t2.`Value_ID`
UNION
SELECT COALESCE(t1.`Value_ID`,t2.`Value_ID`),IF (t1.`date` <= IFNULL(t2.`date`,'31-12-9999'),t1.`date`,t2.`date`)
FROM (SELECT `Value_ID`, MIN(`date`) as `date` FROM tab1 GROUP BY `Value_ID`) t1 
LEFT JOIN (SELECT `Value_ID`, MIN(`date`) as `date` FROM tab2 GROUP BY `Value_ID`) t2 
ON t1.`Value_ID` = t2.`Value_ID`

ORDER BY `Value_ID`


Value_ID
IF (IFNULL(t1.`date`,'31-12-9999') <= t2.`date`,t1.`date`,t2.`date`)


1
01-02-2020

2
01-03-2019

3
01-03-2018

4
07-02-2019

5
10-02-2020