How to use NOT EXISTS & UNION ALL at the same time?

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so i wanna join two of my table together and the result must be like if table A exist then choose table A else Table B

What I have tried:

Select 
      id, 
      name,
      age

FROM TableA as a
WHERE NOT EXISTS ( SELECT * FROM TableB WHERE a.id = b.id)

UNION ALL

Select 
      id, 
      name,
      age

FROM TableB as b;

is it the correct way to use NOT EXISITS and UNION ALL?

Posted 31-Aug-20 23:03pm

BBO001

Updated 31-Aug-20 23:31pm

v2

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Richard Deeming · Answer 1 · 2020-08-31T23:32:00

Quote:
if table A exist then choose table A else Table B

The code you've shown would do the opposite - it will only return the record from table A if there isn't a matching record in table B.

Try something like this:

SQL

SELECT
    id,
    name,
    age
FROM
    TableA

UNION ALL

SELECT
    id,
    name,
    age
FROM
    TableB As b
WHERE
    NOT EXISTS
    (
        SELECT *
        FROM TableA As a
        WHERE a.id = b.id
    )
;