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Editorial Team
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Asked: 2026-06-17T23:56:49+00:00

I’m re-posting a more clear version of a previous question I deleted. As the

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I’m re-posting a more clear version of a previous question I deleted.

As the example below shows, I want to combine table_b into table_a only if table_b‘s id doesn’t appear in table_a‘s table_b_id column. Each table should have a null value for any fields it doesn’t have.

Example:

table_a

+----+-------+------------+
| id | name  | table_b_id |
+----+-------+------------+
| 0  | mike  | 1          |
+----+-------+------------+
| 1  | jack  | 1          |
+----+-------+------------+

table_b

+----+-------+
| id | name  |
+----+-------+
| 0  | tom   |
+----+-------+
| 1  | joe   |
+----+-------+

Result:

+-------------+-------+------------+
| original_id | name  | table_b_id |
+-------------+-------+------------+
| 0           | mike  | 1          |
+-------------+-------+------------+
| 0           | tom   |            |
+-------------+-------+------------+
| 1           | jack  | 1          |
+-------------+-------+------------+
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1 Answer

  1. Editorial Team

    Your question can almost directly be translated into a query. You want to see table A queried but supplemented with rows from B for which id of B is not in any table_b_id of A. So the following UNION combines A with the subset of B where id of B is not in table_b_id:

    SELECT table_a.id AS original_id, table_a.name, table_a.table_b_id FROM table_a
UNION ALL
SELECT table_b.id AS original_id, table_b.name, NULL AS table_b_id FROM table_b
  WHERE table_b.id NOT IN (SELECT table_a.table_b_id FROM table_a)

    Note that for the second set we select NULL as table_b_id because A does not have this field.

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