I am trying to merge information contained in 2 different archives but I will try to simplify my problem to its root cause.

I have 2 tables A and B and I want to update a column in A with values from a column of B if the value in B is not null, otherwise I have to preserve the value in A. A and B are not direclty related but I have the information I need to identify which elements of A are also present in B.

To avoid perform all operations in a single and really complex query I've added a Foreign Key in table B where I store, if present, the key value of the corresponding element in A.

At this point my idea was to perform an update joining the tables:


Unfortunately for me this doesn't work in all cases because in table B there can be more rows that refers to a single row of table A. For this scope lets assume that only one of them has a non null value.

Table A
| PK | value |
| 1  |   X   |

Table B
| PK | value | A_FK |
|  n |   Y   |  1   |
|  m |  null |  1   |

In this case the value in table A is not modified and is still X.

I hoped that performing the update process it would have run the update 2 times for my row in A and that my result would have been the following but that's not the case.

Table A
| PK | value |
| 1  |   Y   |

I really would like to avoid resorting to a cursor to iterate over all rows, so can I modify my query to obtain this result?

  • How do you decide to apply n before m or vice versa?
    – Rick James
    Jan 15, 2021 at 18:10
  • 1
    In this case the value in table A is not modified and is still X. No. UPDATE: Each matching row is updated once, even if it matches the conditions multiple times - but it is not defined what matching value will be used. So maybe value will stay unchanged, maybe it will change - noway to predict. can I modify my query to obtain this result? You must create a subquery which will select only one matched row from table B, and use it in UPDATE instead of the table.
    – Akina
    Jan 15, 2021 at 18:42
  • @Akina that's what I was missing. Thanks
    – Zucch
    Jan 18, 2021 at 7:36

1 Answer 1


Combine the rows of B (GROUP BY a_fk); in the process decide whether to keep Y or NULL.

Store the result of that into a temp table.

Then do the original UPDATE, but against the temp table.

So two statements, no looping:

    SELECT ...
        GROUP BY a_fk;
    SET A... = t...;

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