如何在表上放置约束以确保表的子集中只有一个布尔列为真？

Question

I'm wondering how I can constrain a particular boolean column to only be true for a certain subset of rows in a table.

in the example below rows with the same values for tuples (id2, 1d3) should only have one true in the default column:

$> SELECT * FROM records WHERE id2 = 2 AND id3 = 3;
$>   id  |  id2  |  id3  |  name  |  default
   ------+-------+-------+--------+---------
      1  |  2    |   3   |  bob   |  false
      2  |  2    |   3   |  jane  |  false
      3  |  2    |   3   |  jim   |  false
      4  |  2    |   3   |  cory  |  true
      5  |  2    |   3   |  alan  |  false


$> SELECT * FROM records WHERE id2 = 4 AND id3 = 5;
$>   id  |  id2  |  id3  |  name  |  default
   ------+-------+-------+--------+---------
      6  |  4    |   5   |  bill  |  false
      7  |  4    |   5   |  fred  |  false
      8  |  4    |   5   |  frank |  false
      9  |  4    |   5   |  dave  |  true
     10  |  4    |   5   |  ryan  |  false

Answer 1

You can do that with a partial unique index:

create unique index on records (id2, id3) where "default";

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Note that default is a reserved word and thus a very bad choice for a column name.

Answer 2

Have a separate table with one row that contains the mainOfferName of the row that you want. Then query as:

select yt.*, (nt.id is not null) as valueFlag from records yt left join
 newtable nt
 on yt.id = nt.id;

When you want to change the value, change the value in newtable.

Note: you can put the query into a view and read-only users will just work.

And, you could implement this with an update trigger on records . Instead of updating records , it would update newtable.