I have a column: standard BOOLEAN NOT NULL
I would like to enforce one row True, and all others False. The are no FK's or anything else depending on this constraint. I know I can accomplish it with plpgsql, but this seems like a sledgehammer. I would prefer something like a CHECK or UNIQUE constraint. The simpler the better.
One row must be True, they cannot all be False (so the first row inserted would have to be True).
The row will need to be updated, which means I have to wait to check constraints until updates are done, since all rows may be set False first and one row True afterwards.
There is a FK between products.tax_rate_id and tax_rate.id, but it has nothing to do with the default or standard tax rate, which is user selectable to ease creating new products..
PostgreSQL 9.5 if it matters.
Background
The table is the tax rate. One of the tax rates is the default (standard since default is a Postgres command). When a new product is added, the standard tax rate is applied to the product. If there is no standard, the database must either do a guess or all kinds of unneeded checks. The simple solution, I thought, was to make sure there is a standard.
By "default" above, I mean for the presentation layer (UI). There is a user option for changing the default tax rate. I either need to add extra checks to ensure the GUI/user does not try to set the tax_rate_id to NULL, or then just set a default tax rate.