Update rows only when a join is unambiguous - sql

Question

I've got two tables, a and b, both with product_name and value. The value column in a is null.
I'd like to update the a table with values from the b table. Because of a quirk in the data, product_name is not unique in either table.
I only want to set the value when there is one unambiguous match on product_name between the two. When more than one row in a has the same product name, or more than one row matches from b, I'd like to keep the value empty. Is there an efficient way to do this in Postgres?
A simpler version of this would be to first identify unique product names in a. Then, update rows where only a single row in b matches -- but I'm also not sure how to write that constraint.

Answer 1

The simple way:
update a
set value = (select min(b.value) from b where b.product_name = a.product_name)
where product_name in (select product_name from a group by product_name having count(*) = 1)
and product_name in (select product_name from b group by product_name having count(*) = 1)
;

Answer 2

You can use aggregation:
update a
set value = b.value
from (select b.product_name, max(b.value) as value
from b
group by b.product_name
having min(b.value) = max(b.value) -- there is only one value
) b
where b.product_name = a.product_name;
Note that this assumes that b.value is not null. It is easy to include logic for null values, if that is needed.

Answer 3

Just put together [How to Select Every Row Where Column Value is NOT Distinct and #Gordon Linoff's answer:
create table a (
id serial primary key
,product_name text
,value int
);
create table b (
id serial primary key
,product_name text
,value int not null
);
insert into a (product_name) values
('A')
,('B')
,('C')
,('D')
,('E')
,('E');
insert into b (product_name,value) values
('A',1)
,('A',1)
,('B',42)
,('C',1)
,('C',2)
,('E',1)
;
update a
set value = b.value
from (select product_name, min(value) as value
from b
group by b.product_name
having 1 = count(*)
) b
where b.product_name = a.product_name
and a.product_name not in
(select product_name
from a
group by product_name
having 1 < count(*));
#Gordon Lindof your answer fails if product_name and value both dupilcated in b (example product_name=A) and misses the requirement product_name not duplicated in a.