Update a table from another table using subselect

Question

Using postgresql I have a table A with columns company_name , department_name , some other data, and department .

I have another table B with simply company_name , department_name , and did . (This are two tables with a relationship in between, but I made a view from it to make syntax easier.)

I want to set A.department to B.did of the row with matching names. It is a part of a normalization process.

According to this Q&A I tried:

UPDATE A 
SET department=did 
FROM A AS A 
INNER JOIN B 
  ON A.company_name = B.company_name AND A.department_name=B.department_name;

But I get the result, that A.department for all rows are set to the same value.

(I have also tried different syntax from here , but get syntax errors as expected.)

Why isn't that working and more specifically, why aren't the rows matched properly. When I try a SELECT on that join, I get the expected result.

Answer 1

In Postgres, you want to write the query like this:

UPDATE A 
    SET department = B.did 
FROM B 
WHERE A.company_name = B.company_name AND
      A.department_name = B.department_name;

When you mention the A table in the FROM clause, it is a separate reference to A , nothing to do with the table being updated. The simple solution is to refer to the table only once.