updating values of a table from another table through a join changes all values to 1 single value - sql

Question

I have 3 tables: raw_sales, sales and details. raw_sales is being populated using COPY from a txt file. All the fields in raw_sales are either string or text. After importing, we run an sql to populate sales and details. There is a foreign key (sale_id) in details. Here's a sample INSERT command that we use to populate sales and details.
INSERT INTO sales (source, source_identifier)
(SELECT DISTINCT
'FOO' AS source,
"identifier" AS source_identifier
FROM raw_sales
LEFT JOIN sales
ON sales.source_identifier = raw_sales.identifier
AND sales.source = 'FOO'
WHERE sales.id IS NULL
AND identifier IS NOT NULL);
INSERT INTO details (sale_id, description)
(SELECT DISTINCT
sales.id AS sale_id,
"improvements" as description
FROM raw_sales
JOIN sales
ON sales.source_identifier = raw_sales.identifier
AND sales.source = 'FOO'
LEFT JOIN details AS existing
ON existing.sale_id = sales.id
WHERE existing.id IS NULL
AND "improvements" != '');
This seems to work fine. After this, there's another sql that's being ran to update existing tables. The query is as follows
UPDATE details SET
description = "improvements"
FROM raw_sales
JOIN sales
ON sales.source_identifier = raw_sales.identifier
AND sales.source = 'FOO'
JOIN details AS existing
ON existing.sale_id = sales.id
WHERE existing.id IS NOT NULL;
This query updates all rows in the details table to a single value, the first non-empty value from raw_sales table. How can I change the above sql so that it updates the existing records in the details table?

Answer 1

There are several problems with your query:
if details.id is a primary key (field id typically is), then what is the point in comparing it to NOT NULL? You're not using any left joins, there is no way it could possibly be NULL if it truly is an identifier.
UPDATE table t SET ... FROM ... requires linking the table t with something on the FROM section, but you're not, therefore each row of table will be updated to any single random row from the FROM results.
Perhaps you want to do this:
UPDATE details SET
description = "improvements"
FROM raw_sales
JOIN sales ON (sales.source_identifier = raw_sales.identifier AND sales.source = 'FOO')
JOIN details AS existing ON (existing.sale_id = sales.id)
WHERE existing.id = details.id;