[PostgreSQL] How to update data from a sub-query

Problem
I have two tables, project and intent, in my database.

I import data into intent table, but found out I mistype some data in intent.value column. All data in intent.value should start with qa_.

How to iron out this problem using update statement?

How-To
Here has an example to fix this problem:

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update intent set value = source.new_value from( select id as intent_id, value, 'qa_' || value as new_value, project_id as proj_id from intent where project_id= (select id from project where identifier='test') and substring(value, 1, 3) <> 'qa_' ) as source where id = source.intent_id and project_id = source.proj_id