Help with SQL query

I am not sure if this is the right place to ask for this but I would appreciate help from anyone who is comfortable with MySQL.

We used to have a plugin to answer user queries (faqtastic). Now its developer has stopped development since long and we want to use the plugin availble at wpmudev.

But there are thousands of answered questions in the faqtastic table that we dont want to lose and we dont want to load the redundant plugin too. So we decided to migrate data from the faqtastic table to the wp_posts table.

The faqtastic plugin keeps the question, answer and user in its table but creates a post in the main wp_posts tables with shortcode [faq answer] in wp_posts.post_content linking the post id to the question in the faqtastic table.

Since the post for each question is already there, we just need to concat and copy the data from the faqtastic table to the wp_posts table.

I am not good at writing sql queries and dont want to corrupt the db with a wrong query. I created these queries but they give syntax errors. Can someone please help me with the correct syntax?

UPDATE wp_posts
SET post_content = SELECT CONCAT('Posted by: ', wp_faqtastic_questions.author_name, '<br/><br/>', wp_faqtastic_questions.question, '<hr/>', wp_faqtastic_questions.answer)
  FROM wp_faqtastic_questions WHERE wp_faqtastic_questions.page_id= wp_posts.id;

or

update wp_posts
INNER JOIN wp_faqtastic_questions
        on wp_posts.id=wp_faqtastic_questions.page_id
SET wp_posts.post_content = SELECT CONCAT('Posted by: ', wp_faqtastic_questions.author_name, '<br/><br/>', wp_faqtastic_questions.question, '<hr/>', wp_faqtastic_questions.answer)
WHERE wp_posts.post_content='[faq answer]';

or

something else