Add a paragraph before 'read more' as an excerpt to a WordPress post

wp_insert_excerpt.sql

/*Select the needed part (excluding <p> tags and <!-- wp:paragraph --> line*/
SELECT
	REPLACE(REPLACE(SUBSTRING_INDEX(SUBSTRING_INDEX(post_content, '\n', 2), '\n', -1), '<p>', ''), '</p>', '') as excerpt
FROM
	wp_posts wp
WHERE
	ID = 1771;
  
/*Insert that part as an excerpt for a certain post*/
UPDATE
	wp_posts
SET
	post_excerpt = (
	SELECT
		REPLACE(REPLACE(SUBSTRING_INDEX(SUBSTRING_INDEX(post_content, '\n', 2), '\n', -1), '<p>', ''), '</p>', '')
	FROM
		wp_posts
	WHERE
		ID = 1771)
WHERE
	ID = 1771;

/* Insert the excerpt for all posts without one */
UPDATE
	wp_posts wp
JOIN (
	SELECT
		ID as id,
		REPLACE(REPLACE(SUBSTRING_INDEX(SUBSTRING_INDEX(post_content, '\n', 2), '\n', -1), '<p>', ''), '</p>', '') as excerpt
	FROM
		wp_posts
	WHERE
		post_status = 'publish'
		AND post_excerpt = ''
		AND post_type = 'post'
) a
ON
	wp.id = a.id SET
	wp.post_excerpt = a.excerpt;