Wordpress code: What does pmwp in SQL statements mean?

Wordpress code: What does pmwp in an SQL statement mean? See the following code: WordPress & nbsp; it has been in use for a long time. some & nbsp; Post & nbsp; have been deleted, but the corresponding & nbsp; Post & nbsp; meta & nbsp; if data still exists, how can I delete these isolated & nbsp; Post & nbsp; Meta & nbsp; wordpress code: What does pm wp in an SQL statement mean?
See the following code:
WordPress has been in use for a long time and some posts have been deleted, but the corresponding Post meta data still exists. how can we delete these isolated Post Meta data? Run the following SQL statement in the database management software:
DELETE pm
FROM wp_postmeta pm
Left join wp_posts wp ON wp. ID = pm. post_id
WHERE wp. ID IS NULL

---- What does "pm" and "wp" mean?
------ Solution --------------------
For SQL commands, pm is the alias of table wp_postmeta, and wp is the table name of table wp_posts.

A little review of database knowledge can be recalled.