How does this SQL query to update a row if exists, or insert if not, work?
Are you sure that is updating using only that piece of code?
What is happing is that you are doing left join with the table_name (the table to insert new records) and filtering only for rows that doesn't exist in that table. (WHERE B.id IS NULL)
Is like doing "not exists", only in a different way.
I hope my answer could help you.
Regards.
The LEFT JOIN/IS NULL means that the query is INSERTing records that don't already exist. That's assuming the table defined on the INSERT clause is the same as the one in the LEFT JOIN clause - careful about abstracting...
I'm interested to know what %s is