MySQL "INSERT ... WHERE NOT EXISTS" if you don't have/know a primary/unique key

MySQL_INSERT_WHERE_NOT_EXISTS_non_indexed

INSERT INTO my_table (unique_but_non_indexed_col1, col2, col3)
SELECT * FROM (SELECT 'unique_val1', 'val2', 'val3') AS tmp
WHERE NOT EXISTS (
  SELECT id FROM my_table WHERE unique_but_non_indexed_col1='unique_val1'
) LIMIT 1;

"my_table", "unique_but_non_indexed_col1", "col2", and "col3" are placeholders for the table and the columns needed, respectively.
"tmp" is not a placeholder per se but the name of a temporary table (could be anything).
"id" is the name of the primary key field in "my_table" ("id" is the convention) - it is not a placeholder.

Probably not very performant, but useful for run-rarely scripts