Write SQL query to solve given problem: What are the names of all the publishers who have published at least 30 books?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT T2.publisher_name FROM book AS T1 INNER JOIN publisher AS T2 ON T1.publisher_id = T2.publisher_id GROUP BY T2.publisher_name HAVING COUNT(T2.publisher_name) >= 30