Finding MySQL tables that do not have a PRIMARY key

Some web hosts (such ad DigitalOcean managed databases) are now enforcing that all tables should have a primary key, as lack of a primary key on a table causes performance and replication issues.

To check if any of your tables lack a primary key, you can run the SQL query below. Ideally this query should return an empty result set.

USE INFORMATION_SCHEMA;
SELECT 
    TABLES.table_name
FROM TABLES
LEFT JOIN KEY_COLUMN_USAGE AS c 
ON (
       TABLES.TABLE_NAME = c.TABLE_NAME
   AND c.CONSTRAINT_SCHEMA = TABLES.TABLE_SCHEMA
   AND c.constraint_name = 'PRIMARY'
)
WHERE 
    TABLES.table_schema <> 'information_schema'
AND TABLES.table_schema <> 'performance_schema'
AND TABLES.table_schema <> 'mysql'
AND c.constraint_name IS NULL;

Source / further reading