Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_59731 ( "Name (year commissioned)" text, "Owner/operator" text, "Length" text, "Maximum diameter" text, "From/to" text, "Licence number" text )

### Question ###
What is the license number where the maximum diameter is 400 mm?

### Accurate SQL ###

SELECT "Licence number" FROM table_59731 WHERE "Maximum diameter" = '400 mm'