Why is "LIMIT 0" even allowed in MySQL SELECT statements?

limit 0 can be used to get the same columns types of other tables

create table newtable
select col1 from table1 limit 0;

That way, a hard-coded description of the columns types for newtable is not needed, ensuring that the columns types will still match even if a change occurs in the description of table1 before creating newtable

It also works with a more complete statement, involving indexes, engine, multiple tables, etc

create table newtable (primary key (col1)) engine=memory
select col1,col2,col3 from table1,table2 limit 0;

From the MySQL documentation

LIMIT 0 quickly returns an empty set. This can be useful for checking the validity of a query. When using one of the MySQL APIs, it can also be employed for obtaining the types of the result columns.