I worked with MySQL for a long time, so I know how useful to limit the number of returning rows of a query. For example,

Unfortunately, Oracle does not have an operator like LIMIT. On the other hand, we can emulate it by using ROWNUM pseudo column. ROWNUM will tell the row number.

So we can limit the number of returning rows:

Unfortunately, if we add ORDER BY, GROUP BY operators to the query, this method will not work, because rownum will show the row number before “sorting”. So we use rownum in a subquery: