2 different small query vs 1 query with subquery

I have table like this ``` name | personal_number ----------------------------------------- Jon | 222 Alex | 555 Jon | 222 Jimmy | 999 ``` I need get every name, which personal_number repeates in table more than 1, that is result must be: ``` Jon Jon ``` So, Variant 1): ``` SELECT name FROM mytable WHERE personal_number IN ( SELECT personal_number FROM mytable GROUP BY personal_number HAVING COUNT(*) > 1 ) ``` Variant 2): ``` SELECT personal_number FROM mytable GROUP BY personal_number HAVING COUNT(*) > 1 ) ``` Then, using php, retrieved personal_numbers join as string (soemthing like this `'222', '222'` ) and run other query ``` SELECT name FROM mytable WHERE personal_number IN( here joined string ) ``` Variant 2 works approximately 10 times faster, than variant 1, this is surprise for me, I was thinking that one query will be faster, but... (In table is 500 000 rows, column `personal_number` not indexed) So, what you mean about cases like this? why variant 2 is many faster than variant 1 ?