Vamos a verlo con un ejemplo. Utilizaré la base de datos World. Supongamos que quiero saber qué paises tienen ciudades que superen los 7.000.000 de habitantes:
mysql> select country.name from country INNER JOIN city ON CountryCode=Code where city.population>7000000;
+--------------------+
| name |
+--------------------+
| Brazil |
| United Kingdom |
| Indonesia |
| India |
| India |
| Japan |
| China |
| China |
| South Korea |
| Mexico |
| Pakistan |
| Turkey |
| Russian Federation |
| United States |
+--------------------+
14 rows in set (0.00 sec)

mysql> select DISTINCT country.name from country INNER JOIN city ON CountryCode=Code where city.population>7000000;
+--------------------+
| name |
+--------------------+
| Brazil |
| United Kingdom |
| Indonesia |
| India |
| Japan |
| China |
| South Korea |
| Mexico |
| Pakistan |
| Turkey |
| Russian Federation |
| United States |
+--------------------+
12 rows in set (0.00 sec)