SQL
Joins
Return a list of city–airport code pairs from the `city_airport` table where the airport name **is available** in the `airport_names` table:  
```sql
SELECT *
FROM city_airport
SEMI JOIN airport_names
USING (iata);
```  
| city_name | iata |
|-----------|------|
| Amsterdam | AMS  |
| Rotterdam | RTM  |  
This query is equivalent with:  
```sql
SELECT *
FROM city_airport
WHERE iata IN (SELECT iata FROM airport_names);
```