SELECT
Name
FROM country
WHERE Code IN (
SELECT CountryCode
FROM countrylanguage
WHERE Language = 'Russian'
)
| Name |
| Azerbaijan |
| Belarus |
| Estonia |
| Finland |
| Georgia |
| Israel |
| Kazakstan |
| Kyrgyzstan |
| Lithuania |
| ... |
| When processing this query, first it runs the subquery: SELECT CountryCode FROM countrylanguage WHERE Language = 'Russian' Its result is passed to the WHERE clause of the outer query in the comma-delimited format required by the IN operator. |