表连接

SELECT
    country.Name,
    countrylanguage.Language
FROM country, countrylanguage
WHERE
    country.Code = countrylanguage.CountryCode


Name Language
Aruba Dutch
Aruba English
Aruba Papiamento
Aruba Spanish
Afghanistan Balochi
Afghanistan Dari
Afghanistan Pashto
Afghanistan Turkmenian
Afghanistan Uzbek
...

SELECT 语句的 FROM 子句里列了两张表：country 和 countrylanguage。
两张表由 WHERE 子句按国家代码正确地连接起来。