Filtering by Subquery 按子查询筛选

  1. SELECT
  2.     Name
  3. FROM country
  4. WHERE Code IN (
  5.     SELECT CountryCode
  6.     FROM countrylanguage
  7.     WHERE Language = 'Russian'
  8. )

Subqueries as Calculated Fields 子查询作为计算字段

SELECT
    Name,
    (
        SELECT Count(*)
        FROM countrylanguage
        WHERE CountryCode = Code
    ) AS languages
FROM country

Fully Qualified Column Name 完全限定列名

SELECT
    Name,
    (
        SELECT Count(*)
        FROM countrylanguage
        WHERE
            countrylanguage.CountryCode = country.Code
    ) AS languages
FROM country