列别名引发错误：没有这样的列

Question

此查询有望在Mysql中运行：

SELECT category AS category_name, COUNT(id) AS total_books,
(SELECT COUNT(language) FROM books WHERE language = 'lang1' AND category = category_name) AS lang1,
(SELECT COUNT(language) FROM books WHERE language = 'lang2' AND category = category_name) AS lang2,
(SELECT COUNT(language) FROM books WHERE language = 'lang3' AND category = category_name) AS lang3
FROM books GROUP BY category

但是在SQLite中，它引发错误：房间[SQLITE_ERROR] SQL错误或缺少数据库（没有这样的列：category_name）

为什么会这样？对于此查询，SQLite是否需要与Mysql不同的方法？

Answer 1

MySQL允许使用别名似乎更为宽松。您可以尝试使用更明确的别名来重写查询，如下所示：

SELECT
    category AS category_name,
    COUNT(id) AS total_books,
    (SELECT COUNT(language) FROM books b2
     WHERE language = 'lang1' AND b2.category = b1.category) AS lang1,
    (SELECT COUNT(language) FROM books b2
     WHERE language = 'lang2' AND b2.category = b1.category) AS lang2,
    (SELECT COUNT(language) FROM books b2
     WHERE language = 'lang3' AND b2.category = b1.category) AS lang3
FROM books b1
GROUP BY category;

但是写一个更好的方法可能是用条件聚合代替相关的子查询：

SELECT
    category AS category_name,
    COUNT(id) AS total_books,
    COUNT(CASE WHEN language = 'lang1' THEN 1 END) AS lang1,
    COUNT(CASE WHEN language = 'lang2' THEN 1 END) AS lang2,
    COUNT(CASE WHEN language = 'lang3' THEN 1 END) AS lang3
FROM books
GROUP BY category;