如何改进 MySQL 中的 SELECT 子查询？

Question

我有一个 table 学生和第二本能读的书。我正在尝试获取学生名单以及他们拥有的书籍数量。

"SELECT 
    StudentID, 
    FullName,
    book_count.ct
FROM 
    students
LEFT JOIN (
    SELECT
        StudentID,
        COUNT(DISTINCT id) AS ct
    FROM
        books
    GROUP BY
        StudentID
    ) AS book_count
ON 
    book_count.StudentID = students.StudentID
ORDER BY
    students.FullName ASC
LIMIT 0, 30;

此查询到运行大约需要 6 秒，并且随着添加更多书籍，它会变得越来越慢。查看查询配置文件 >90% 的时间花在 "copying to temp table" 上。问题是我的子 select 查询获取书籍搜索的数量并计算整本书的总数 table 而不管我可能会查找多少学生，在我的例子中是 30 个。我该如何提高这方面的表现？谢谢。

Answer 1

试试这个：

SELECT StudentID, FullName,COUNT(books.id) AS book_count
FROM students
LEFT JOIN books ON books.StudentID = students.StudentID
GROUP BY StudentID
ORDER BY students.FullName ASC
LIMIT 0, 30;

Answer 2

将其更改为

"SELECT 
    StudentID, 
    FullName,
    ( SELECT
        COUNT(id)
    FROM
        books
    where books.StudentID = students.StudentID
    ) AS book_count
FROM 
    students
ORDER BY
    students.FullName ASC
LIMIT 0, 30;"

在学生列的全名上添加索引和书籍中的 StudentID table.

如何改进 MySQL 中的 SELECT 子查询？

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