Question

我正在执行两个查询。一个返回记录数和一个获取结果的记录。但是我只想要记录总数的查询，它比获取结果的查询需要相同的时间。为什么？这是我的SQL。我正在使用SQL 2014：

(SELECT count(*) as id
        FROM (
        SELECT DISTINCT title, version
        FROM book AS b
        WHERE b.title IS NOT NULL AND NOT EXISTS (SELECT * FROM user AS u WHERE u.column1 = b.column1)
        group by title, version
        EXCEPT
        SELECT DISTINCT title, version
        FROM book AS b
        WHERE b.title IS NOT NULL AND EXISTS (SELECT * FROM user AS u WHERE u.column2 = b.column1)
        group by title, version
    ) c )

Answer 1

试试这个：

(SELECT count(*) as id
        FROM (
        SELECT DISTINCT title, version
        FROM book  b left join user u on b.column1=u.column1
        WHERE b.title IS NOT NULL and u.column1 is null
        EXCEPT
        SELECT DISTINCT title, version
        FROM book b join user u on b.column1=u.column2
        WHERE b.title IS NOT NULL  
    ) c )

Answer 2

嗯。嗯。。。为了提高性能，请尝试删除group by和select distinct（except无论如何都这样做）：

SELECT count(*) as id
FROM (SELECT title, version
      FROM book b
      WHERE b.title IS NOT NULL AND NOT EXISTS (SELECT 1 FROM user u WHERE u.column1 = b.column1)
      EXCEPT
      SELECT title, version
      FROM book b
      WHERE b.title IS NOT NULL AND EXISTS (SELECT 1 FROM user u WHERE u.column2 = b.column1)
    ) c;

下一步：确保您在user(column2)和user(column1)上有索引。

有关数据的更多信息，可能会提出进一步的建议。

使用Except子句选择count需要太长时间

2 个答案: