Corrected SQL Code:
SELECT DISTINCT [author_id] AS id
FROM Views WITH (NOLOCK)
WHERE author_id = viewer_id
ORDER BY [author_id];
Explanation:
-
SELECT DISTINCT [author_id] AS id:
We are selecting theauthor_id
column from theViews
table, but we are using theDISTINCT
keyword to ensure that only uniqueauthor_id
s are returned. TheAS id
renames the column toid
in the result set.DISTINCT
ensures that if the same author views their own article multiple times, only one entry is returned for that author.AS id
renames theauthor_id
column toid
for simplicity.
我们从
Views
表中选择author_id
列,并使用DISTINCT
关键字确保只返回唯一的author_id
。AS id
将列重命名为id
。DISTINCT
确保如果同一个作者多次查看自己的文章,结果集中只返回该作者的一条记录。AS id
将author_id
列重命名为id
,使结果更加简洁。
-
FROM Views WITH (NOLOCK):
TheFROM
clause specifies theViews
table as the source of data. TheWITH (NOLOCK)
hint is used to allow the query to read data without locking the table, which can improve performance in high-concurrency environments. However, usingNOLOCK
can result in reading uncommitted or dirty data.FROM
子句指定Views
表为数据源。WITH (NOLOCK)
提示允许查询在不锁定表的情况下读取数据,这可以提高高并发环境下的性能。然而,使用NOLOCK
可能会导致读取未提交或脏数据。 -
WHERE author_id = viewer_id:
TheWHERE
clause filters the rows to only include cases where theauthor_id
is equal to theviewer_id
, meaning the author is viewing their own article.WHERE
子句筛选出author_id
等于viewer_id
的行,即作者查看自己文章的情况。 -
ORDER BY [author_id]:
TheORDER BY
clause ensures that the result set is sorted byauthor_id
. This provides an ordered list of authors who viewed their own articles.ORDER BY
子句确保结果集按author_id
排序。这提供了一个按顺序列出的查看自己文章的作者列表。
Key Concepts:
-
DISTINCT:
TheDISTINCT
keyword removes duplicate entries. This is crucial in this case because an author could view their own article multiple times. WithoutDISTINCT
, the query would return multiple rows for the sameauthor_id
if they viewed their article more than once.DISTINCT:
DISTINCT
关键字删除重复的条目。这在此处至关重要,因为作者可能多次查看自己的文章。如果没有DISTINCT
,查询会为同一作者返回多行记录。 -
NOLOCK:
TheWITH (NOLOCK)
hint allows the query to avoid locking the table during reads, improving performance. However, it may lead to reading uncommitted or dirty data, which could cause inconsistent results.NOLOCK:
WITH (NOLOCK)
提示允许查询在读取时不锁定表,从而提高性能。然而,它可能导致读取未提交或脏数据,可能导致结果不一致。 -
ORDER BY:
Sorting the result byauthor_id
ensures the result is returned in a consistent and meaningful order.ORDER BY:
按author_id
排序确保结果按顺序返回,提供一致且有意义的顺序。
Warnings:
-
Reading Dirty Data:
When usingWITH (NOLOCK)
, be cautious because it may read uncommitted or dirty data. This is acceptable in environments where performance is prioritized over data accuracy, but be aware of potential inconsistencies.读取脏数据:
使用WITH (NOLOCK)
时要谨慎,因为它可能会读取未提交的脏数据。这在性能优先于数据准确性的环境中是可以接受的,但要注意潜在的不一致性。
Summary:
This query retrieves a list of unique author_id
s where the authors have viewed their own articles. The DISTINCT
keyword ensures only unique entries are returned, and the result is sorted by author_id
. The WITH (NOLOCK)
hint is used to improve performance by avoiding table locks, but be cautious of potential dirty data reads.
中文总结:
该查询检索了唯一的 author_id
列表,表示作者查看了自己撰写的文章。DISTINCT
确保只返回唯一的条目,结果按 author_id
排序。WITH (NOLOCK)
提示用于提高性能,避免锁表,但需要注意潜在的脏数据读取问题。
Tips:
-
Indexing:
Consider creating an index on theauthor_id
andviewer_id
columns to improve query performance, especially if theViews
table is large.索引:
考虑在author_id
和viewer_id
列上创建索引,以提高查询性能,尤其是在Views
表很大的情况下。 -
When to Avoid NOLOCK:
If you need accurate and consistent data, avoid usingNOLOCK
, especially in environments where data integrity is critical (e.g., financial systems).何时避免使用 NOLOCK:
如果需要准确和一致的数据,避免使用NOLOCK
,特别是在数据完整性至关重要的环境中(如金融系统)。
Similar SQL Problems:
-
LeetCode 176: Second Highest Salary
Involves retrieving unique data, similar to howDISTINCT
is used here to remove duplicates. -
LeetCode 181: Employees Earning More Than Their Managers
Filters data based on conditions, much like filtering authors who viewed their own articles. -
LeetCode 183: Customers Who Never Order
Focuses on filtering data using specific conditions, similar to how this query filters rows whereauthor_id = viewer_id
. -
LeetCode 196: Delete Duplicate Emails
Deals with filtering and removing duplicates, which is related to howDISTINCT
is used in this problem. -
LeetCode 184: Department Highest Salary
Involves filtering specific data from a table, similar to how this query retrieves specific authors.
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