性能比较：使用in和like的SQL查询

Question

Which query runs faster?

Considering it returns 200 records and authorname is atleast 20 chars long and authorname is fulltext indexed

select * from quotestable 
where quotesauthor like (select Authorname from  Authortable where authorid =45)

.

select * from quotestable 
where quotesauthor in (select Authorname from  Authortable where authorid =45)

Answer 1

It's not a question of "faster". They have different meanings.

The first query can only run if the subquery returns 0 or 1 records (and should normally use TOP 1 to guarantee this). However, it can do wildcard matching on the results. The second query can run if the subquery returns any number of records, but will not do wildcard matching.

It sounds like what you should really have here is a JOIN:

SELECT q.* 
FROM quotestable q 
INNER JOIN AuthorTable a ON q.quotesauthor = a.authorname
WHERE a.authorid = 45

... assuming of course that AuthorID or AuthorName is unique in AuthorTable . This will also allow to use LIKE with wildcards for the matching condition, in the case where the quotesauthor field might not always be a direct match with AuthorTable.AuthorName .

While I'm here, it's also strange to me that AuthorName would be full-text indexed. A traditional index, rather than fulltext, would be more helpful for this query. The only reason to use fulltext here is if you have full names like 'John Milton' in that field, and want to be able to do things like search on last name only or first name only. But even in that case, it seems like you'd be much better served by storing those as their own fields and removing the fulltext index. Fulltext indexes work best on longer fields, like descriptions or articles/posts.

Answer 2

Not the same. One test equality, other test like expression...

try this for equality

select * from quotestable f1 
where exists 
(
select * from  Authortable f2
where f2.authorid =45 and f1.quotesauthor =f2.Authorname
)

try this for like

select * from quotestable f1 
where exists 
(
select * from  Authortable f2
where f2.authorid =45 and f1.quotesauthor like '%' + f2.Authorname + '%'
)