mysql在table1上退出条件连接

Question

I'd like to do a left join using only certain rows of the first table in mysql. Currently I do something like:

SELECT students.* FROM students             
LEFT JOIN courses                           
ON students.id = courses.id                 
WHERE students.id = 6

But will mysql first select rows from table1 (students) satisfying students.id = 6, before doing the left join? If not, is there a way to force mysql do to so? Thanks.

Answer 1

Yes there is, try this:

SELECT students.* FROM students             
LEFT JOIN courses                           
ON students.id = courses.id                 
HAVING students.id = 6
LIMIT 1

Answer 2

It seems to me that you are trying to optimize for the DB. If the query is not slow, and your testing data set is a reasonable proximation of the production DB, then it is not best practice to do so, for many reasons.

With that said, (maybe I am not right, so ignore me)

I think your trying to say, "How can i speed things up, by limiting the number of rows that the DB looks at during the query"

The best way to do this, is ensure you have proper indexes on the tables, on the fields that your going to query. Indexes are extremely fast. If you do not already index the .id fields of both tables, add those indexes to the DB, and that may solve your issue.