How do I query one table, but get related information from another table?

for example i have two tables:

posts {id,user_id,date}
design{user_id, bg_color}

and i have a post id of 18 for开发者_如何学JAVA example, i want to retrieve the design's table by the same user_id of that post(which is 18)

how can i query this?

sorry for this stupid, im confused ? :))

SELECT design.user_id, design.bg_color
FROM posts
JOIN design
  ON posts.user_id = design.user_id
WHERE posts.id = 18

In other words, we first select from posts where id is 18;
then we join design rows on which the user_id equals the user_id of the posts row.
We retrieve the corresponding user_id and bg_color from the design table.

Note that if there are multiple rows with the same design.user_id, you will get multiple rows back - example:

posts.id  | posts.user_id
 1           5
 3           2
 18          9


design.user_id | design.bg_color
 2                '#aaffcc'
 5                'red'
 5                'blue'
 9                '#000000'
 9                '#cafe00'

query result:

posts.id | posts.user_id | design.user_id | design.bg_color
 18         9               9                '#000000'
 18         9               9                '#cafe00'

SELECT design.*
  FROM design
  JOIN posts on posts.user_id = design.user_id
 where posts.ID = 18

You mean this?:

SELECT design.*
FROM design
    JOIN posts ON design.user_id = posts.user_id
WHERE posts.id = ?givenid

Keep in mind that this could, based on the information you've given us, be a many-to-one relation. Are you looking for a single record or multiple records from the design table?

SELECT `design`.* FROM `design`, `posts` WHERE `posts`.`user_id` = `design`.`user_id` AND `posts`.`id` = 18

    select * from posts join 
design on 
posts.user_id=design.user_id 
where posts.id=18

Your question is a little confusing, but I think this will do the trick:

SELECT * FROM `posts` LEFT JOIN `design` USING (`user_id`) WHERE `posts`.`id` = 18