如何从第二张表查询第一张表的每个结果？

Question

I'm not really sure how to do this - I only posses a limited knowledge of joins in MySQL, and from what I read I don't think they'd be of much use here.

Essentially, I have 2 tables:

images                          votes
---------------------------     ------------------------------
image_id | name | square_id     vote_id | image_id | vote_type 
---------------------------     ------------------------------
1          img1   14            1         4          1
2          img2   3             2         17         0
3          img7   72            3         2          1
...                             ...
n          imgn   1478          n         n          1

What I'd like to do is get the details of each image and the number of votes cast (plus the vote_type ) on each image where a certain condition is true (such as where each image has a certain square_id ). Executing the first part of this query is easy:

SELECT `image_id`, `name` FROM `images` WHERE `square_id` = :boundParameter;

But, I'm unsure of how to get each vote_id and vote_type for each image that meets the original WHERE condition in my query.

How would I accomplish this?

Answer 1

This is actually a really simple join between tables. You really, really should read this Q&A that I put together about SQL and queries and joins between two tables.

In the meantime, this will probably give you what you want:

select
    img.image_id,
    img.name,
    img.square_id,
    count(vot.vote_id) as numberVotes,
    vot.vote_type
from
    images img
        join votes vot
            on img.image_id=vot.image_id
group by
    img.image_id,
    img.name,
    img.square_id,
    vot.vote_type

So, with that, first off, you don't want multiple queries, you want to use one query here. Running multiple connections/queries to a database is a MUCH higher overhead than running one query that fetches all the data you want.

Answer 2

Try it like this:

SELECT images.image_id, images.name , votes.vote_id , votes.vote_type 
FROM images
LEFT JOIN votes ON images.image_id = votes.image_id
WHERE `square_id` = :boundParameter;

Answer 3

SELECT images.image_id,
       images.name,
       votes.vote_id,
       votes.vote_type
FROM images
LEFT JOIN votes ON images.image_id = votes.image_id
WHERE images.square_id = :boundParameter;