使用 where 条件连接 2 个表，但显示 table2 中的第一行

Question

I have 2 tables. One is movie table, where all movie data is stored and another one is relation table, where I can see which movie is attached to which category. If I want to show movies from category 6 I join them like this:

SELECT t1.title, t2.category FROM movies t1
JOIN movie_categories t2 ON t1.movieid = t2.movieid
WHERE t2.category = 6

And it works just fine. However, movie SEF url are generated based on the first row from table2 (actually it should connect to table3, a category table, where i will find sef alias name, but for now I dont need that, I just want to know how to get first category). So lets say, movie have 3 categories, which will be looking like this:

Table 2 [relations table]
id movieid category
 1   1      3   
 2   1      6   
 3   1      2   

Table 1 [movie table]
movieid
   1      
   2     

The query I showed before returns results like this:

title category
bla1      6
bla2      6

However to create sef url, I need to get the first category id from relations table which is 3 in this case.

I'd like to know how to construct query to achieve the result I described.

Answer 1

ASSUMPTION: by MIN you mean the lowest ID in movie_Categories and then display it's category value.

1. Generate a set of data which is the lowest ID for each movie (subquery)
2. Join it back to the movie list
3. Then join your movie categories back to the generated set with the min value
4. Join the movie categories again back to the base movie set to get the limiting set for category.
5. Then limit the data base the category desired.

.

drop table movie;
drop table movie_categories;
create table movie (movieid int, Title varchar(10));
create table movie_categories (Id int, movieID int, category int);

insert into movie values (1, 'The');
insert into movie values (2, 'End');
insert into movie_categories values (1,1,3);
insert into movie_categories values (2,1,6);
insert into movie_categories values (3,1,2);

--To aid in understanding you may want to select * and see why this works and remove the where clause.
--Basically the two joins to Movie_Categories is once for the LOWEST ID and once again to get the limiting category.
SELECT M.Title, MC1.Category
FROM Movie M
INNER JOIN (SELECT min(ID) ID, MovieID
            from movie_categories mc
            GROUP BY MovieID) L
 on L.MovieID = M.MovieID
INNER JOIN Movie_Categories MC1 
 on  L.MovieID = MC1.MovieID
INNER JOIN Movie_Categories MC2
 on l.movieid = mc2.movieid
 and L.ID = MC1.ID
where mc2.category = 6

Results in:

title category
The     3

Note: the 2nd title isn't listed because there are no records in movie_category which match a category of 6.