从表中选择与其他表记录匹配的行

Question

I have two SQL tables:

items table

item_id     name       timestamp
--------------------------------------------
a           apple      2014-01-01
b           banana     2014-01-06
c           tomato     2013-12-25
d           chicken    2014-01-23
e           cheese     2014-01-02
f           carrot     2014-01-16

items_to_categories table

cat_id      item_id
--------------------------------------------
1           a
5           c
2           e
3           a
4           f
5           d
5           b
5           a

Knowing cat_id ( eg 5 ), I need to get 2 latest items (based on the timestamp ) that belongs to that cat_id .

If I first get 2 rows from items_to_categories table:

SELECT item_id FROM items_to_categories WHERE cat_id = 5 LIMIT 2;
>> returns 'c' and 'd'

And then use returned items ids to query items table, I am not making sure returned items will be the latest ones (order by timestamp ).

The ideal result what I need to get selecting 2 latest items by cat_id ( eg 5 ) would be:

d           chicken    2014-01-23
b           banana     2014-01-06

Answer 1

SELECT t1.item_id, t1.name, t1.timestamp 
   FROM items t1 
     LEFT JOIN items_to_categories t2 ON t1.item_id = t2.item_id 
       WHERE cat_id = 5 
       ORDER BY t1.timestamp DESC 
       LIMIT 2;

Use above query.

Answer 2

SELECT top 2 I.item_id, I.name, I.timestamp 
FROM items I 
JOIN items_to_categories IC ON I.item_id = IC.item_id 
WHERE IC.cat_id in (Select top 1 from items_to_categories order by timestamp desc)
ORDER BY I.timestamp DESC

Answer 3

You can try something like this

SELECT
items.item_id
items.item,
items.item_timestamp
FROM
items_to_categories
INNER JOIN items ON items_to_categories.item_id = items.item_id
WHERE
items_to_categories.cat_id = 5
ORDER BY items.item_timestamp desc
limit 2

By the way, timestamp is a reserved word, you really should avoid using it for a field name

Hope it will helps

Answer 4

Try this using join to get the required result

SELECT a . * 
FROM items a
INNER JOIN items_to_categories b ON a.item_id = b.cat_id
ORDER BY a.timestamp DESC LIMIT 2