SQL根据初始表的ID联接另一个表

Question

I really just can't seem to grasb the sytanx of SQL Joins, etc ... so I'm needing some help with this (which I think is quite simple)

I'm querying bid_tag as follows:

SELECT paid_date, term, pet_id FROM bid_tag WHERE active = 1

And I need to use the pet_id to then grab some information from another table wp_postmeta , where the table is actually in a meta_key meta_value structure (WordPress) ...

So I need to grab the meta_value of meta_key "bid_name", for example... amongst other values.

TABLE
id | meta_key | meta_value
1    bid_name    Max

That ID is the same ID that I need to connect the initial table to...

I really appreciate it!

Answer 1

Join on both the id and the meta_key to select your desired value. If you want values that go with other keys in the meta table as well, one way is to join multiple times:

SELECT bt.paid_date, bt.term, bt.pet_id,
    bn.meta_value as bidname,
    bo.meta_value as bidother
FROM bid_tag bt
INNER JOIN wp_postmeta bn on bn.id = bt.pet_id and bn.meta_key = 'bid_name'
INNER JOIN wp_postmeta bo on bo.id = bt.pet_id and bo.meta_key = 'bid_other'
WHERE bt.active = 1

If the values won't necessarily be present in the meta table, use a LEFT OUTER JOIN instead and it will return null for whatever is missing.

Answer 2

Here's how your query would go:

SELECT B.meta_key, B.meta_value, A.pet_id
FROM bid_tag A
JOIN wp_postmeta B
ON B.id = A.pet_id;

Answer 3

there're some ways to use join:
1)use inner join or equally just join :

select 
  --some columns
from bid_tag
join wp_postmeta on
  bid_tag.pet_id = wp_postmeta.id
where
  wp_postmeta.meta_key = 'bid_name'

The result will have only that records which pet_id exists in wp_postmeta . The other will be omitted. And if wp_postmeta have a couple of records with the same id (and exists pet_id equal this id ) then you'll get a couple records with this pet_id ( BUT with different meta_key and meta_value ).
2)use left join :

select 
  --some columns
from bid_tag
left join wp_postmeta on
  bid_tag.pet_id = wp_postmeta.id
where
  wp_postmeta.meta_key = 'bid_name'

In this case behavior will be the same except one thing - even if there aren't any id for a pet_id the result will contain record from wp_postmeta with this pet_id . But meta_key and meta_value in this case will be null .