MySQL在一个查询中将2个表连接在一起

Question

I've got 2 queries which I want to join and I've never really done it before. Hopefully one of you bright sparks can help.

It's displaying businesses for sale. The first query is fine.

SELECT DISTINCT * FROM businesses WHERE business_asking_price > 1 AND business_location = 11 ORDER BY business_id DESC

The above searches for all businesses within a certain price as well as finding all the businesses in London.

So far so good.

But the headache I'm having is the TYPE of business. One business (let's say a hairdressers) can have multiple types of business type categories. In this case it's Health/Beauty and Hair Salons.

So the categories are in a different table called bus_parents:

bus_parent_id  |   bus_parent_parent | bus_parent_child

176            |          56         |         42
177            |          56         |         43
181            |          56         |         46
202            |          56         |         57

bus_parent_id is obviously the unique id

bus_parent_parent is the business type id

bus_parent_child is the business id

So i really want to wrap the two queries together so it finds all the businesses with the first query, then narrows them down further so it only shows businesses within a bus_parent_parent as well.

Hopefully I've made myself clear as to what I'm trying to do.

Is it a JOIN that I'm after or something else?

Answer 1

Try below SQL with JOIN:

SELECT DISTINCT businesses.* 
FROM businesses 
INNER JOIN bus_parents ON businesses.business_id = bus_parents.bus_parent_child 
WHERE businesses.business_asking_price > 1 
AND businesses.business_location = 11 
AND bus_parents.bus_parent_parent = 56 
ORDER BY businesses.business_id DESC

It will retrieve all the businesses, whose type id = 56, you can change this parameter according to your need.

Based on your description for each field in bus_parents I have used JOIN to retrieve the desired result. (narrow down on business type id)

Answer 2

If i've understood you correctly, I think you want something like this:

SELECT DISTINCT * FROM businesses 
LEFT JOIN business_type ON business_type.bus_parent_child = business.id 
WHERE business_asking_price > 1 AND business_location = 11 
AND business_type.bus_parent_parent = <Your desired parent> 
ORDER BY business_id DESC

This is joining business to business_type on business_type.bus_parent_child = business.id

With an additional WHERE clause on the type.bus_parent_parent

Answer 3

SELECT DISTINCT businesses.* 
FROM businesses a
INNER JOIN bus_parents b ON a.business_id = b.bus_parent_parent 
WHERE a.business_asking_price > 1 
AND a.business_location = 11 
ORDER BY a.business_id DESC

Answer 4

SELECT * FROM businesses b WHERE business_asking_price > 1 AND business_location = 11 

and (select bus_parent_id from bus_parents where bus_parent_child=b.bussiness_id and bus_parent_parent=in(56,57,58) limit 1) != NULL

ORDER BY business_id DESC

here i assume bussiness_id is a primary key column of businesses table.

You should avoid using distinct on large queries as it can be quite taxing(slow) on large tables. especially if you request many columns.