MySQL子查询还是多个查询？

Question

I'm trying to figure out the best way to get data from a MySQL database and process it. I have 2 tables 'objects', and 'objects_metadata'. rows in the objects_metadata table belong to rows in the objects table and the link is defined by a 'parent_id' column in objects_metadata that corresponds to an 'id' column in objects. (SQLFiddle below).

The Scenario

When I search against these tables I'm always looking for rows from the objects table. I sometimes have to query the objects_metadata table to get the right results. I do this by defining boundaries such as "hasMetadataWithValue". This boundary would run the following query by itself:

SELECT * FROM objects 
INNER JOIN objects_metadata ON objects.id=objects_metadata.parent_id 
WHERE objects_metadata.type_id = ? AND objects_metadata.value = ?

Another example boundary "notSelf" would use a query such as:

SELECT * FROM objects WHERE objects.id != ?

My scenario caters for multiple boundaries at a time. For a row from the objects table to be selected it MUST pass all boundaries. (ie if each boundary query was run independently the row would appear in every set of results)

I'm wondering if anyone has any thoughts on the best way to do this?

• Use each boundary's query as a subquery in a single query on the database (my original goal)
• Run each boundary's query as a full query and then use PHP to process the results

I would prefer to make the database do most of the work and spit out the results simply to avoid running a bunch of queries instead of a single one. Here's the tricky part, I've tried to create a full query using subqueries, but I'm not getting the hang of it at all. My latest attempt is below:

SELECT * FROM objects 
WHERE type_id = 7 
AND confirmed = 1 
AND (SELECT * FROM objects WHERE objects.id != 1) 
AND (SELECT * FROM objects LEFT JOIN objects_metadata ON objects.id=objects_metadata.parent_id WHERE objects_metadata.type_id = 8 AND objects_metadata.value ='male') 
LIMIT 0,20

I can see that the way I'm trying to use these subqueries is obviously wrong, but I can't figure out what the right way is.

SQL Fiddle is here

Any insights into the best way of doing this would be much appreciated.

Answer 1

I think you can just put those 'boundaries' inside your joined query.

SELECT 
    * 
FROM objects LEFT JOIN objects_metadata 
    ON objects.id = objects_metadata.parent_id
WHERE 
    objects_metadata.type_id = 8
    AND objects.confirmed=1 
    AND ( objects.id!=1 )
    AND ( objects_metadata.type_id=8 AND objects_metadata.value='male' ) 
LIMIT 0,20

SQL Fiddle: http://sqlfiddle.com/#!2/0ee42/34

Just mind the same column names for both tables, so you have to specify the exact table as well (eg, objects_metadata.type_id = 8). If I completely misunderstand your question let me know! :)