MySql查询结果不正确

Question

I have 2 tables (import_size,export_size) both contain these 2 columns (size_id,weight) now I'm trying to get the sum of weights for each size in each table, my query looks like this

SELECT sum(i.weight) imports,sum(e.weight) exports,s.size 
       FROM size s 
       LEFT JOIN import_size i on s.id=i.size_id 
       LEFT JOIN export_size e on s.id=e.size_id
       GROUP BY s.size

now say for example in the import_size table for size 1 we have only one record but for the same size in export_size table we have 2 records, the sum of weights resulting from import_size is multiplied by 2, any thoughts?

for example:

import table has 1 record:

size_id 1 - weight 2 

export table has 2 records:

size_id 1 - weight 2  
size_id 1 - weight 3 

the query result:

imports 4 - exports 5 - size 1 

however it should be

imports 2 - exports 5 - size 1

Answer 1

Generally speaking, a join creates a row for each permutation that match the conditions. You are joining 3 tables. You are saying that for size 1, these tables have:

• size : 1 row
• import_size : 1 row
• export_size : 2 rows

This gives us 1*1*2=2 permutations, which means we get 2 rows. Obviously each contains a different export_size record, but what about size and import_size ? The two rows use the same record from both of them. That means that import_size.weight appears twice - and that's why it's summed twice.

BTW - if both tables had 2 entries for size 1, you'd get 4 permutations and both import and export sizes will be doubled.

The solution: do the aggregation separately for each table and then do the join(not tested):

SELECT i.imports, e.exports, s.size
    FROM size AS s
    LEFT JOIN (SELECT size_id,sum(weight) AS imports FROM import_size GROUP BY size_id) AS i ON s.id=i.size_id
    LEFT JOIN (SELECT size_id,sum(weight) AS exports FROM export_size GROUP BY size_id) AS e ON s.id=e.size_id