MySQL从两个查询中获取匹配结果？

Question

I have some php code that generates multiple MySql queries and combines the results. I am trying to find a way to combine the queries so the only fields that match in both queries are outputed.

Example of PHP Codes Output.

SELECT LoadData_3 pow FROM tblLoadData WHERE LoadData_1 = 'test1' AND LoadData_2 = 'test2' 
UNION ALL
SELECT LoadData_3 pow FROM tblLoadData WHERE LoadData_1 = 'test3' AND LoadData_2 = 'test4'

I want only the LoadData_3 fields that are the same in both queries.

Answer 1

Use the INTERSECT operation:

(SELECT LoadData_3 pow 
  FROM tblLoadData 
 WHERE LoadData_1 = 'test1' 
   AND LoadData_2 = 'test2')
INTERSECT                     -- return the intersection of the two queries
(SELECT LoadData_3 pow 
  FROM tblLoadData 
 WHERE LoadData_1 = 'test3' 
   AND LoadData_2 = 'test4')

EDIT

For MySQL you can use INNER JOIN USING (..)

SELECT pow -- Use DISTINCT pow instead if you want unique values (like INTERSECT)
  FROM (SELECT LoadData_3 pow 
          FROM tblLoadData 
         WHERE LoadData_1 = 'test1' 
           AND LoadData_2 = 'test2') T1
  INNER JOIN
       (SELECT LoadData_3 pow 
          FROM tblLoadData 
         WHERE LoadData_1 = 'test3' 
           AND LoadData_2 = 'test4') T2
  USING (pow)

Or using a WHERE IN clause:

SELECT LoadData_3 pow -- Use DISTINCT pow instead if you want unique values (like INTERSECT)
  FROM tblLoadData 
 WHERE LoadData_1 = 'test1' 
   AND LoadData_2 = 'test2'
   AND LoadData_3 IN (SELECT LoadData_3 pow 
                        FROM tblLoadData 
                       WHERE LoadData_1 = 'test3' 
                         AND LoadData_2 = 'test4')

Answer 2

Big Edit!

I've spent too much time in Oracle, and I skimmed over that article too quickly! The above answer was edited before mine. You can also take a look at the following question:

Alternative to Intersect in MySQL