MySQL-为什么将近2个相同的查询导致2个不同的结果？ 通过...分组

Question

I have 2 queries that are nearly identical, one with a GROUP BY, one without. The results are very different. The GROUP BY query results in over double the non-GROUP BY query result.

Query 1:

SELECT table2.name, COUNT(DISTINCT(op.id))
FROM op INNER JOIN table1 ON table1.EID = op.ID
    INNER JOIN table3 ON table3.id = table1.jobid
    INNER JOIN table2 ON table2.id = table3.CatID
WHERE op.BID = 1
    AND op.ActiveStartDate <= NOW()
    AND op.ActiveEndDate >= NOW()
GROUP BY table2.name
ORDER BY COUNT(*) DESC;

Query 2:

SELECT op.Type, COUNT(DISTINCT op.id)
FROM op
WHERE op.BID = 1
AND op.ActiveStartDate <= NOW()
    AND op.ActiveEndDate >= NOW()
ORDER BY op.Type ASC;

These should result in the same result. When playing around with them, once I remove the "GROUP BY" from query 1, the result is the same. If I put the "GROUP BY" back into Query 1, the result is more than doubled.

Edit: It seems the additional INNER JOINS are not affecting the results, but rather the GROUP BY in query 1. If I remove the GROUP BY in query 1, the results between the 2 queries are identical. If I add the GROUP BY back into query 1, the results are very different.

Answer 1

I don't know how you think that those are nearly identical queries; they are very different. Anyway, you shouldn't remove the GROUP BY from the first one, but add a GROUP BY on the second query:

SELECT op.Type, COUNT(DISTINCT op.id)
FROM op
WHERE op.BID = 1
AND op.ActiveStartDate <= NOW()
    AND op.ActiveEndDate >= NOW()
GROUP BY op.Type
ORDER BY op.Type ASC;

Of course, this doesn't mean that you'll get the same results anyway, since the first query has 3 extra joins.

Answer 2

the queries are not at all "nearly identical..." in my view. you have INNER JOIN with other tabls that can have duplicates and so the INNER JOIN will increase the number of rows by that number. you can check the explaination in here

INNER JOIN and GROUP BY