使用不同的SQL查询在表中查找重复项会产生不同的结果

Question

I was expecting both queries to give the same outcome. Why is this not the case?

select count(*) as c,number from numbers group by number having count(*) > 1;

select * from numbers as a INNER JOIN numbers  as b 
                                    on a.number = b.number  and a.id < b.id; 

First query return 1661 results where as the second 1911.

Answer 1

Because the first query will always return one row per number.
The second may return more rows with the same number (if you have triplicates etc)

try the below to see the difference

SELECT a.number, count(*) FROM
(select * from numbers as a INNER JOIN numbers  as b 
   on a.number = b.number  and a.id < b.id) c
GROUP BY a.number
-- having count(*) > 1
-- ORDER BY COUNT(*) DESC

Answer 2

The first query is correct, showing you one row for each number.

But the second query shows you all combinations of equal numbers (see the fiddle). If you have more than two records with the same number, the number of records when you JOIN will increase.

If you have:

ID     NUMBER
1        1
2        1
3        1
4        2
5        3

Joining these two tables will result in three times the number 1 being displayed.

Here you can see a SQLFiddle and how your query works.