MySQL按SNO DESC分组按列返回不同的行顺序

Question

I have the following mysql table:

Here in this table there could be houndred of thousands of rows, in which some may be with the same fir_sno [column No. 2]. My problem is that to select only the distinct rows from this table order by sno desc order. And it should return sno : 2, 3. for this i have tried the following sql query but its return row no: 1,3 where i need row no:2, 3.

SELECT DISTINCT p.sno,p.fir_sno,p.short_order,p.date_fixed,
CONCAT(f.complainant,' V/S ',f.accused) AS title,f.under_sections,f.case_FIR_no 
FROM pre_trials p, fir f 
WHERE f.sno = p.fir_sno 
GROUP BY p.fir_sno 
ORDER BY p.sno DESC

Please help me how to do my required job?

Thanks for your precious time.l

Answer 1

您可以检查WHERE f.sno <> p.fir_sno

Answer 2

You are grouping but no aggregate function is being used

Consider this similar example

mysql> create table test (a int, b int, c int);
Query OK, 0 rows affected (0,47 sec)

mysql> insert into test values (1,1,1),(2,1,2),(3,2,3);
Query OK, 3 rows affected (0,08 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> insert into test values (1,1,1),(2,1,2),(3,2,3);

mysql> select a, b, c from test group by b;
+------+------+------+
| a    | b    | c    |
+------+------+------+
|    1 |    1 |    1 |
|    3 |    2 |    3 |
+------+------+------+
2 rows in set (0,00 sec)

What you need, if I understand your question correctly, is to specify an aggregate on column a (in your example sno) in order to get only the last (highest) value per group on column b:

mysql> select max(a), b, c from test group by b;
+--------+------+------+
| max(a) | b    | c    |
+--------+------+------+
|      2 |    1 |    1 |
|      3 |    2 |    3 |
+--------+------+------+
2 rows in set (0,04 sec)

NOTE: This approach will not work in other DBMS as MySQL has a more relaxed group by syntax. For reference: http://dev.mysql.com/doc/refman/5.7/en/group-by-functions.html