SQL - Give Me 3 Hits for Each Type Only

SQL - Give me 3 hits for each type only

select id, title, type
from   (select id, title, type,
               @num := if(@group = type, @num + 1, 1) as row_number,
               @group := type as dummy
        from   your_table
        order by type, title) as x
where  row_number <= 3

(Uses a different article on the same site as Martin Wickman's answer!)

mysql: select the last 10 messages and for each message the last 3 replies

A working example:

EDIT - (see revision for earlier query)

Full table creation and explain plan

Note: The table "datetable" just contains all dates for about 10 years. It is used just to generate rows.

drop table if exists messages;
create table messages (
   message_id int primary key, reply_to int, createdate datetime, index(reply_to));

insert into messages 
select @n:=@n+1, floor((100000 - @n) / 10), a.thedate
from (select @n:=0) n
cross join datetable a
cross join datetable b
limit 1000000;

The above generates 1m messages, and some valid replies. The query:

select m1.message_id, m1.reply_to, m1.createdate, N.N, r.*
from
(
    select m.*, (
         select group_concat(r.message_id order by createdate)
          from messages r
        where r.reply_to = m.message_id) replies
     from messages m
     order by m.message_id
    limit 10
) m1
inner join ( # this union-all query controls how many replies per message
    select 1 N union all
     select 2 union all
     select 3) N
  on (m1.replies is null and N=1) or (N <= length(m1.replies)-length(replace(m1.replies,',','')))
left join messages r
  on r.message_id = substring_index(substring_index(m1.replies, ',', N), ',', -1)

Time: 0.078 sec

Explain plan

id     select_type         table        type      possible_keys    key      key_len ref                rows    Extra
1      PRIMARY             <derived4>   ALL      (NULL)            (NULL)   (NULL)  (NULL)             3    
1      PRIMARY             <derived2>   ALL      (NULL)            (NULL)   (NULL)  (NULL)             10      Using where
1      PRIMARY             r            eq_ref   PRIMARY           PRIMARY  4       func               1    
4      DERIVED             (NULL)       (NULL)   (NULL)            (NULL)   (NULL)  (NULL)             (NULL)  No tables used
5      UNION               (NULL)       (NULL)   (NULL)            (NULL)   (NULL)  (NULL)             (NULL)  No tables used
6      UNION               (NULL)       (NULL)   (NULL)            (NULL)   (NULL)  (NULL)             (NULL)  No tables used
(NULL) UNION RESULT        <union4,5,6> ALL      (NULL)            (NULL)   (NULL)  (NULL)             (NULL)    
2      DERIVED             m            index    (NULL)            PRIMARY  4       (NULL)             1000301    
3      DEPENDENT SUBQUERY  r            ref      reply_to          reply_to 5       test.m.message_id  5       Using where

Mysql Query- for each date max 3 datas

SELECT a.*
FROM Table1 a
INNER JOIN Table1 b ON a.start_date = b.start_date AND a.event_id <= b.event_id
GROUP BY a.event_id,a.start_date
HAVING COUNT(*) <= 3
ORDER BY a.start_date 

How to select only the first rows for each unique value of a column?

A very simple answer if you say you don't care which address is used.

SELECT
    CName, MIN(AddressLine)
FROM
    MyTable
GROUP BY
    CName

If you want the first according to, say, an "inserted" column then it's a different query

SELECT
    M.CName, M.AddressLine,
FROM
    (
    SELECT
        CName, MIN(Inserted) AS First
    FROM
        MyTable
    GROUP BY
        CName
    ) foo
    JOIN
    MyTable M ON foo.CName = M.CName AND foo.First = M.Inserted

Get records with max value for each group of grouped SQL results

There's a super-simple way to do this in mysql:

select * 
from (select * from mytable order by `Group`, age desc, Person) x
group by `Group`

This works because in mysql you're allowed to not aggregate non-group-by columns, in which case mysql just returns the first row. The solution is to first order the data such that for each group the row you want is first, then group by the columns you want the value for.

You avoid complicated subqueries that try to find the max() etc, and also the problems of returning multiple rows when there are more than one with the same maximum value (as the other answers would do)

Note: This is a mysql-only solution. All other databases I know will throw an SQL syntax error with the message "non aggregated columns are not listed in the group by clause" or similar. Because this solution uses undocumented behavior, the more cautious may want to include a test to assert that it remains working should a future version of MySQL change this behavior.

Version 5.7 update:

Since version 5.7, the sql-mode setting includes ONLY_FULL_GROUP_BY by default, so to make this work you must not have this option (edit the option file for the server to remove this setting).

How to get multiple counts with one SQL query?

You can use a CASE statement with an aggregate function. This is basically the same thing as a PIVOT function in some RDBMS:

SELECT distributor_id,
    count(*) AS total,
    sum(case when level = 'exec' then 1 else 0 end) AS ExecCount,
    sum(case when level = 'personal' then 1 else 0 end) AS PersonalCount
FROM yourtable
GROUP BY distributor_id

get first x rows using in()

Here ist a sample how you can do it.

1) sort all results by type

2) then count the row of each type

3) get only ROWs where recno < 3

SELECT  
  @nr:=IF(@lastid = e.TYPE , @nr:=@nr+1 ,1) AS recno,
  @lastid:=TYPE AS last_type, e.* 
  FROM `example`e,
  (SELECT @lastid:=0, @nr:=0) tmp
WHERE TYPE IN(3, 4) 
HAVING recno < 3
ORDER BY TYPE;

Result:

MariaDB [tmp]> SELECT
    ->   @nr:=IF(@lastid = e.TYPE , @nr:=@nr+1 ,1) AS recno,
    ->   @lastid:=TYPE AS last_type, e.*
    ->   FROM `example`e,
    ->   (SELECT @lastid:=0, @nr:=0) tmp
    -> WHERE TYPE IN(3, 4)
    -> HAVING recno < 3
    -> ORDER BY TYPE;
+-------+-----------+----+------+---------+
| recno | last_type | id | type | title   |
+-------+-----------+----+------+---------+
|     1 |         3 |  1 |    3 | sth1    |
|     3 |         3 |  3 |    3 | sth2    |
|     1 |         4 |  2 |    4 | sthelse |
|     3 |         4 |  5 |    4 | sth4    |
+-------+-----------+----+------+---------+
4 rows in set (0.00 sec)

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