SQLのフィールド値ごとの行数を制限するにはどうすればよいですか？

残念ながら、mysqlには分析関数がありません。したがって、変数を操作する必要があります。自動インクリメントフィールドがあるとします。

mysql> create table mytab (
    -> id int not null auto_increment primary key,
    -> first_column int,
    -> second_column int
    -> ) engine = myisam;
Query OK, 0 rows affected (0.05 sec)

mysql> insert into mytab (first_column,second_column)
    -> values
    -> (1,1),(1,4),(2,10),(3,4),(1,4),(2,5),(1,6);
Query OK, 7 rows affected (0.00 sec)
Records: 7  Duplicates: 0  Warnings: 0

mysql> select * from mytab order by id;
+----+--------------+---------------+
| id | first_column | second_column |
+----+--------------+---------------+
|  1 |            1 |             1 |
|  2 |            1 |             4 |
|  3 |            2 |            10 |
|  4 |            3 |             4 |
|  5 |            1 |             4 |
|  6 |            2 |             5 |
|  7 |            1 |             6 |
+----+--------------+---------------+
7 rows in set (0.00 sec)

mysql> select
    -> id,
    -> first_column,
    -> second_column,
    -> row_num
    -> from (
    -> select *,
    -> @num := if(@first_column = first_column, @num:= @num + 1, 1) as row_num,
    -> @first_column:=first_column as c
    -> from mytab order by first_column,id) as t,(select @first_column:='',@num:
=0) as r;
+----+--------------+---------------+---------+
| id | first_column | second_column | row_num |
+----+--------------+---------------+---------+
|  1 |            1 |             1 |       1 |
|  2 |            1 |             4 |       2 |
|  5 |            1 |             4 |       3 |
|  7 |            1 |             6 |       4 |
|  3 |            2 |            10 |       1 |
|  6 |            2 |             5 |       2 |
|  4 |            3 |             4 |       1 |
+----+--------------+---------------+---------+
7 rows in set (0.00 sec)

mysql> select
    -> id,
    -> first_column,
    -> second_column,
    -> row_num
    -> from (
    -> select *,
    -> @num := if(@first_column = first_column, @num:= @num + 1, 1) as row_num,
    -> @first_column:=first_column as c
    -> from mytab order by first_column,id) as t,(select @first_column:='',@num:
=0) as r
    -> having row_num<=2;
+----+--------------+---------------+---------+
| id | first_column | second_column | row_num |
+----+--------------+---------------+---------+
|  1 |            1 |             1 |       1 |
|  2 |            1 |             4 |       2 |
|  3 |            2 |            10 |       1 |
|  6 |            2 |             5 |       2 |
|  4 |            3 |             4 |       1 |
+----+--------------+---------------+---------+
5 rows in set (0.02 sec)