拥有示例数据对于获得帮助非常重要。要确定数据的外观仍然很困难。让我们假设数据如下所示：

create table table1 (
      client varchar(10), 
      product varchar(10)
     );

    insert into table1 values
    ('xxx', 'A'),
    ('bbb', 'A'), 
    ('bbb', 'A'), 
    ('ccc', 'A'), 
    ('ddd', 'A'),
    ('ddd', 'A'),
    ('ddd', 'A'),
    ('ddd', 'A'),
    ('tt', 'A'), 
    ('tt', 'A'), 
    ('tt', 'A'), 
    ('tt', 'A'), 
    ('bdad', 'A'), 
    ('bdad', 'A'), 
    ('bdad', 'A'), 
    ('bdad', 'A');

我不能访问DB2数据库，但是这个查询对大多数的dbms类型都有效。你可能需要调整以适合DB2。

select purchased as most_common_value
    from (
      select client, count(*) as purchased
      from table1
      where product = 'A'
      group by client
      )z
    group by purchased
    order by count(client) desc
    limit 1

查询的输出为：

most_common_value
    4