聚合函数扫描一组记录，然后返回单行记录。这组记录可以是一个表或者视图、或者一个子查询的结果。OceanBase 支持的聚合函数详情请参考手册《OceanBase SQL参考（MySQL模式）》。

    聚合函数通常跟 GROUP BY 子句一起使用，按照一个或多个列的值分组，然后每组返回单笔记录。

    示例：分组统计每个仓库的销售额

    MySQL 租户中，聚合函数跟 GROUP BY 子句一起使用的时候，对 select_list 里的列没有要求。这个可能会导致结果集很奇怪。如果要求 select_list 里的列跟 GROUP BY 子句中的列保持一致，需要设置 MySQL 命令行下的 sql_mode 为 ‘ONLY_FULL_GROUP_BY’。SQL 查询如下：

    1. obclient> SELECT ol_w_id
    2.     , count(*)              order_count
    3.     , sum(ol_amount)            sum_amount
    4.     , round(avg(ol_amount),2)   avg_amount
    5.     , min(ol_amount)            min_amount
    6.     ,max(ol_amount)             max_amount
    7. FROM ordl
    8. GROUP BY ol_w_id  
    9. ORDER BY ol_w_id ;
    11. +---------+-------------+------------+------------+------------+------------+
    12. | ol_w_id | order_count | sum_amount | avg_amount | min_amount | max_amount |
    13. +---------+-------------+------------+------------+------------+------------+
    14. |       1 |         297 |  917174.33 |    3088.13 |       0.00 |    9876.11 |
    15. |       2 |         329 | 1153354.23 |    3505.64 |       0.00 |    9979.34 |
    16. +---------+-------------+------------+------------+------------+------------+
    17. 2 rows in set (0.01 sec)
    19. obclient> SELECT ol_w_id, ol_d_id
    20.     ,  count(*)                 order_count
    21.     , sum(ol_amount)            sum_amount
    22.     , round(avg(ol_amount),2)   avg_amount
    23.     , min(ol_amount)            min_amount
    24.     , max(ol_amount)            max_amount 
    25. FROM ordl  
    26. GROUP BY ol_w_id  
    27. ORDER BY ol_w_id
    28. ;
    30. +---------+---------+-------------+------------+------------+------------+------------+
    31. | ol_w_id | ol_d_id | order_count | sum_amount | avg_amount | min_amount | max_amount |
    32. +---------+---------+-------------+------------+------------+------------+------------+
    33. |       1 |       1 |         297 |  917174.33 |    3088.13 |       0.00 |    9876.11 |
    34. |       2 |       1 |         329 | 1153354.23 |    3505.64 |       0.00 |    9979.34 |
    35. +---------+---------+-------------+------------+------------+------------+------------+
    36. 2 rows in set (0.00 sec)
    38. obclient> show variables like '%sql_mode%';
    39. +---------------+-------------------------------------------------------+
    40. | Variable_name | Value                                                 |
    41. +---------------+-------------------------------------------------------+
    42. | sql_mode      | PIPES_AS_CONCAT,STRICT_TRANS_TABLES,STRICT_ALL_TABLES |
    43. +---------------+-------------------------------------------------------+
    44. 1 row in set (0.00 sec)
    46. obclient> SET SESSION sql_mode='STRICT_ALL_TABLES,ONLY_FULL_GROUP_BY';                                                                                                                                    Query OK, 0 rows affected (0.00 sec)
    49. obclient> SELECT ol_w_id, ol_d_id
    50.     , count(*)                  order_count
    51.     , sum(ol_amount)            sum_amount
    52.     , round(avg(ol_amount),2)   avg_amount
    53.     , min(ol_amount)            min_amount
    54.     , max(ol_amount)            max_amount 
    55. FROM ordl  
    56. GROUP BY ol_w_id  
    57. ORDER BY ol_w_id
    58. ;
    60. ERROR 1055 (42000): 'tpccdb.ordl.ol_d_id' is not in GROUP BY
    61. obclient>