组内的mysql限制？

Question

我想限制组内记录的大小，这是我的试用版，如何正确操作？

mysql> select * from accounts limit 5 group by type;

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'group by type' at line 1

Answer 1

聚合函数(以及它需要的 GROUP BY)的要点是将多行变成一行。因此，如果您真的只想要前 5 个储蓄账户和前 5 个支票账户以及前 5 个美元账户等，您需要的更像是这样:

标准:特定账户类型的前 5 名账户余额

SELECT account_type, account_balance FROM accounts WHERE account_type='savings' 
   ORDER BY account_balance DESC LIMIT 5
UNION
SELECT account_type, account_balance FROM accounts WHERE account_type='chequing' 
   ORDER BY account_balance DESC LIMIT 5
UNION
SELECT account_type, account_balance FROM accounts WHERE account_type='USD' 
   ORDER BY account_balance DESC LIMIT 5;

这不是很漂亮，但是如果您使用脚本构建 SQL，然后代入 account_types 并将查询串联在一起就很简单。