从一组MySQL的统计记录

Question

当我执行以下命令它告诉我

＃1247 - 参考 'StockCount' 不支持（参照组功能）

SELECT stockSymbol, count(*) as StockCount 
FROM US500 
group by stockSymbol 
AND StockCount > 100 
ORDER BY stockSymbol DESC 

打算什么为什么它不工作。

Answer 1

SELECT和任何WHERE子句在任何聚合GROUP BY函数之前应用。

如果你尝试这个，你会得到关于未知列StockCount错误的WHERE子句中：

SELECT stockSymbol, count(*) as StockCount 
FROM US500 
WHERE StockCount > 100 
GROUP BY stockSymbol 
ORDER BY stockSymbol DESC 

相反，使用HAVING发生后SELECT，WHERE，并GROUP BY适用于从汇总结果的条件GROUP BY：

SELECT stockSymbol, count(*) as StockCount 
FROM US500 
/* WHERE clause would go here */ 
GROUP BY stockSymbol 
HAVING StockCount > 100 
ORDER BY stockSymbol DESC 

ORDER BY最后发生，所以你也可以参考聚合列那里，如果哟你想：

SELECT stockSymbol, count(*) as StockCount 
FROM US500 
GROUP BY stockSymbol 
HAVING StockCount > 100 
ORDER BY StockCount DESC 

您的查询中的子句的顺序应该给你一些他们处理的顺序的想法。例如。 GROUP BY子句必须在WHERE子句和HAVING必须在GROUP BY之后。