Question

当我执行下面的命令时，它告诉我

＃1247 - 参考＆＃39; StockCount＆＃39;不支持（参考群组功能）

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

为什么它不起作用。

Answer 1

SELECT和任何WHERE子句在任何聚合GROUP BY函数之前应用。

如果您尝试这样做，StockCount子句中的未知列WHERE会出现错误：

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

相反，请使用在HAVING，SELECT和WHERE之后发生的GROUP BY来应用条件来汇总来自GROUP BY的结果：

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

ORDER BY最后出现，因此您还可以在那里引用聚合列：

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

查询中子句的顺序应该可以让您了解它们的处理顺序。例如。 GROUP BY子句必须位于WHERE子句之后，HAVING必须位于GROUP BY之后。

MySQL计算组中的记录

1 个答案: