mysql - 如何让 MySQL 识别自定义列名？

Question

SELECT Y/X as Z
FROM (
    SELECT
    (count(county) FROM `cleanup_site_list_2011` WHERE county='kings') as X,
    (count(county) FROM `cleanup_site_list_2011` WHERE county='kings' AND people != 0) as Y
    FROM `cleanup_site_list_2011`
) as innertable

在上面的示例中，两个“SELECT ...”表示任何描述的嵌套查询(X 和 Y 将是整数)。当我尝试使用此语句时出现错误

#1054 - Unknown column 'Y' in 'field list'

如何让 MySQL 识别我创建的列的名称？另外，一旦数学计算出来，我将如何使除法结果为 float ？还是它会自动执行此操作？

Answer 1

处理此问题的一种方法是在子查询中为列提供“自定义名称”，然后在周围查询中使用这些自定义名称:

SELECT 
  Y/X as Z -- moved to outer query
FROM (
  SELECT 
    (...) as X, -- assign custom names in subquery
    (...) as Y, 
  FROM `table`
) as innertable;

---

对于第二个问题:

mysql> select 4 / 3;
+--------+
| 4 / 3  |
+--------+
| 1.3333 |
+--------+
1 row in set (0.00 sec)

这能回答吗？ (MySQL 5)

更多信息 here .