MySQL从十进制转换为字符串

Question

为什么CAST在此MySQL查询中不起作用？

SELECT  MAX(Signups) AS Max, 
        MIN(Signups) AS Min,
        CAST(ROUND(AVG(Signups),2) AS VARCHAR(3)) AS Avg
FROM
(
    SELECT COUNT(1) AS Signups,
    DATE_FORMAT(JoinDate, "%Y-%m-%d") AS Date
    FROM users
    GROUP BY Date
) z 

为什么会出现此错误？

#1064 - 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 
'VARCHAR(3)) AS Avg FROM ( ' at line 2

Answer 1

这可能是由于MySQL错误＃34564：CAST不接受varchar type。

尝试强制转换为其他类型，如下所示：

CAST(ROUND(AVG(Signups),2) AS CHAR(3)) AS Avg