如强制转换函数和运算符中所述:
结果的类型可以是以下值之一:
BINARY[(N)] CHAr[(N)] DATE DATETIME DECIMAL[(M[,D])] SIGNED [INTEGER] TIME UNSIGNED [INTEGER]
因此,您应该使用:
SELECt CAST(PROD_CODE AS UNSIGNED) FROM PRODUCT

如强制转换函数和运算符中所述:
结果的类型可以是以下值之一:
BINARY[(N)] CHAr[(N)] DATE DATETIME DECIMAL[(M[,D])] SIGNED [INTEGER] TIME UNSIGNED [INTEGER]
因此,您应该使用:
SELECt CAST(PROD_CODE AS UNSIGNED) FROM PRODUCT