mysql类型转换

CAST (expr AS type)

BINARY          #  二进制字符串
CHAR(n)         #  普通字符串
DATE            #  日期字符串
DATETIME        #  时间日期字符串
DECIMAL(m,n)    #  精确浮点数
SIGNED          #  整数（有正负）
TIME            #  时间字符串
UNSIGNED        #  整数（正数）

mysql> select cast('25' as signed);
+----------------------+
| cast('25' as signed) |
+----------------------+
|                   25 |
+----------------------+