语法
TRUNCATE(X,D)
Returns the number X, truncated to D decimal places. If D is 0, the result has no decimal point or fractional part. D can be negative to cause D digits left of the decimal point of the value X to become zero.
返回数字X，截断到D小数位。 如果D为0，结果没有小数点或小数部分。 D是负数，导致值X的小数点左边的D数字变为零。
实例：
SELECT truncate(1314.1314, 3); # 1314.131 SELECT truncate(1314.1314, 1); # 1314.1 SELECT truncate(1314.1314, 0); # 1314 SELECT truncate(1314.1314, -1); # 1310 SELECT truncate(1314.1314, -3); # 1000 SELECT truncate(-1314.1314, -1); # -1310 SELECT truncate(-1314.1314, -3); # -1000 SELECT truncate(1314.1314*100, -3); # 131000
