在SQL中，如何获取整数的最大值？

Question

我正在尝试从MySQL数据库中找出整数（有符号或无符号）的最大值。有没有办法从数据库本身撤回此信息？

有没有我可以使用的内置常量或函数（标准SQL或MySQL专用）。

在http://dev.mysql.com/doc/refman/5.0/en/numeric-
types.html上，
它列出了值-
但是数据库有一种方法可以告诉我。

以下是MAX_BIGINT-我想要的是MAX_INT。

SELECT CAST( 99999999999999999999999 AS SIGNED ) as max_int;
# max_int | 9223372036854775807

Answer 1

在Mysql中，有一个便宜的技巧可以做到这一点：

mysql> select ~0;
+----------------------+
| ~0                   |
+----------------------+
| 18446744073709551615 |
+----------------------+

波浪号是按位取反。结果值是bigint。参见：http :
//dev.mysql.com/doc/refman/5.1/en/bit-functions.html#operator_bitwise-
invert

对于其他整数型，可以使用正确的bitshift运算符，>>如下所示：

SELECT ~0 as max_bigint_unsigned
,      ~0 >> 32 as max_int_unsigned
,      ~0 >> 40 as max_mediumint_unsigned
,      ~0 >> 48 as max_smallint_unsigned
,      ~0 >> 56 as max_tinyint_unsigned
,      ~0 >> 1  as max_bigint_signed
,      ~0 >> 33 as max_int_signed
,      ~0 >> 41 as max_mediumint_signed
,      ~0 >> 49 as max_smallint_signed
,      ~0 >> 57 as max_tinyint_signed
\G

*************************** 1. row ***************************
   max_bigint_unsigned: 18446744073709551615
      max_int_unsigned: 4294967295
max_mediumint_unsigned: 16777215
 max_smallint_unsigned: 65535
  max_tinyint_unsigned: 255
     max_bigint_signed: 9223372036854775807
        max_int_signed: 2147483647
  max_mediumint_signed: 8388607
   max_smallint_signed: 32767
    max_tinyint_signed: 127
1 row in set (0.00 sec)