65535 error in unsigned type field in MySQL

$sql = "UPDATE ' dzxbbs_common_member ' SET newprompt=newprompt-1 WHERE uid= ' {$uid} '";

The type of newprompt in the ' dzxbbs_common_member ' table is the type of smallint unsigned length 6.

When the Newprompt value is 0 o'clock, execute this SQL statement and get an unexpected result newprompt the field value becomes 65535.

The server version that appears on this issue is the MySQL 5.1.38-log.

To resolve this need to set this parameter sql_mode= "No_unsigned_subtraction" enables this option after two UNSIGNED types are subtracted to return the signed type.

My local test results were an error.
#1690 –bigint UNSIGNED value is out of the range in ' (' Test '. ' Test '. ' View '-1) '

Server version: MySQL 5.5.16

Open source Electronic Business system Ecshop Commodity inventory also has the problem of suddenly becoming 65535

Add: Unsigned related field length

unsigned is not a negative number, use this type can increase the length of the data!
For example, if the tinyint max is 127, then the tinyint unsigned maximum can reach 127 * 2
The unsigned property is only for integers, and the binary property is used only for char and varchar.

Type	Description
tinyint	a very small integer
smallint	smaller integers
Mediumint	Medium size integers
Int	Standard integer
bigint	Larger integer
Float	Single-precision floating-point numbers
Double	Double-precision floating point numbers
Decimal	Floating-point numbers for a string

The name and the range of values for each numeric type are shown in table 2.

Type Description	Take value range
Tinyint[(m)]	Signed values: 128 to 127 (-27 to 27-1) Unsigned values: 0 to 255 (0 to 28-1) 1 bytes
smallint [(M)]	Signed values: 32768 to 32767 (-215 to 215-1) Unsigned value: 0 to 65535(0 to 6-1) 2 bytes
Mediumint[(m)]	Signed values:-8388608 to 8388607 (-22 3 to 22 3-1) Unsigned values: 0 to 16777215 (0 to 4-1) 3 bytes
Int[(m)]	Signed values: 2147683648 to 2147683647 (-231 to 231-1) Unsigned values: 0 to 4294967295 (0 to 232-1) 4 bytes
Bigint[(m)]	Signed values: 9223372036854775808 to 9223373036854775807 (-263 to 263-1) Unsigned values: 0 to 18446744073709551615 (0 to 264–1) 8 bytes
Float[(M, D)]	Min Non 0 value: ±1.175494351e-38
double[(M,D)]	Min Non 0 value: ±2.2250738585072014e-308
Decimal (M, D)	variable; the range of values depends on M and D

Table 2: Range of values for numeric column types

The amount of storage required for the various types of values is shown in table 3.

Type Description	Storage requirements
Tinyint[(m)]	1 bytes
smallint [(M)]	2 bytes
Mediumint[(m)]	3 bytes
Int[(m)]	4 bytes
Bigint[(m)]	8 bytes
Float[(M, D)]	4 bytes
Double[(M, D)]	8 bytes
Decimal (M, D)	M-byte ( mysql < 3.23), m+2 bytes ( mysql > 3.23)