Question

1. My table has few fields with an amount column of type decimal.
2. This column will have either a deposited amount (a positive value) or a withdraw amount (a negative value).
3. I store the positive value as just 120 and the negative value as -50.
4. I sum the column and got the result as expected.
5. Mysql version is: 5.1.33-community.

6. When i checked mysql documentation about decimal i confused with their description.

7. Before MySQL 5.0.3, if you inserted
   +0003.1 into a DECIMAL(5,1) column, it was stored as +0003.1. As of MySQL
   5.0.3, it is stored as 3.1. For negative numbers, a literal –
   character is no longer stored.
   Applications that rely on the older
   behavior must be modified to account
   for this change. http://dev.mysql.com/doc/refman/5.0/en/precision-math-decimal-changes.html

8. When i listed the rows in phpmyadmin i could see the negative sign and when i calculated the result was as expected. but the documentation said no sign will appear.

9. Is it good to store negative number in decimal(haven’t i studied in school? confused)?… or do we have to use float.

10. I learn’t that float would complicate calculations and was advised to stick with decimal for certain conditions.

    I would like to have suggestions for this.

Answer 1

From what I understand, the documentation is saying it won’t store a literal “-” character, which means it’s probably now doing what the other signed INTEGER fields have always done and it’s storing a sign bit to denote negative numbers instead.

You’re still seeing a minus sign preceding the number because it’s being generated by MySQL as a result of that sign bit.

If you don’t understand the sign bit, you can consider how a signed byte can store numbers from -128 to 127, while an unsigned byte can store numbers from 0 to 255. That’s because one of the 8 bits in a signed number is being used to store +/- (1 is negative, 0 is positive), while the remaining bits offer numbers up to 2^7 (-128 or 127).

So, for example, if the bits 1111 had a sign bit they would equal -7 (negative+4+2+1), but if they were unsigned they’d equal 15 (8+4+2+1). It’s still the same amount of bits being stored.

You may wonder why the negative bound in a signed number can use the 8th bit, while the positive bound is limited to the sum of the 7 bits (1 less than the 8th bit). This is because 10000000 is considered to be both negative and the 8th bit simultaneously, because its representation of -0 otherwise is redundant with 00000000 which represents 0. There’s no distinction between negative and positive zero, so a negative most significant bit is always the value of that bit itself (but negative).