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I have a table MACRecord in MYSQL. I have 3 fields declared as TINYINT(4) (RSSI1, RSSI2, RSSI3) with a default value of 0. I use it to store signed negative values i-e -90, -84 etc. I am trying to understand how much length each number is taking in the column. I have a c++ code to output length of each column in bytes as follows:

lengths = mysql_fetch_lengths(result);
num_rows = mysql_num_rows(result);

for(i = 3; i < num_fields; i++)
{
     if (strstr(fields[i].name, "RSSI") != NULL) 
     {
         printf("Column %u is %lu bytes in length.\n",
                    i, lengths[i]);
     }

So now for example if columns 3, 4 and 5 which are my RSSI's columns contain values such as -90, 0(default), -83, i get the output as follows:

Column 3 is 3 bytes in length

Column 4 is 1 bytes in length.

Column 5 is 3 bytes in length

I don't understand why having a value of for example -90 in my RSSI's columns which were declared as TINYINT(4) which can only store 1 byte values is displaying as having 3 bytes of data? The default value of 0 makes sense. If I describe my table I can see that the data type is TINYINT but still it says values 3 bytes in length. 

| Field     | Type        | Null | Key | Default | Extra  |   
| RSSI1     | tinyint(4)  | YES  |     | 0       |        |
| RSSI2     | tinyint(4)  | YES  |     | 0       |        |
| RSSI3     | tinyint(4)  | YES  |     | 0       |        |

I am a bit confused. Any help will be appreciated.

Ravinder Reddy
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srai
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1 Answers1

1

don't understand why having a value of for example -90 in my RSSI's columns which were declared as TINYINT(4) which can only store 1 byte values

in binary ...

is displaying as having 3 bytes of data?

in decimal.

"-90" is three bytes in decimal, and -90 is one byte of binary.

user207421
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  • Hi, I am still not sure about this. My confusion is when i declared RSSI column to be a TINYINT data type, i expected it should only store 1 byte of data. Why does it say 3 byte for a storage of a value such as -70 or -90? Can you please provide further clarification? Thanks – srai May 12 '14 at 06:09
  • Is it storing the sign i-e (-ive - ) in one byte and the next 2 numbers as separate 2 bytes? Is that is why it is giving me 3 bytes? But this does not make sense to me. -90 can be stored in 1 byte as 1010010? – srai May 12 '14 at 06:43
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    I've told you. "-90" is three bytes. "-", "9", and "0", in decimal, whereas 1010010 is binary. – user207421 May 12 '14 at 06:50