Question

i have to do a select query in a posting table where a specific bit of an integer is set.
The integer represents a set of categories in a bitmask:
E.g.

1 => health
2 => marketing
3 => personal
4 => music
5 => video
6 => design
7 => fashion
8 => ......

Data example:

id | categories | title
1  | 11         | bla bla
2  | 48         | blabla, too

I need a mysql query that selects postings, that are marked with a specific category.
Let’s say “all video postings”
This means i need a result set of postings where the 5th bit of the catgories column is set (e.g. 16,17,48 ….)

SELECT * FROM postings WHERE ....????

Any ideas ?

Answer 1

You can use bitwise operators like this. For video (bit 5):

WHERE categories & 16 = 16

Substitute the value 16 using the following values for each bit:

1 = 1
2 = 2
3 = 4
4 = 8
5 = 16
6 = 32
7 = 64
8 = 128

This goes from least significant bit to highest, which is opposite of the way most programmers think. They also start at zero.