The question is okay (though I'd personally prefer to ignore the bitwise operators - too much people with a string backgroun in procedurall programming and a weak level of DBMS skill will want to use them).
But I really don't understand why this question is in the category "Design and Theory". Clearly, it should be in the "T-SQL" category, right?
Hi Adrian,
Try to visualize a life without bitmasks instead.....
Remember the rules for first normal form - every column should only hold atomic values. Clearly, a bit mask violates that rule. If you want to store 8 yes/no values, use 8 columns instead. Define them as bit, if you must (I am not as opposed to the bit datatype as some, but not a big supporter either), and SQL Server will still use only 1 byte to store them. Or store them as CHAR(1) with a CHECK constraint to limit the population to 'Y' of 'N', and gain a lot of clarity at the price a mere 7 extra bytes.
What are some instances where a bitmask would be useful?
I've been programming for a while now and never really understood their purpose.
Thanks!
Bonnie
Hi Bonnie,
Usage of bitmasks was very common and useful up until the seventies, when memory footprint off applications and data came at a premium. If you have to run a program on your home computer with only 32 KB (yes, KILObytes) of memory -half of which was reserved for video memory, keyboard memory, and firmware-, you get to appreciate the value of combining multiple values in a single bytes. And when data has to be stored on a casette recorder with a recording andd playback speed of 300 bits per second, you get to apppreciate the savings even more.
Same for professional development. Those computers often had a whopping 64 kB to their disposal - still good reason to save a few bytes. Storage was on floppy disks, or, for the lucky, on hard disks. One of the earliest hard disk bragged a storage capacity of 5 MB, and came at a price of $ 50,000. Try telling your boss: "The good news is, I decomposed the bit mask in the main file for orders into seperate fields; the bad news is, we need to add a second 5 MB hard disk..."
In modern computer development, bit masks are still used quite often in low-level languages like assembly language, or C. These languages are commonly used for the most performance-critical bits of applications, and since many processors have instructions in their instruction sets for handling bit masks, using bit masks can yield a performance boost.
But in modern third-and-higher generation languages, bit masks have no place at all. And in databases, they should be outright forbidden.
Sometimes I do, sometimes I don't have that luxury. I am well aware of how much some third-party DBs suck, and how awful it can be to be stuck with them.
That makes knowledge of what bitmasks are and how they can be operated a Good Thing for those in a similar position as you currently are - but it still doesn't make it right to advertise the use of bitmasks as if they are a Good Thing in themselves.
(Note - I don't think that the QOTD actually "advertises" the use of bitmasks, but it does teach people about them without the "don't do this unless forced to by crappy third-party software" disclaimer that I think ought to be attached here).
Hugo,
Thanks for the quick response and the practical advice!
I thought I may have been missing out on a good shortcut, but I will stay away from them.
