Store multiple bit values in a single table column

Question

I need to store some sort of day-of-week scheduling in database, where I can schedule a record (say it represents a task) for a day or multiple days of the week. I need to h

Answer 1

I'm not sure what you mean by easy querying. It is all easy once you wrap it in a function. An option for easy ad hoc querying might be to have easily readable string fields and use some string manipulation processes work it out. Example:

UPDATE T SET Schedule = 'M|W|H'

SELECT * FROM T WHERE CHARINDEX(Schedule, 'M') > -1

Answer 2

You are talking about a "bit mask". These are handy devices as you can apply binary math on them to easily check the values, but they do take a little setup. To expand on that a little more it would look something like this.

-- You will want to work with constants
DECLARE @Mon INT, @Tue INT, @Wed INT, @Thu INT, @Fri INT, @Sat INT, @Sun INT

SET @Mon = POWER(2,0) -- 1
SET @Tue = POWER(2,1) -- 2
SET @Wed = POWER(2,2) -- 4
SET @Thu = POWER(2,3) -- 8
SET @Fri = POWER(2,4) -- 16
SET @Sat = POWER(2,5) -- 32
SET @Sun = POWER(2,6) -- 64

-- Set Monday and Wednesday
UPDATE T SET Schedule = @Mon | @Wed

-- Find all tasks scheduled on Tuesday
SELECT * FROM T WHERE Schedule & @Tue = @Tue

-- Find all tasks scheduled on Tuesday and Saturday
SELECT * FROM T WHERE Schedule & @Tue | @Sat = @Tue | @Sat

Answer 3

From the title, I was led here with a situation of having existing bit fields in a table and needing to convert to a single integer field.

In following the example of days-of-week, let's say have 7 individual bit fields that you want to convert into a (bit-mask) integer field named Schedule:

UPDATE T
SET Schedule = Mon * POWER(2,0) +
               Tue * POWER(2,1) +
               Wed * POWER(2,2) +
               Thu * POWER(2,3) +
               Fri * POWER(2,4) +
               Sat * POWER(2,5) +
               Sun * POWER(2,6)

Answer 4

You could store this as a bitfield and then use boolean logic operators to retrieve the values

for example:

CREATE TABLE [dbo].[testBF](
    [field1] [varchar](max) NOT NULL,
    [field2] [varchar](max) NOT NULL,
    [bitfield] [int] NOT NULL CONSTRAINT [DF_testBF_bitfield]  DEFAULT ((0))
) ON [PRIMARY]

Then for selects:

SELECT field1, field2,
       CASE WHEN (bitfield & 1) = 1 THEN 'monday ' ELSE '' END + 
       CASE WHEN (bitfield & 2) = 2 THEN 'tuesday ' ELSE '' END +
       CASE WHEN (bitfield & 4) = 4 THEN 'wednesday ' ELSE '' END +
       CASE WHEN (bitfield & 8) = 8 THEN 'thursday ' ELSE '' END +
       CASE WHEN (bitfield & 16) = 16 THEN 'friday' ELSE '' END as [days of week]
FROM testBF

To find all days that contain tuesday flag (tuesday is the 2nd bit or 2^1 or 2)

SELECT * 
FROM aTable
WHERE (bitfield & 2) = 2

or

SELECT * 
FROM aTable
WHERE (bitfield & 2) != 0

Note, the template in the second case will work for any bit -- that is for friday (the 5th bit or 2^4 or 16) would be

SELECT * 
FROM aTable
WHERE (bitfield & 16) != 0

Finally the general case... pass in a number (1 for monday) you get

SELECT * 
FROM aTable
WHERE (bitfield & POWER(2,@inNumOfWeekday-1)) != 0

---

This seems like a lot of work to me, when you could just save it as 5 (or 7 bit fields) but that is how you could do it.

For more examples look at the gist I wrote for another question:

https://gist.github.com/1846338

and the answer:

https://stackoverflow.com/a/9302106/215752