Update one certain bit in a bitmask field (SQL Server) - sql-server

Question

I have an int column containing 8 bits in my SQL Server database. How can I update certain bit without affecting others?
For example, I have a value
11010000
and I want to set bit1 and bit2 to 1, so it would become
11010011
Looked through bitwise operators but couldn't find proper solution.
My goal is not only to update certain bit, but also to avoid database locks.
So when transaction1 updates bit1 in a certain record, another transaction2 could update bit2 in the same field of the same record at the same time.
Is this possible? Or is using 8 separate bit columns the only way?

Answer 1

What you are looking for are the Bitwise operations. To always turn on the proper bit, use a bitwise OR. So, to turn on bits 1 and 2 (total value = 3), use an statement like this (assumes the value in #value is 208, or 11010000 binary):
SET #value = #value | 3
-- or, the alternate form
SET #value |= 3
Other operators are the Bitwise AND (SET #value = #value & 3), and the bitwise NOT (SET #Value = #value ~ 3), and the Bitwise XOR (SET #value = #value ^ 3).
That said, having eight bit fields is easier logically for a new programmer. I don't need to find something special to see that the field ShowCurrencySymbol is the flag for showing a currency symbol, as opposed to finding out what the fifth bit in a byte does. And, since the fields are compacted internally so that eight one-bit, non-nullable fields = one byte of space used (adding NULL takes two bits per bit).
Finally, you can't have two transactions simultaneously update a field on the same row. While one update is occurring, the row will be locked, preventing the other update from processing. If you really want something like this, you will have to use a much more expansive method - a separate BitValues or Flags table, something like this:
CREATE TABLE Flags (
RowID int not null,
FlagName varchar(16) Not Null,
BitValue bit not null,
CONSTRAINT PK_Flags PRIMARY KEY (RowID, FlagName)
);
Then you read and write your flags from this table separate from the row.

Answer 2

A pseudo-bitset value is when you have an integer or string that, when selected or printed, looks like a bitset. Something like this:
DECLARE #bits varchar(8) = '1010'
SELECT Right(8, REPLICATIE('0', 8) + #bits
Value returned: 00001010
For this type of bitset, you need to remember that you are not setting the bits, but are representing the bits instead, and you need to set the representation of the bits in the same way. For example, the function below can be used to set bits in a string representation of your bitstring:
CREATE FUNCTION dbo.SetBitInString
(
#Source char(8),
#Position int,
#Action char(1) = 'S',
#Value bit = NULL
)
RETURNS Char(8)
AS
BEGIN
DECLARE #work Char(8) = '00000000'
SET #work = Right(#work + ISNULL(LTRIM(RTRIM(#Source)), ''), 8)
SET #Work =
CASE #Action
WHEN 'S'-- Set
THEN Stuff(#Work, #Position, 1, '1')
WHEN 'R' -- Reset
THEN Stuff(#Work, #Position, 1, '0')
WHEN 'X' -- XOR value with position
THEN STUFF(#Work, #Position, 1, CAST(CAST(SubString(#Work, #Position, 1) as int) ^ ISNULL(#Value, 0) as CHAR(1)))
WHEN 'Q'
THEN '1'
---- add other options as needed - this is a quick example.
ELSE #Action
END
IF (#Action = #Work)
RAISERROR('Bad Action (%s) Submitted', 13, 1, #Action)
RETURN #Work
END
Reading a bit is a simple SUBSTRING. Constants can be defined for the meaning of each bit (e.g. DECLARE #ShowTotalPrice = 4 -- The 4th bit is the Show Total Price flag)
This should give you enough to go on if you want to use this style of setting, where the displayed value is your bitset represented in 1s and 0s.