SQL: best way to store yes/no values? Looking after performance in huge databases
Semantically, I suggest you use bit if it's available to you. When looking at the column, any other developer can immediately determine that a boolean value is stored in it. If you don't have bit, try using tinyint. Ensuring that 1 is the only true value and 0 is the only false value will bring consistency. Otherwise, you could end up with a messy mixture of true/false, yes/no, valid/invalid, y/n, and/or t/f.
Comparing bit or tinyint values probably isn't slower than comparing strings, and even if it were slower than comparing strings, I can't imagine it having a significant effect on overall speed.
Is there something you don't like about the 'bit' data type?
The most commonly supported means is to use CHAR(1) - on most databases, it takes the same amount of space as BIT (assuming BIT is available, 1 byte) but supports more values (26 if case insensitive, 52 if not) if there's any chance of supporting more values. Unlike BIT, CHAR(1) is human readable. Also, BIT isn't supported on every database.