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copied!<p>The best option is 0 and 1 (as numbers - another answer suggests 0 and 1 as <em>CHAR</em> for space-efficiency but that's a bit too twisted for me), using NOT NULL and a check constraint to limit contents to those values. (If you need the column to be nullable, then it's not a boolean you're dealing with but an enumeration with three values...)</p> <p>Advantages of 0/1:</p> <ul> <li>Language independent. 'Y' and 'N' would be fine if everyone used it. But they don't. In France they use 'O' and 'N' (I have seen this with my own eyes). I haven't programmed in Finland to see whether they use 'E' and 'K' there - no doubt they're smarter than that, but you can't be sure.</li> <li>Congruent with practice in widely-used programming languages (C, C++, Perl, Javascript)</li> <li>Plays better with the application layer e.g. Hibernate</li> <li>Leads to more succinct SQL, for example, to find out how many bananas are ready to eat <code>select sum(is_ripe) from bananas</code> instead of <code>select count(*) from bananas where is_ripe = 'Y'</code> or even (yuk) <code>select sum(case is_ripe when 'Y' then 1 else 0) from bananas</code></li> </ul> <p>Advantages of 'Y'/'N':</p> <ul> <li>Takes up less space than 0/1</li> <li>It's what Oracle suggests, so might be what some people are more used to</li> </ul> <p>Another poster suggested 'Y'/null for performance gains. If you've <em>proven</em> that you need the performance, then fair enough, but otherwise avoid since it makes querying less natural (<code>some_column is null</code> instead of <code>some_column = 0</code>) and in a left join you'll conflate falseness with nonexistent records.</p>

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