Generalities
In programming languages that have a built-in Boolean data type, such as Pascal and Java, the comparison operators such as '>' and '≠' are usually defined to return a Boolean value. Conditional and iterative commands may be defined to test Boolean-valued expressions.
Languages without an explicit Boolean data type, like C90 and Lisp, may still represent truth values by some other data type. Lisp uses an empty list for false, and any other value for true. C uses an integer type, where relational expressions like i > j and logical expressions connected by && and || are defined to have value 1 if true and 0 if false, whereas the test parts of if, while, for, etc., treat any non-zero value as true. Indeed, a Boolean variable may be regarded (and be implemented) as a numerical variable with a single binary digit (bit), which can store only two values. It is worth noting that the implementation of booleans in computers are most likely represented as a full byte, rather than a bit; this is usually due to the ways computers transfer blocks of information.
Most programming languages, even those that do not have an explicit Boolean type, have support for Boolean algebraic operations such as conjunction (AND, &, *), disjunction (OR, |, +), equivalence (EQV, =, ==), exclusive or/non-equivalence (XOR, NEQV, ^, !=), and not (NOT, ~, !).
In some languages, like Ruby, Smalltalk, and Alice the "true" and "false" values belong to separate classes -- e.g. "True" and "False", respective -- so there is no single Boolean "type".
In SQL, which uses a three-valued logic for explicit comparisons because of its special treatment of Nulls, the Boolean data type (introduced in SQL:1999) is also defined to include more than two truth values, so that SQL "Booleans" can store all logical values resulting from the evaluation of predicates in SQL. A column of Boolean type can also be restricted to just TRUE and FALSE though.
In the lambda calculus model of computing, Boolean values can be represented as Church booleans.
Read more about this topic: Boolean Data Type