all other values are considered Boolean true by default. Classes can define how their instances are treated in a Boolean context through the special method __nonzero__ (Python 2) or __bool__ (Python 3). For containers, __len__ (the special method for determining the length of containers) is used if the explicit Boolean conversion method is not defined. In Ruby, in contrast, only nil (Ruby's null value) and a special false object are false; all else (including the integer 0 and empty arrays) is true. In JavaScript, the empty string (""), null, undefined, NaN, +0, −0 and false are sometimes called falsy (of which the complement is truthy) to distinguish between strictly type-checked and coerced Booleans. As opposed to Python, empty containers (Arrays, Maps, Sets) are considered truthy. Languages such as PHP also use this approach. SQL Booleans appear in SQL when a condition is needed, such as clause, in form of predicate which is produced by using operators such as comparison operators, operator, etc. However, apart from and , these operators can also yield a third state, called , when comparison with is made. The SQL92 standard introduced and operators which evaluate a predicate, which predated the introduction of boolean type in SQL:1999. The SQL:1999 standard introduced a data type as an optional feature (T031). When restricted by a constraint, a SQL behaves like Booleans in other languages, which can store only and values. However, if it is nullable, which is the default like all other SQL data types, it can have the special null value also. Although the SQL standard defines three literals for the type – and it also says that the and "may be used interchangeably to mean exactly the same thing". This has caused some controversy because the identification subjects to the equality comparison rules for NULL. More precisely is not but . As of 2012 few major SQL systems implement the T031 feature. Firebird and PostgreSQL are notable exceptions, although PostgreSQL implements no literal; can be used instead. The treatment of boolean values differs between SQL systems. For example, in Microsoft SQL Server, boolean value is not supported at all, neither as a standalone data type nor representable as an integer. It shows the error message "An expression of non-boolean type specified in a context where a condition is expected" if a column is directly used in the clause, e.g. , while a statement such as yields