Class Exists

django/db/models/expressions.py:1887–1911

class Exists(Subquery):
    template = "EXISTS(%(subquery)s)"
    output_field = fields.BooleanField()
    empty_result_set_value = False

    def __init__(self, queryset, **kwargs):
        super().__init__(queryset, **kwargs)
        self.query = self.query.exists()

    def select_format(self, compiler, sql, params):
        # Wrap EXISTS() with a CASE WHEN expression if a database backend
        # (e.g. Oracle) doesn't support boolean expression in SELECT or GROUP
        # BY list.
        if not compiler.connection.features.supports_boolean_expr_in_select_clause:
            sql = "CASE WHEN {} THEN 1 ELSE 0 END".format(sql)
        return sql, params

    def as_sql(self, compiler, *args, **kwargs):
        try:
            return super().as_sql(compiler, *args, **kwargs)
        except EmptyResultSet:
            features = compiler.connection.features
            if not features.supports_boolean_expr_in_select_clause:
                return "1=0", ()
            return compiler.compile(Value(False))


@deconstructible(path="django.db.models.OrderBy")