    ///
    /// The second form uses a base expression and then a series of "when" clauses that match on a
    /// literal value.
    ///
    /// CASE expression
    ///     WHEN value THEN result
    ///     [WHEN ...]
    ///     [ELSE result]
    /// END
    Case {
        /// Optional base expression that can be compared to literal values in the "when" expressions
        expr: Option<Box<Expr>>,
        /// One or more when/then expressions
        when_then_expr: Vec<(Box<Expr>, Box<Expr>)>,