/// A parsed `SELECT-FROM-WHERE` statement in the current SQL subset.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Select {
    /// Output expressions requested by the query.
    pub projection: Vec<SelectItem>,
    /// Source tables and their optional aliases.
    pub from: Vec<TableRef>,
    /// Optional filter predicate.
    pub selection: Option<Expr>,
}

/// One source entry in a `FROM` list.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct TableRef {
    /// The predicate-backed table name.
    pub name: String,
    /// Optional table alias used for qualification.
    pub alias: Option<String>,
}

/// One item in a `SELECT` projection list.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum SelectItem {
    /// `*`
    Wildcard,
    /// A projected expression, optionally renamed with `AS`.
    Expr { expr: Expr, alias: Option<String> },
}

/// A SQL expression in the current subset.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Expr {
    /// A column reference.
    Identifier(String),
    /// A literal value.
    Literal(Literal),
    /// A binary expression.
    Binary {
        left: Box<Expr>,
        op: BinaryOp,
        right: Box<Expr>,
    },
}

/// A SQL literal in the current subset.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Literal {
    /// A string literal.
    String(String),
    /// The `NULL` literal.
    Null,
}

/// A binary operator in the current subset.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum BinaryOp {
    /// Equality.
    Eq,
}