Enum JoinConstraintKind 

pub enum JoinConstraintKind {
    On(Box<Expr>),
    Using(Vec<String>),
    Natural,
}

How a JOIN matches rows. SQLR-5 originally shipped ON only; the USING / NATURAL increment adds the two name-based constraints. ON carries its predicate straight from the parser. USING and NATURAL defer their equality synthesis to the executor because they need table schemas (which column names exist, and — for NATURAL — which are shared) that the parser doesn’t have. The executor turns both into the same left.col = right.col [AND …] predicate the ON path already evaluates. CROSS JOIN is rewritten to ON true at parse time (no schema needed) and so reuses the On variant directly.

Variants

On(Box<Expr>)

ON <expr> (and the parse-time rewrite of CROSS JOIN to ON true). Evaluated per-row over the multi-table scope. Boxed to keep this enum small — Expr dwarfs the other variants.

Using(Vec<String>)

USING (col[, col…]) — equality on each named column, plus the SQLite convention that each named column appears once in SELECT *. Columns are validated and the predicate is synthesized at execution time.

Natural

NATURAL — the shared column names of the two sides are discovered at execution time, then treated exactly like USING (<shared cols>). No shared columns ⇒ a cross product.

Trait Implementations

impl Clone for JoinConstraintKind

fn clone(&self) -> JoinConstraintKind

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for JoinConstraintKind

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.