code: func (nr *nameResolver) resolveColumnNameInOnCondition(cn *ast.ColumnNameExpr) {
  	ctx := nr.currentContext()
  	join := ctx.joinNodeStack[len(ctx.joinNodeStack)-1]
  	tableSources := appendTableSources(nil, join)
  	if !nr.resolveColumnInTableSources(cn, tableSources) {
  		nr.Err = errors.Errorf("unknown column name %s", cn.Name.Name.O)
  	}
  }
docs: // resolveColumnNameInOnCondition resolves the column name in current join.