now expressions with database fields on both sides of the operator are compiled into Closures and hence are valid

class/ExpressionBuilder.php

@@ -1,5 +1,14 @@
 <?php
 
+class FieldName {
+    public string $fieldName;
+
+    public function __construct(string $fieldName)
+    {
+        $this->fieldName = $fieldName;
+    }
+}
+
 class ExpressionBuilder
 {
     // Automatic typecast.
@@ -11,13 +20,15 @@ class ExpressionBuilder
             } else { // is it a float?
                 return (double)$rval;
             }
-        } else { // it's a string
+        } else if (str_starts_with($rval, '"') && str_ends_with($rval, '"')){ // it's a string
             return substr($rval, 1, strlen($rval) - 2); // strip leading and trailing quotes
+        } else { // it must be a column name
+            return new FieldName($rval);
         }
     }
 
     // Divide expression into operands and operators.
-    static function splitCriterion(string $crstr): array
+    static function splitCriterion(string $crstr): array|Closure
     {
         preg_match("/([<>=!]+|LIKE|NOT LIKE|IN|NOT IN|CONTAINS|NOT CONTAINS|BETWEEN|NOT BETWEEN|EXISTS)/", $crstr, $matches, PREG_OFFSET_CAPTURE);
 
@@ -44,7 +55,38 @@ class ExpressionBuilder
             $right = self::automaticTypecast($right);
         }
 
-        return [$left, $op, $right];
+        // handle "default" criteria and closures
+        if (is_a($right, FieldName::class)) { // field name
+            return function($a) use ($left, $right, $op): bool {
+                $X = &$a[$left];
+                $Y = &$a[$right->fieldName];
+                switch ($op) {
+                    case "=": {
+                        return $X == $Y;
+                    }
+                    case "!=": {
+                        return $X != $Y;
+                    }
+                    case "<": {
+                        return $X < $Y;
+                    }
+                    case ">": {
+                        return $X > $Y;
+                    }
+                    case "<=": {
+                        return $X <= $Y;
+                    }
+                    case ">=": {
+                        return $X >= $Y;
+                    }
+                    default: {
+                        return false;
+                    }
+                }
+            };
+        } else { // default critera
+            return [$left, $op, $right];
+        }
     }
 
     // Build SleekDB query expression. Processes encapsulated expressions recursively as well.