Handle system variables

Author: JanJakes

tests/WP_SQLite_Driver_Translation_Tests.php

@@ -294,6 +294,23 @@ public function testDataTypes(): void {
 		);
 	}
 
+	public function testSystemVariables(): void {
+		$this->assertQuery(
+			'SELECT NULL',
+			'SELECT @@sql_mode'
+		);
+
+		$this->assertQuery(
+			'SELECT NULL',
+			'SELECT @@SESSION.sql_mode'
+		);
+
+		$this->assertQuery(
+			'SELECT NULL',
+			'SELECT @@GLOBAL.sql_mode'
+		);
+	}
+
 	private function assertQuery( $expected, string $query ): void {
 		$driver = new WP_SQLite_Driver( new PDO( 'sqlite::memory:' ) );
 		$driver->query( $query );

wp-includes/sqlite-ast/class-wp-sqlite-driver.php

@@ -862,6 +862,13 @@ private function translate( $ast ) {
 				throw $this->not_supported_exception(
 					sprintf( 'data type: %s', $child->value )
 				);
+			case 'systemVariable':
+				// @TODO: Emulate some system variables, or use reasonable defaults.
+				//        See: https://dev.mysql.com/doc/refman/8.4/en/server-system-variable-reference.html
+				//        See: https://dev.mysql.com/doc/refman/8.4/en/server-system-variables.html
+
+				// When we have no value, it's reasonable to use NULL.
+				return 'NULL';
 			default:
 				return $this->translate_sequence( $ast->get_children() );
 		}