diff --git a/src/Database/SqlPreprocessor.php b/src/Database/SqlPreprocessor.php
index 210253052..64fac25ba 100644
--- a/src/Database/SqlPreprocessor.php
+++ b/src/Database/SqlPreprocessor.php
@@ -312,6 +312,7 @@ private function formatAssigns(array $items): string
 	 */
 	private function formatWhere(array $items, string $mode): string
 	{
+		$default = '1=1';
 		$res = [];
 		foreach ($items as $k => $v) {
 			if (is_int($k)) {
@@ -319,27 +320,28 @@ private function formatWhere(array $items, string $mode): string
 				continue;
 			}
 
-			[$k, $operator] = explode(' ', $k . ' ');
+			[$k, $operator] = explode(' ', $k, 2) + [1 => ''];
 			$k = $this->delimit($k);
 			if (is_array($v)) {
-				if ($v) {
-					$res[] = $k . ' ' . ($operator ? $operator . ' ' : '') . 'IN (' . $this->formatList(array_values($v)) . ')';
-				} elseif ($operator === 'NOT') {
+				$kind = ['' => true, 'IN' => true, 'NOT' => false, 'NOT IN' => false][$operator] ?? null;
+				if ($v || $kind === null) {
+					$res[] = $k . ' ' . ($kind === null ? $operator : ($kind ? 'IN' : 'NOT IN')) . ' (' . $this->formatList(array_values($v)) . ')';
+
 				} else {
-					$res[] = '1=0';
+					$default = $kind ? '1=0' : '1=1';
+					if ($kind === ($mode === self::ModeAnd)) {
+						return "($default)";
+					}
 				}
+
 			} else {
 				$v = $this->formatValue($v);
-				$operator = $v === 'NULL'
-					? ($operator === 'NOT' ? 'IS NOT' : ($operator ?: 'IS'))
-					: ($operator ?: '=');
+				$operator = ['' => ['=', 'IS'], 'NOT' => ['!=', 'IS NOT']][$operator][$v === 'NULL'] ?? $operator;
 				$res[] = $k . ' ' . $operator . ' ' . $v;
 			}
 		}
 
-		return $items
-			? '(' . implode(') ' . strtoupper($mode) . ' (', $res) . ')'
-			: '1=1';
+		return '(' . implode(') ' . strtoupper($mode) . ' (', $res ?: [$default]) . ')';
 	}
 
 
diff --git a/tests/Database/SqlPreprocessor.phpt b/tests/Database/SqlPreprocessor.phpt
index 1b62d83ff..6594552bf 100644
--- a/tests/Database/SqlPreprocessor.phpt
+++ b/tests/Database/SqlPreprocessor.phpt
@@ -135,11 +135,11 @@ test('Auto-detects operator in WHERE conditions', function () use ($preprocessor
 
 	[$sql, $params] = $preprocessor->process(['SELECT id FROM tbl WHERE', [
 		'col_null NOT' => null,
-		'x.col_val NOT' => 'a', // not supported
+		'x.col_val NOT' => 'a',
 		'col_arr NOT' => [1, 2],
 	]]);
 
-	Assert::same(reformat('SELECT id FROM tbl WHERE ([col_null] IS NOT NULL) AND ([x].[col_val] NOT ?) AND ([col_arr] NOT IN (?, ?))'), $sql);
+	Assert::same(reformat('SELECT id FROM tbl WHERE ([col_null] IS NOT NULL) AND ([x].[col_val] != ?) AND ([col_arr] NOT IN (?, ?))'), $sql);
 });
 
 
@@ -148,12 +148,12 @@ test('Supports explicit operators in WHERE conditions', function () use ($prepro
 		'col_is =' => 1,
 		'col_not <>' => 1,
 		'col_like LIKE' => 'a',
-		'col_like NOT LIKE' => 'a', // not supported
+		'col_like NOT LIKE' => 'a',
 		'col_null =' => null, // always false
-		'col_arr =' => [1, 2], // not supported
+		'col_arr =' => [1, 2],
 	]]);
 
-	Assert::same(reformat('SELECT id FROM tbl WHERE ([col_is] = ?) AND ([col_not] <> ?) AND ([col_like] LIKE ?) AND ([col_like] NOT ?) AND ([col_null] = NULL) AND ([col_arr] = IN (?, ?))'), $sql);
+	Assert::same(reformat('SELECT id FROM tbl WHERE ([col_is] = ?) AND ([col_not] <> ?) AND ([col_like] LIKE ?) AND ([col_like] NOT LIKE ?) AND ([col_null] = NULL) AND ([col_arr] = (?, ?))'), $sql);
 });
 
 
@@ -166,7 +166,7 @@ test('Redundant WHERE operators', function () use ($preprocessor) {
 	[$sql, $params] = $preprocessor->process(['SELECT id FROM tbl WHERE', [
 		'col_arr NOT IN' => [],
 	]]);
-	Assert::same(reformat('SELECT id FROM tbl WHERE ()'), $sql); // buggy
+	Assert::same(reformat('SELECT id FROM tbl WHERE (1=1)'), $sql);
 });
 
 
@@ -175,7 +175,7 @@ test('Empty WHERE conditions', function () use ($preprocessor) {
 		'col_empty' => [],
 		'foo',
 	]]);
-	Assert::same(reformat('SELECT id FROM tbl WHERE (1=0) AND (?)'), $sql);
+	Assert::same(reformat('SELECT id FROM tbl WHERE (1=0)'), $sql);
 
 	[$sql, $params] = $preprocessor->process(['SELECT id FROM tbl WHERE', [
 		'col_empty' => [],
@@ -190,12 +190,12 @@ test('Empty WHERE conditions', function () use ($preprocessor) {
 	[$sql, $params] = $preprocessor->process(['SELECT id FROM tbl WHERE', [
 		'col_empty NOT' => [],
 	]]);
-	Assert::same(reformat('SELECT id FROM tbl WHERE ()'), $sql); // buggy
+	Assert::same(reformat('SELECT id FROM tbl WHERE (1=1)'), $sql);
 
 	[$sql, $params] = $preprocessor->process(['SELECT id FROM tbl WHERE', [
 		'col_empty NOT IN' => [],
 	]]);
-	Assert::same(reformat('SELECT id FROM tbl WHERE ()'), $sql); // buggy
+	Assert::same(reformat('SELECT id FROM tbl WHERE (1=1)'), $sql);
 });
 
 
@@ -204,7 +204,7 @@ test('Empty WHERE conditions joined with OR', function () use ($preprocessor) {
 		'col_empty' => [],
 		new SqlLiteral('foo'),
 	]]);
-	Assert::same(reformat('SELECT id FROM tbl WHERE (1=0) OR (foo)'), $sql);
+	Assert::same(reformat('SELECT id FROM tbl WHERE (foo)'), $sql);
 
 	[$sql, $params] = $preprocessor->process(['SELECT id FROM tbl WHERE ?or', [
 		'col_empty' => [],
@@ -215,13 +215,13 @@ test('Empty WHERE conditions joined with OR', function () use ($preprocessor) {
 		'col_empty' => [],
 		'col_empty NOT' => [],
 	]]);
-	Assert::same(reformat('SELECT id FROM tbl WHERE (1=0)'), $sql); // buggy
+	Assert::same(reformat('SELECT id FROM tbl WHERE (1=1)'), $sql);
 
 	[$sql, $params] = $preprocessor->process(['SELECT id FROM tbl WHERE ?or', [
 		'col_empty IN' => [],
 		'col_empty NOT IN' => [],
 	]]);
-	Assert::same(reformat('SELECT id FROM tbl WHERE (1=0)'), $sql); // buggy
+	Assert::same(reformat('SELECT id FROM tbl WHERE (1=1)'), $sql);
 });
 
 
@@ -369,7 +369,7 @@ test('WHERE conditions with SQL literals', function () use ($preprocessor) {
 test('empty WHERE conditions array', function () use ($preprocessor) {
 	[$sql, $params] = $preprocessor->process(['SELECT id FROM author WHERE', []]);
 
-	Assert::same(reformat('SELECT id FROM author WHERE 1=1'), $sql);
+	Assert::same(reformat('SELECT id FROM author WHERE (1=1)'), $sql);
 	Assert::same([], $params);
 });