diff --git a/src/Sql/Predicate/In.php b/src/Sql/Predicate/In.php
index 97e5d5b4..c42623e5 100644
--- a/src/Sql/Predicate/In.php
+++ b/src/Sql/Predicate/In.php
@@ -111,29 +111,22 @@ public function getSQL(DriverInterface $driver = null, Params $params = null): s
         }
 
         $values = [];
-        $hasNull = false;
+        $has_null = false;
         foreach ($this->valueList as $value) {
             if (null === $value) {
-                $hasNull = true;
+                $has_null = true;
                 continue;
             }
             $values[] = $this->getValueSQL($params, $value, null, 'in');
         }
 
-        $ivl_sql = "(" . (empty($values) ? Sql::NULL : implode(", ", $values)) . ")";
+        $in_value_list = empty($values) ? Sql::NULL : implode(", ", $values);
 
-        $null_sql = "";
-        if ($hasNull) {
-            $null_sql = " " . (
-                static::$not
-                ? Sql::AND . " {$identifier} " . Sql::IS_NOT . " " . Sql::NULL
-                : Sql::OR . " {$identifier} " . Sql::IS . " " . Sql::NULL
-            );
-        }
+        $sql = "{$identifier} {$operator} ({$in_value_list})";
 
-        $sql = "{$identifier} {$operator} {$ivl_sql}{$null_sql}";
-        if ($hasNull) {
-            $sql = "({$sql})";
+        if ($has_null) {
+            $null_sql = static::$not ? "AND {$identifier} IS NOT NULL" : "OR {$identifier} IS NULL";
+            $sql = "({$sql} {$null_sql})";
         }
 
         return $this->sql = $sql;
diff --git a/src/Sql/Statement/Delete.php b/src/Sql/Statement/Delete.php
index fda97da9..da230ed7 100644
--- a/src/Sql/Statement/Delete.php
+++ b/src/Sql/Statement/Delete.php
@@ -80,8 +80,7 @@ public function getSQL(DriverInterface $driver = null, Params $params = null): s
             );
         }
 
-        $this->sql = Sql::DELETE . " " . Sql::FROM . " {$table} {$where_sql}";
-        return $this->sql;
+        return $this->sql = "DELETE FROM {$table} {$where_sql}";
     }
 
     /**
diff --git a/src/Sql/Statement/Insert.php b/src/Sql/Statement/Insert.php
index 4310b0a5..bdab6e8f 100644
--- a/src/Sql/Statement/Insert.php
+++ b/src/Sql/Statement/Insert.php
@@ -334,7 +334,7 @@ public function getSQL(DriverInterface $driver = null, Params $params = null): s
 
     protected function generateSQL(DriverInterface $driver, Params $params): string
     {
-        $insert  = $this->ignore ? Sql::INSERT_IGNORE : Sql::INSERT;
+        $INSERT  = $this->ignore ? Sql::INSERT_IGNORE : Sql::INSERT;
         $table   = $driver->quoteIdentifier($this->table);
         $columns = $this->getColumnsSQL($driver);
         $values  = $this->getValuesSQL($driver, $params);
@@ -351,10 +351,10 @@ protected function generateSQL(DriverInterface $driver, Params $params): string
         $column_list = empty($columns) ? "" : "{$columns} ";
 
         if ($this->select instanceof Select) {
-            return "{$insert} " . Sql::INTO . " {$table} {$column_list}{$values}";
+            return "{$INSERT} INTO {$table} {$column_list}{$values}";
         }
 
-        return "{$insert} " . Sql::INTO . " {$table} {$column_list}" . Sql::VALUES . " {$values}";
+        return "{$INSERT} INTO {$table} {$column_list}VALUES {$values}";
     }
 
     private function getColumnsSQL(DriverInterface $driver): string
diff --git a/src/Sql/Statement/Select.php b/src/Sql/Statement/Select.php
index 47c4f863..d98f6059 100644
--- a/src/Sql/Statement/Select.php
+++ b/src/Sql/Statement/Select.php
@@ -572,10 +572,11 @@ private function getFromSQL(DriverInterface $driver, Params $params, bool $prett
         }
 
         if (!empty($this->alias)) {
-            $from = trim("{$from} " . $driver->quoteAlias($this->alias));
+            $from = "{$from} {$driver->quoteAlias($this->alias)}";
+            $from = trim($from);
         }
 
-        return Sql::FROM . " {$from}";
+        return "FROM {$from}";
     }
 
     /**
@@ -794,7 +795,9 @@ private function getGroupBySQL(DriverInterface $driver): string
             $groupBy[$key] = $this->getIdentifierSQL($identifier, $driver);
         }
 
-        $this->sqls['group'] = $sql = Sql::GROUP_BY . " " . implode(", ", $groupBy);
+        $grouping_element_list = implode(", ", $groupBy);
+
+        $this->sqls['group'] = $sql = "GROUP BY {$grouping_element_list}";
         return $sql;
     }
 
@@ -902,7 +905,9 @@ private function getOrderBySQL(DriverInterface $driver): string
             $sqls[] = $this->getIdentifierSQL($identifier, $driver) . " {$direction}";
         }
 
-        $this->sqls['order'] = $sql = Sql::ORDER_BY . " " . implode(", ", $sqls);
+        $sort_specification_list = implode(", ", $sqls);
+
+        $this->sqls['order'] = $sql = "ORDER BY {$sort_specification_list}";
         return $sql;
     }
 
@@ -955,15 +960,15 @@ private function getLimitSQL(DriverInterface $driver, Params $params): string
         // PostgreSQL also supports OFFSET without LIMIT
         if (isset($this->limit)) {
             $limit = $params->create($this->limit, PDO::PARAM_INT, 'limit');
-            $sql = Sql::LIMIT . " {$limit}";
+            $sql = "LIMIT {$limit}";
         }
 
         if (isset($this->offset) && $this->offset > 0) {
             if (!isset($sql)) {
-                $sql = Sql::LIMIT . " " . PHP_INT_MAX;
+                $sql = "LIMIT " . PHP_INT_MAX;
             }
             $offset = $params->create($this->offset, PDO::PARAM_INT, 'offset');
-            $sql .= " " . Sql::OFFSET . " {$offset}";
+            $sql .= " OFFSET {$offset}";
         }
 
         return $sql ?? '';
@@ -1195,7 +1200,7 @@ public function __get(string $name)
         }
 
         if ('where' === $name) {
-            if (!isset($this->where)) {
+            if ($this->where === null) {
                 $this->where = new Where();
                 $this->where->setParent($this);
             }
@@ -1207,7 +1212,7 @@ public function __get(string $name)
         }
 
         if ('having' === $name) {
-            if (!isset($this->having)) {
+            if ($this->having === null) {
                 $this->having = new Having();
                 $this->having->setParent($this);
             }
diff --git a/src/Sql/Statement/Update.php b/src/Sql/Statement/Update.php
index 330c7709..a89d4978 100644
--- a/src/Sql/Statement/Update.php
+++ b/src/Sql/Statement/Update.php
@@ -161,7 +161,9 @@ private function getBaseSQL(DriverInterface $driver, Params $params): string
             $set[]  = "{$column} = {$marker}";
         }
 
-        return Sql::UPDATE . " {$table} " . Sql::SET . " " . implode(", ", $set);
+        $set_clause_list = implode(", ", $set);
+
+        return "UPDATE {$table} SET {$set_clause_list}";
     }
 
     /**
@@ -178,7 +180,7 @@ public function __get(string $name)
         }
 
         if ('where' === $name) {
-            if (!isset($this->where)) {
+            if ($this->where === null) {
                 $this->where = new Where();
                 $this->where->setParent($this);
             }
@@ -191,7 +193,7 @@ public function __get(string $name)
     public function __clone()
     {
         parent::__clone();
-        if (isset($this->where)) {
+        if ($this->where instanceof Where) {
             $this->where = clone $this->where;
             $this->where->setParent($this);
         }