Respect an explicit null modifier when changing a column

src/Database/Schema/BlueprintState.php

@@ -65,8 +65,10 @@ protected function preserveExistingColumnAttributes(Fluent $column): void
                 continue;
             }
 
+            $attributes = $column->getAttributes();
+
             foreach ($this->preservedAttributes as $attribute) {
-                if (!isset($column[$attribute]) && isset($existing[$attribute])) {
+                if (!array_key_exists($attribute, $attributes) && isset($existing[$attribute])) {
                     $column[$attribute] = $existing[$attribute];
                 }
             }

src/Database/Schema/Grammars/Concerns/MySqlBasedGrammar.php

@@ -43,10 +43,12 @@ public function compileChange(Blueprint $blueprint, Fluent $command)
                 $oldColumn = new ColumnDefinition($oldColumn);
             }
 
+            $attributes = $column->getAttributes();
+
             foreach ($this->modifiers as $modifier) {
                 if (method_exists($this, $method = "modify{$modifier}")) {
                     $mod = strtolower($modifier);
-                    $col = isset($oldColumn->{$mod}) && !isset($column->{$mod}) ? $oldColumn : $column;
+                    $col = isset($oldColumn->{$mod}) && !array_key_exists($mod, $attributes) ? $oldColumn : $column;
                     $sql .= $this->{$method}($blueprint, $col);
                 }
             }

src/Database/Schema/Grammars/PostgresGrammar.php

@@ -38,14 +38,16 @@ public function compileChange(Blueprint $blueprint, Fluent $command)
                 $oldColumn = new ColumnDefinition($oldColumn);
             }
 
+            $attributes = $column->getAttributes();
+
             foreach ($this->modifiers as $modifier) {
                 if ($modifier === 'Collate') {
                     continue;
                 }
 
                 if (method_exists($this, $method = "modify{$modifier}")) {
                     $mod = strtolower($modifier);
-                    $col = isset($oldColumn->{$mod}) && ! isset($column->{$mod}) ? $oldColumn : $column;
+                    $col = isset($oldColumn->{$mod}) && ! array_key_exists($mod, $attributes) ? $oldColumn : $column;
                     $constraints = (array) $this->{$method}($blueprint, $col);
 
                     foreach ($constraints as $constraint) {

src/Database/Schema/Grammars/SqlServerGrammar.php

@@ -43,10 +43,12 @@ public function compileChange(Blueprint $blueprint, Fluent $command)
                 $oldColumn = new ColumnDefinition($oldColumn);
             }
 
+            $attributes = $column->getAttributes();
+
             foreach ($this->modifiers as $modifier) {
                 if (method_exists($this, $method = "modify{$modifier}")) {
                     $mod = strtolower($modifier);
-                    $col = isset($oldColumn->{$mod}) && !isset($column->{$mod}) ? $oldColumn : $column;
+                    $col = isset($oldColumn->{$mod}) && !array_key_exists($mod, $attributes) ? $oldColumn : $column;
                     $sql .= $this->{$method}($blueprint, $col);
                 }
             }

tests/Database/Schema/Grammars/MySqlSchemaGrammarTest.php

@@ -70,4 +70,19 @@ public function testQuoteInDefaultValue()
         $statements = $this->runBlueprint($initialBlueprint);
         $this->assertSame("alter table `users` add `name` varchar(255) not null default 'O''Brian'", $statements[0]);
     }
+
+    public function testChangeCanRemoveDefaultWithNull()
+    {
+        $initialBlueprint = $this->getBlueprint('users');
+        $initialBlueprint->string('name')->default('admin');
+
+        $statements = $this->runBlueprint($initialBlueprint);
+        $this->assertSame("alter table `users` add `name` varchar(255) not null default 'admin'", $statements[0]);
+
+        $changedBlueprint = $this->getBlueprint('users');
+        $changedBlueprint->string('name')->nullable()->default(null)->change();
+
+        $statements = $this->runBlueprint($changedBlueprint);
+        $this->assertSame('alter table `users` modify `name` varchar(255) null', $statements[0]);
+    }
 }

tests/Database/Schema/Grammars/PostgresSchemaGrammarTest.php

@@ -79,4 +79,19 @@ public function testQuoteInDefaultValue()
         $parts = explode(', ', $statements[0]);
         $this->assertSame("alter table \"users\" add column \"name\" varchar(255) not null default 'O''Brian'", $parts[0]);
     }
+
+    public function testChangeCanRemoveDefaultWithNull()
+    {
+        $initialBlueprint = $this->getBlueprint('users');
+        $initialBlueprint->string('name')->default('admin');
+
+        $statements = $this->runBlueprint($initialBlueprint);
+        $this->assertSame("alter table \"users\" add column \"name\" varchar(255) not null default 'admin'", $statements[0]);
+
+        $changedBlueprint = $this->getBlueprint('users');
+        $changedBlueprint->string('name')->nullable()->default(null)->change();
+
+        $statements = $this->runBlueprint($changedBlueprint);
+        $this->assertSame('alter table "users" alter column "name" type varchar(255), alter column "name" drop not null, alter column "name" drop default, alter column "name" drop identity if exists', $statements[0]);
+    }
 }

tests/Database/Schema/Grammars/SQLiteSchemaGrammarTest.php

@@ -152,4 +152,16 @@ public function testChangeRebuildsTableExactlyOnce(): void
 
         $this->assertSame(1, $rebuilds, 'A single ->change() must rebuild the table exactly once.');
     }
+
+    public function testChangeCanRemoveDefaultWithNull(): void
+    {
+        [$pdo, , $builder] = $this->bootSqlite("CREATE TABLE users (id integer primary key, name varchar not null default 'bob')");
+
+        // An explicit null default must remove the existing default, not keep it.
+        $builder->table('users', fn (Blueprint $table) => $table->string('name')->nullable()->default(null)->change());
+
+        $info = $this->columnInfo($pdo, 'users', 'name');
+        $this->assertSame(0, (int) $info['notnull']);
+        $this->assertNull($info['dflt_value']);
+    }
 }

tests/Database/Schema/Grammars/SqlServerSchemaGrammarTest.php

@@ -71,4 +71,20 @@ public function testQuoteInDefaultValue()
         $statements = $this->runBlueprint($initialBlueprint);
         $this->assertSame("alter table \"users\" add \"name\" nvarchar(255) not null default 'O''Brian'", $statements[0]);
     }
+
+    public function testChangeCanRemoveDefaultWithNull()
+    {
+        $initialBlueprint = $this->getBlueprint('users');
+        $initialBlueprint->string('name')->default('admin');
+
+        $statements = $this->runBlueprint($initialBlueprint);
+        $this->assertSame('alter table "users" add "name" nvarchar(255) not null default \'admin\'', $statements[0]);
+
+        $changedBlueprint = $this->getBlueprint('users');
+        $changedBlueprint->string('name')->nullable()->default(null)->change();
+
+        $statements = $this->runBlueprint($changedBlueprint);
+        $this->assertCount(2, $statements);
+        $this->assertSame('alter table "users" alter column "name" nvarchar(255) null', $statements[1]);
+    }
 }