From 9367af4b1523741bba1ebb3bd5788c835fd49f1e Mon Sep 17 00:00:00 2001
From: DarkDef <darkdef@gmail.com>
Date: Sun, 25 Apr 2021 20:19:14 +0300
Subject: [PATCH] Fix for issue #18604

---
 framework/db/mssql/QueryBuilder.php           |  5 +--
 tests/framework/db/mssql/QueryBuilderTest.php | 32 +++++++++++++++++++
 2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/framework/db/mssql/QueryBuilder.php b/framework/db/mssql/QueryBuilder.php
index df399712c1..66e169cb46 100644
--- a/framework/db/mssql/QueryBuilder.php
+++ b/framework/db/mssql/QueryBuilder.php
@@ -184,9 +184,10 @@ class QueryBuilder extends \yii\db\QueryBuilder
         $type = $this->getColumnType($type);
 
         if (preg_match('/\s+DEFAULT\s+(["\']?\w*["\']?)/i', $type, $matches)) {
+            $value = strtolower($matches[1]) === 'null' ? null : $matches[1];
             $type = preg_replace('/\s+DEFAULT\s+(["\']?\w*["\']?)/i', '', $type);
             $sqlAfter[] = $this->dropConstraintsForColumn($table, $column, 'D');
-            $sqlAfter[] = $this->addDefaultValue("DF_{$constraintBase}", $table, $column, $matches[1]);
+            $sqlAfter[] = $this->addDefaultValue("DF_{$constraintBase}", $table, $column, $value);
         } else {
             $sqlAfter[] = $this->dropConstraintsForColumn($table, $column, 'D');
         }
@@ -213,7 +214,7 @@ class QueryBuilder extends \yii\db\QueryBuilder
     public function addDefaultValue($name, $table, $column, $value)
     {
         return 'ALTER TABLE ' . $this->db->quoteTableName($table) . ' ADD CONSTRAINT '
-            . $this->db->quoteColumnName($name) . ' DEFAULT ' . $this->db->quoteValue($value) . ' FOR '
+            . $this->db->quoteColumnName($name) . ' DEFAULT ' . ($value === null ? 'NULL' : $this->db->quoteValue($value)) . ' FOR '
             . $this->db->quoteColumnName($column);
     }
 
diff --git a/tests/framework/db/mssql/QueryBuilderTest.php b/tests/framework/db/mssql/QueryBuilderTest.php
index 624ca723b9..df16edc3ae 100644
--- a/tests/framework/db/mssql/QueryBuilderTest.php
+++ b/tests/framework/db/mssql/QueryBuilderTest.php
@@ -656,6 +656,38 @@ ALTER TABLE [foo1] ADD CONSTRAINT [UQ_foo1_bar] UNIQUE ([bar])";
         $this->assertEquals(false, $schema->getColumn('bar')->allowNull);
     }
 
+    public function testAlterColumnWithNull()
+    {
+        $qb = $this->getQueryBuilder();
+
+        $expected = "ALTER TABLE [foo1] ALTER COLUMN [bar] int NULL
+DECLARE @tableName VARCHAR(MAX) = '[foo1]'
+DECLARE @columnName VARCHAR(MAX) = 'bar'
+
+WHILE 1=1 BEGIN
+    DECLARE @constraintName NVARCHAR(128)
+    SET @constraintName = (SELECT TOP 1 OBJECT_NAME(cons.[object_id])
+        FROM (
+            SELECT sc.[constid] object_id
+            FROM [sys].[sysconstraints] sc
+            JOIN [sys].[columns] c ON c.[object_id]=sc.[id] AND c.[column_id]=sc.[colid] AND c.[name]=@columnName
+            WHERE sc.[id] = OBJECT_ID(@tableName)
+            UNION
+            SELECT object_id(i.[name]) FROM [sys].[indexes] i
+            JOIN [sys].[columns] c ON c.[object_id]=i.[object_id] AND c.[name]=@columnName
+            JOIN [sys].[index_columns] ic ON ic.[object_id]=i.[object_id] AND i.[index_id]=ic.[index_id] AND c.[column_id]=ic.[column_id]
+            WHERE i.[is_unique_constraint]=1 and i.[object_id]=OBJECT_ID(@tableName)
+        ) cons
+        JOIN [sys].[objects] so ON so.[object_id]=cons.[object_id]
+         WHERE so.[type]='D')
+    IF @constraintName IS NULL BREAK
+    EXEC (N'ALTER TABLE ' + @tableName + ' DROP CONSTRAINT [' + @constraintName + ']')
+END
+ALTER TABLE [foo1] ADD CONSTRAINT [DF_foo1_bar] DEFAULT NULL FOR [bar]";
+        $sql = $qb->alterColumn('foo1', 'bar', $this->integer()->null()->defaultValue(NULL));
+        $this->assertEquals($expected, $sql);
+    }
+
     public function testAlterColumnWithCheckConstraintOnDb()
     {
         $connection = $this->getConnection();