diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 20eb6f5..b00b525 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -9,6 +9,7 @@ Yii Framework 2 Change Log
 - Bug #13362: Fixed return value of `yii\caching\MemCache::setValues()`  (masterklavi)
 - Enh #13963: Added tests for yii\behaviors\TimestampBehavior (vladis84)
 - Enh #13994: Refactored `yii\filters\RateLimiter`. Added tests (vladis84)
+- Bug #13842: Fixed ambiguous error sql while using unique validator (vladis84)
 - Enh #13820: Add new HTTP status code 451 (yyxx9988)
 - Bug #13671: Fixed error handler trace to work correctly with XDebug (samdark)
 - Bug #13657: Fixed `yii\helpers\StringHelper::truncateHtml()` skip extra tags at the end (sam002)
diff --git a/framework/helpers/ActiveQueryHelper.php b/framework/helpers/ActiveQueryHelper.php
new file mode 100644
index 0000000..f136c56
--- /dev/null
+++ b/framework/helpers/ActiveQueryHelper.php
@@ -0,0 +1,107 @@
+<?php
+
+namespace yii\helpers;
+
+use yii\db\ActiveQuery;
+use yii\base\InvalidConfigException;
+
+class ActiveQueryHelper
+{
+    /**
+     * Table names calculate on from.
+     * @param ActiveQuery $query
+     * @return string[] table names
+     */
+    public static function getTableNames(ActiveQuery $query)
+    {
+        $tableNames = [];
+        $from = $query->from;
+
+        if (empty($from)) {
+            $tableNames[] = self::getTableNameForModel($query);
+        } elseif (is_array($from)) {
+            $tableNames = array_values($from);
+        } elseif (is_string($from)) {
+            $tableNames = preg_split('/\s*,\s*/', trim($from), -1, PREG_SPLIT_NO_EMPTY);
+        } else {
+            self::generateNotSupportedTypeFrom($from);
+        }
+
+        // Clear table alias.
+        foreach ($tableNames as &$tableName) {
+            $tableName = preg_replace('/^(\w+)\s*.*$/', '$1', $tableName);
+        }
+
+        return $tableNames;
+    }
+
+    /**
+     * Tables alias calculate on from.
+     * @param ActiveQuery $query
+     * @return string[] table alias
+     */
+    public static function getTablesAlias(ActiveQuery $query)
+    {
+        $tablesAlias = [];
+        $from = $query->from;
+
+        if (empty($from)) {
+            return [self::getTableNameForModel($query)];
+        }
+
+        if (is_string($from)) {
+           $tableNames = preg_split('/\s*,\s*/', trim($from), -1, PREG_SPLIT_NO_EMPTY);
+        } elseif (is_array($from)) {
+            $tableNames = $from;
+        } else {
+            self::generateNotSupportedTypeFrom($from);
+        }
+
+        foreach ($tableNames as $alias => $tableName) {
+            if (is_string($alias)) {
+                $tablesAlias[] = $alias;
+            } else {
+                $tablesAlias[] = self::getAliasForTableName($tableName);
+            }
+        }
+
+        return $tablesAlias;
+    }
+
+    /**
+     * @param string $tableName
+     * @return string
+     */
+    private static function getAliasForTableName($tableName)
+    {
+        $cleanedTableName = preg_replace('/\'|\"|`|as/u', '', trim($tableName));
+
+        $alias = preg_replace('/^.+\s+(\w+)$/', '$1', $cleanedTableName);
+
+        return $alias;
+    }
+
+    /**
+     * Table name get Model.
+     * @param ActiveQuery $query
+     * @return type
+     */
+    private static function getTableNameForModel(ActiveQuery $query)
+    {
+        /* @var $modelClass ActiveRecord */
+        $modelClass = $query->modelClass;
+
+        return $modelClass::tableName();
+    }
+
+    /**
+     * @param mixed $from
+     * @throws InvalidConfigException
+     */
+    private static function generateNotSupportedTypeFrom($from)
+    {
+        $error = sprintf('Not supported type "$s"', gettype($from));
+
+        throw new InvalidConfigException($error);
+    }
+}
diff --git a/framework/validators/UniqueValidator.php b/framework/validators/UniqueValidator.php
index 7446484..d17b5be 100644
--- a/framework/validators/UniqueValidator.php
+++ b/framework/validators/UniqueValidator.php
@@ -14,6 +14,7 @@ use yii\db\ActiveRecord;
 use yii\db\ActiveQueryInterface;
 use yii\db\ActiveRecordInterface;
 use yii\helpers\Inflector;
+use yii\helpers\ActiveQueryHelper;
 
 /**
  * UniqueValidator validates that the attribute value is unique in the specified database table.
@@ -249,14 +250,15 @@ class UniqueValidator extends Validator
 
         // Add table prefix for column
         $targetClass = $this->getTargetClass($model);
-        $tableName = $targetClass::tableName();
-        $conditionsWithTableName = [];
+        $query = $targetClass::find();
+        $tableAlias = ActiveQueryHelper::getTablesAlias($query)[0];
+        $prefixedConditions = [];
         foreach ($conditions as $columnName => $columnValue) {
-            $prefixedColumnName = "{$tableName}.$columnName";
-            $conditionsWithTableName[$prefixedColumnName] = $columnValue;
+            $prefixedColumn = "{$tableAlias}.{$columnName}";
+            $prefixedConditions[$prefixedColumn] = $columnValue;
         }
 
-        return $conditionsWithTableName;
+        return $prefixedConditions;
     }
 
     /**
diff --git a/tests/framework/helpers/ActiveQueryHelperTest.php b/tests/framework/helpers/ActiveQueryHelperTest.php
new file mode 100644
index 0000000..f78e592
--- /dev/null
+++ b/tests/framework/helpers/ActiveQueryHelperTest.php
@@ -0,0 +1,92 @@
+<?php
+
+namespace yii\helpers;
+
+use yiiunit\TestCase;
+use yii\db\ActiveQuery;
+use yii\helpers\ActiveQueryHelper;
+use yiiunit\data\ar\Profile;
+
+/**
+ * Generated by PHPUnit_SkeletonGenerator on 2017-03-25 at 03:32:13.
+ */
+class ActiveQueryHelperTest extends TestCase
+{
+    public function testGetTableNames_notFilledFrom()
+    {
+        $query = new ActiveQuery(Profile::className());
+
+        $tableNames = ActiveQueryHelper::getTableNames($query);
+
+        $this->assertEquals([Profile::tableName()], $tableNames);
+    }
+
+    public function testGetTableNames_isFromArray()
+    {
+        $query = new ActiveQuery(null);
+        $query->from = ['prf' => 'profile', 'usr' => 'user'];
+
+        $tableNames = ActiveQueryHelper::getTableNames($query);
+
+        $this->assertEquals(['profile', 'user'], $tableNames);
+    }
+
+    public function testGetTableNames_isFromString()
+    {
+        $query = new ActiveQuery(null);
+        $query->from = 'profile AS \'prf\', user "usr", `order`, "customer"';
+
+        $tableNames = ActiveQueryHelper::getTableNames($query);
+
+        $this->assertEquals(['profile', 'user', '`order`', '"customer"'], $tableNames);
+    }
+
+    public function testGetTableNames_isFromObject_generateException()
+    {
+        $query = new ActiveQuery(null);
+        $query->from = new \stdClass;
+
+        $this->setExpectedException('\yii\base\InvalidConfigException');
+
+        ActiveQueryHelper::getTableNames($query);
+    }
+
+    public function testGetTablesAlias_notFilledFrom()
+    {
+        $query = new ActiveQuery(Profile::className());
+
+        $tablesAlias = ActiveQueryHelper::getTablesAlias($query);
+
+        $this->assertEquals([Profile::tableName()], $tablesAlias);
+    }
+
+    public function testGetTablesAlias_isFromArray()
+    {
+        $query = new ActiveQuery(null);
+        $query->from = ['prf' => 'profile', 'usr' => 'user'];
+
+        $tablesAlias = ActiveQueryHelper::getTablesAlias($query);
+
+        $this->assertEquals(['prf', 'usr'], $tablesAlias);
+    }
+
+    public function testGetTablesAlias_isFromString()
+    {
+        $query = new ActiveQuery(null);
+        $query->from = 'profile AS \'prf\', user "usr", service srv, order';
+
+        $tablesAlias = ActiveQueryHelper::getTablesAlias($query);
+
+        $this->assertEquals(['prf', 'usr', 'srv', 'order'], $tablesAlias);
+    }
+
+    public function testGetTablesAlias_isFromObject_generateException()
+    {
+        $query = new ActiveQuery(null);
+        $query->from = new \stdClass;
+
+        $this->setExpectedException('\yii\base\InvalidConfigException');
+
+        ActiveQueryHelper::getTablesAlias($query);
+    }
+}