diff --git a/application/Espo/ORM/Query/Part/Join.php b/application/Espo/ORM/Query/Part/Join.php
index 57dcc0e4ad..041f15248c 100644
--- a/application/Espo/ORM/Query/Part/Join.php
+++ b/application/Espo/ORM/Query/Part/Join.php
@@ -29,6 +29,8 @@
 
 namespace Espo\ORM\Query\Part;
 
+use Espo\ORM\Query\Select;
+use LogicException;
 use RuntimeException;
 
 /**
@@ -38,11 +40,18 @@ use RuntimeException;
  */
 class Join
 {
+    /** A table join. */
+    public const TYPE_TABLE = 0;
+    /** A relation join. */
+    public const TYPE_RELATION = 1;
+    /** A sub-query join. */
+    public const TYPE_SUB_QUERY = 3;
+
     private ?WhereItem $conditions = null;
     private bool $onlyMiddle = false;
 
     private function __construct(
-        private string $target,
+        private string|Select $target,
         private ?string $alias = null
     ) {
         if ($target === '' || $alias === '') {
@@ -51,10 +60,10 @@ class Join
     }
 
     /**
-     * Get a join target. A relationName or table.
-     * A relationName is in camelCase, a table is in CamelCase.
+     * Get a join target. A relation name, table or sub-query.
+     * A relation name is in camelCase, a table is in CamelCase.
      */
-    public function getTarget(): string
+    public function getTarget(): string|Select
     {
         return $this->target;
     }
@@ -75,16 +84,51 @@ class Join
         return $this->conditions;
     }
 
+    /**
+     * Is a sub-query join.
+     */
+    public function isSubQuery(): bool
+    {
+        return !is_string($this->target);
+    }
+
+    /**
+     * Is a table join.
+     */
     public function isTable(): bool
     {
-        return $this->target[0] === ucfirst($this->target[0]);
+        return is_string($this->target) && $this->target[0] === ucfirst($this->target[0]);
     }
 
+    /**
+     * Is a relation join.
+     */
     public function isRelation(): bool
     {
-        return !$this->isTable();
+        return !$this->isSubQuery() && !$this->isTable();
     }
 
+    /**
+     * Get a join type.
+     *
+     * @return self::TYPE_TABLE|self::TYPE_RELATION|self::TYPE_SUB_QUERY
+     */
+    public function getType(): int
+    {
+        if ($this->isSubQuery()) {
+            return self::TYPE_SUB_QUERY;
+        }
+
+        if ($this->isRelation()) {
+            return self::TYPE_RELATION;
+        }
+
+        return self::TYPE_TABLE;
+    }
+
+    /**
+     * Is only middle table to be joined.
+     */
     public function isOnlyMiddle(): bool
     {
         return $this->onlyMiddle;
@@ -93,18 +137,23 @@ class Join
     /**
      * Create.
      *
-     * @param string $target
-     * A relation name or table. A relationName should be in camelCase, a table in CamelCase.
-     * When joining a table, conditions should be specified.
-     * When joining a relation, conditions will be applied automatically.
+     * @param string|Select $target
+     * A relation name, table or sub-query. A relation name should be in camelCase, a table in CamelCase.
+     * When joining a table or sub-query, conditions should be specified.
+     * When joining a relation, conditions will be applied automatically, additional conditions can
+     * be specified as well.
+     * @param ?string $alias An alias.
      */
-    public static function create(string $target, ?string $alias = null): self
+    public static function create(string|Select $target, ?string $alias = null): self
     {
         return new self($target, $alias);
     }
 
     /**
      * Create with a table target.
+     *
+     * @param string $table A table name. Should start with an upper case letter.
+     * @param ?string $alias An alias.
      */
     public static function createWithTableTarget(string $table, ?string $alias = null): self
     {
@@ -113,12 +162,26 @@ class Join
 
     /**
      * Create with a relation target. Conditions will be applied automatically.
+     *
+     * @param string $relation A relation name. Should start with a lower case letter.
+     * @param ?string $alias An alias.
      */
     public static function createWithRelationTarget(string $relation, ?string $alias = null): self
     {
         return self::create(lcfirst($relation), $alias);
     }
 
+    /**
+     * Create with a sub-query.
+     *
+     * @param Select $subQuery A sub-query.
+     * @param string $alias An alias.
+     */
+    public static function createWithSubQuery(Select $subQuery, string $alias): self
+    {
+        return new self($subQuery, $alias);
+    }
+
     /**
      * Clone with an alias.
      */
@@ -146,6 +209,10 @@ class Join
      */
     public function withOnlyMiddle(bool $onlyMiddle = true): self
     {
+        if (!$this->isRelation()) {
+            throw new LogicException("Only-middle is compatible only with relation joins.");
+        }
+
         $obj = clone $this;
         $obj->onlyMiddle = $onlyMiddle;
 
diff --git a/application/Espo/ORM/Query/SelectingBuilderTrait.php b/application/Espo/ORM/Query/SelectingBuilderTrait.php
index 776e9f19be..7ac7ce81e3 100644
--- a/application/Espo/ORM/Query/SelectingBuilderTrait.php
+++ b/application/Espo/ORM/Query/SelectingBuilderTrait.php
@@ -35,6 +35,7 @@ use Espo\ORM\Query\Part\Order;
 use Espo\ORM\Query\Part\Join;
 
 use InvalidArgumentException;
+use LogicException;
 use RuntimeException;
 
 trait SelectingBuilderTrait
@@ -156,42 +157,55 @@ trait SelectingBuilderTrait
     /**
      * Add JOIN.
      *
-     * @param Join|string $target
-     * A relation name or table. A relation name should be in camelCase, a table in CamelCase.
+     * @param Join|string|Select $target A relation name, table or sub-query. A relation name should be in camelCase,
+     *     a table in CamelCase.
      * @param ?string $alias An alias.
      * @param WhereItem|array<string|int, mixed>|null $conditions Join conditions.
      */
-    public function join($target, ?string $alias = null, $conditions = null): self
-    {
+    public function join(
+        $target,
+        ?string $alias = null,
+        WhereItem|array|null $conditions = null
+    ): self {
+
         return $this->joinInternal('joins', $target, $alias, $conditions);
     }
 
     /**
      * Add LEFT JOIN.
      *
-     * @param Join|string $target
-     * A relation name or table. A relation name should be in camelCase, a table in CamelCase.
+     * @param Join|string|Select $target A relation name, table or sub-query. A relation name should be in camelCase,
+     *     a table in CamelCase.
      * @param ?string $alias An alias.
      * @param WhereItem|array<string|int, mixed>|null $conditions Join conditions.
      */
-    public function leftJoin($target, ?string $alias = null, $conditions = null): self
-    {
+    public function leftJoin(
+        $target,
+        ?string $alias = null,
+        WhereItem|array|null $conditions = null
+    ): self {
+
         return $this->joinInternal('leftJoins', $target, $alias, $conditions);
     }
 
     /**
      * @param 'leftJoins'|'joins' $type
-     * @param Join|string $target
-     * A relation name or table. A relation name should be in camelCase, a table in CamelCase.
-     * @param string|null $alias An alias.
-     * @param WhereItem|array<string|int, mixed>|null $conditions Join conditions.
-     *
      * @todo Support USE INDEX in Join.
+     * $target can be an array for backward compatibility.
+     * @param Join|string|Select $target $target
+     * @param WhereItem|array<string|int, mixed>|null $conditions
      */
-    private function joinInternal(string $type, $target, ?string $alias = null, $conditions = null): self
-    {
+    private function joinInternal(
+        string $type,
+        $target,
+        ?string $alias = null,
+        WhereItem|array|null $conditions = null
+    ): self {
+
         $onlyMiddle = false;
 
+        /** @var string|Join|array<int, mixed> $target */
+
         if ($target instanceof Join) {
             $alias = $alias ?? $target->getAlias();
             $conditions = $conditions ?? $target->getConditions();
@@ -199,11 +213,8 @@ trait SelectingBuilderTrait
             $target = $target->getTarget();
         }
 
-        /** @phpstan-var mixed $conditions */
-        /** @phpstan-var mixed $target */
-
-        if ($conditions !== null && !is_array($conditions) && !$conditions instanceof WhereItem) {
-            throw new InvalidArgumentException("Conditions must be WhereItem or array.");
+        if ($target instanceof Select && !$alias) {
+            throw new LogicException("Sub-query join can't be used w/o alias.");
         }
 
         $noLeftAlias = false;
@@ -228,7 +239,12 @@ trait SelectingBuilderTrait
             return $this;
         }
 
-        if (is_null($alias) && is_null($conditions) && $this->hasJoinAliasInternal($type, $target)) {
+        if (
+            is_null($alias) &&
+            is_null($conditions) &&
+            is_string($target) &&
+            $this->hasJoinAliasInternal($type, $target)
+        ) {
             return $this;
         }
 
diff --git a/application/Espo/ORM/QueryComposer/BaseQueryComposer.php b/application/Espo/ORM/QueryComposer/BaseQueryComposer.php
index 2276a76b49..71e2f2adf3 100644
--- a/application/Espo/ORM/QueryComposer/BaseQueryComposer.php
+++ b/application/Espo/ORM/QueryComposer/BaseQueryComposer.php
@@ -2931,10 +2931,10 @@ abstract class BaseQueryComposer implements QueryComposer
             $itemParams = [];
 
             if (is_array($item)) {
-                $relationName = $item[0];
+                $target = $item[0];
 
                 if (count($item) > 1) {
-                    $alias = $item[1] ?? $relationName;
+                    $alias = $item[1] ?? $target;
 
                     if (count($item) > 2) {
                         $itemConditions = $item[2] ?? [];
@@ -2945,12 +2945,16 @@ abstract class BaseQueryComposer implements QueryComposer
                     }
                 }
                 else {
-                    $alias = $relationName;
+                    $alias = $target;
+                }
+
+                if ($target instanceof Select && !is_string($alias)) {
+                    throw new LogicException("Sub-query join can't be w/o alias");
                 }
             }
             else {
-                $relationName = $item;
-                $alias = $relationName;
+                $target = $item;
+                $alias = $target;
             }
 
             $conditions = [];
@@ -2965,7 +2969,7 @@ abstract class BaseQueryComposer implements QueryComposer
 
             $sql = $this->getJoinItemPart(
                 $entity,
-                $relationName,
+                $target,
                 $isLeft,
                 $conditions,
                 $alias,
@@ -3119,7 +3123,7 @@ abstract class BaseQueryComposer implements QueryComposer
      */
     protected function getJoinItemPart(
         Entity $entity,
-        string $name,
+        string|Select $target,
         bool $isLeft = false,
         array $conditions = [],
         ?string $alias = null,
@@ -3127,27 +3131,44 @@ abstract class BaseQueryComposer implements QueryComposer
         array $params = []
     ): string {
 
-        $prefix = ($isLeft) ? 'LEFT ' : '';
+        $prefixPart = $isLeft ? 'LEFT ' : '';
 
-        if (!$entity->hasRelation($name)) {
-            $alias = !$alias ?
-                $this->sanitize($name) :
-                $this->sanitizeSelectAlias($alias);
+        if (!is_string($target) || !$entity->hasRelation($target)) {
+            if ($alias === '') {
+                throw new LogicException("Empty alias.");
+            }
 
-            $table = $this->toDb($this->sanitize($name));
+            if (!is_string($target)) {
+                if ($alias === null) {
+                    throw new LogicException();
+                }
 
-            $sql = $prefix . "JOIN " . $this->quoteIdentifier($table) . " AS " . $this->quoteIdentifier($alias);
+                $alias = $this->sanitizeSelectAlias($alias);
+            }
+            else {
+                $alias = $alias === null ?
+                    $this->sanitize($target) :
+                    $this->sanitizeSelectAlias($alias);
+            }
 
-            if (empty($conditions)) {
+            $targetPart = is_string($target) ?
+                $this->quoteIdentifier($this->toDb($this->sanitize($target))) :
+                '(' . $this->composeSelecting($target) . ')';
+
+            $aliasPart = $this->quoteIdentifier($alias);
+
+            $sql = $prefixPart . "JOIN $targetPart AS $aliasPart";
+
+            if ($conditions === []) {
                 return $sql;
             }
 
             $sql .= " ON";
 
-            $joinSqlList = [];
+            $conditionParts = [];
 
             foreach ($conditions as $left => $right) {
-                $joinSqlList[] = $this->buildJoinConditionStatement(
+                $conditionParts[] = $this->buildJoinConditionStatement(
                     $entity,
                     $alias,
                     $left,
@@ -3157,12 +3178,12 @@ abstract class BaseQueryComposer implements QueryComposer
                 );
             }
 
-            $sql .= " " . implode(" AND ", $joinSqlList);
+            $sql .= " " . implode(" AND ", $conditionParts);
 
             return $sql;
         }
 
-        $relationName = $name;
+        $relationName = $target;
 
         $keySet = $this->helper->getRelationKeys($entity, $relationName);
 
@@ -3247,16 +3268,16 @@ abstract class BaseQueryComposer implements QueryComposer
                 $middleDeletedColumn = $this->quoteColumn("$midAlias.deleted");
 
                 $sql =
-                    "{$prefix}JOIN ".$this->quoteIdentifier($relTable)." AS " .
+                    "{$prefixPart}JOIN ".$this->quoteIdentifier($relTable)." AS " .
                     $this->quoteIdentifier($midAlias) . "$indexPart " .
                     "ON $leftKeyColumn = $middleKeyColumn" .
                     " AND " .
                     "$middleDeletedColumn = " . $this->quote(false);
 
-                $joinSqlList = [];
+                $conditionParts = [];
 
                 foreach ($conditions as $left => $right) {
-                    $joinSqlList[] = $this->buildJoinConditionStatement(
+                    $conditionParts[] = $this->buildJoinConditionStatement(
                         $entity,
                         $midAlias,
                         $left,
@@ -3265,8 +3286,8 @@ abstract class BaseQueryComposer implements QueryComposer
                     );
                 }
 
-                if (count($joinSqlList)) {
-                    $sql .= " AND " . implode(" AND ", $joinSqlList);
+                if (count($conditionParts)) {
+                    $sql .= " AND " . implode(" AND ", $conditionParts);
                 }
 
                 if (!$onlyMiddle) {
@@ -3274,7 +3295,7 @@ abstract class BaseQueryComposer implements QueryComposer
                     $middleDistantKeyColumn = $this->quoteColumn("$midAlias." . $this->toDb($distantKey));
                     $rightDeletedColumn = $this->quoteColumn("$alias.deleted");
 
-                    $sql .= " {$prefix}JOIN " . $this->quoteIdentifier($distantTable) . " AS " .
+                    $sql .= " {$prefixPart}JOIN " . $this->quoteIdentifier($distantTable) . " AS " .
                         $this->quoteIdentifier($alias)
                         . " ON $rightKeyColumn = $middleDistantKeyColumn"
                         . " AND "
@@ -3293,19 +3314,19 @@ abstract class BaseQueryComposer implements QueryComposer
                 $leftDeletedColumn = $this->quoteColumn("$alias.deleted");
 
                 $sql =
-                    "{$prefix}JOIN " . $this->quoteIdentifier($distantTable) . " AS "
+                    "{$prefixPart}JOIN " . $this->quoteIdentifier($distantTable) . " AS "
                     . $this->quoteIdentifier($alias) . " ON "
                     . "$leftIdColumn = $rightIdColumn AND "
                     . "$leftDeletedColumn = " . $this->quote(false);
 
-                $joinSqlList = [];
+                $conditionParts = [];
 
                 foreach ($conditions as $left => $right) {
-                    $joinSqlList[] = $this->buildJoinConditionStatement($entity, $alias, $left, $right, $params);
+                    $conditionParts[] = $this->buildJoinConditionStatement($entity, $alias, $left, $right, $params);
                 }
 
-                if (count($joinSqlList)) {
-                    $sql .= " AND " . implode(" AND ", $joinSqlList);
+                if (count($conditionParts)) {
+                    $sql .= " AND " . implode(" AND ", $conditionParts);
                 }
 
                 return $sql;
@@ -3326,27 +3347,27 @@ abstract class BaseQueryComposer implements QueryComposer
                 $leftDeletedColumn = $this->quoteColumn("$alias.deleted");
 
                 $sql =
-                    "{$prefix}JOIN " . $this->quoteIdentifier($distantTable)
+                    "{$prefixPart}JOIN " . $this->quoteIdentifier($distantTable)
                     . " AS "
                     . $this->quoteIdentifier($alias) . " ON "
                     . "$leftIdColumn = $rightIdColumn AND "
                     . "$leftTypeColumn = " . $this->quote($entity->getEntityType()) . " AND "
                     . "$leftDeletedColumn = " . $this->quote(false);
 
-                $joinSqlList = [];
+                $conditionParts = [];
 
                 foreach ($conditions as $left => $right) {
-                    $joinSqlList[] = $this->buildJoinConditionStatement($entity, $alias, $left, $right, $params);
+                    $conditionParts[] = $this->buildJoinConditionStatement($entity, $alias, $left, $right, $params);
                 }
 
-                if (count($joinSqlList)) {
-                    $sql .= " AND " . implode(" AND ", $joinSqlList);
+                if (count($conditionParts)) {
+                    $sql .= " AND " . implode(" AND ", $conditionParts);
                 }
 
                 return $sql;
 
             case Entity::BELONGS_TO:
-                return $prefix . $this->getBelongsToJoinItemPart($entity, $relationName, $alias, $params);
+                return $prefixPart . $this->getBelongsToJoinItemPart($entity, $relationName, $alias, $params);
         }
 
         return '';
diff --git a/tests/unit/Espo/ORM/MysqlQueryComposerTest.php b/tests/unit/Espo/ORM/MysqlQueryComposerTest.php
index 5ac8b05b7e..dc91b8bb70 100644
--- a/tests/unit/Espo/ORM/MysqlQueryComposerTest.php
+++ b/tests/unit/Espo/ORM/MysqlQueryComposerTest.php
@@ -52,6 +52,7 @@ use Espo\ORM\Query\Part\Condition;
 use Espo\ORM\Query\Select;
 use Espo\ORM\Query\Update;
 
+use LogicException;
 use RuntimeException;
 
 require_once 'tests/unit/testData/DB/Entities.php';
@@ -986,6 +987,89 @@ class MysqlQueryComposerTest extends \PHPUnit\Framework\TestCase
         );
     }
 
+    public function testJoinSubQuery1(): void
+    {
+        $sql =
+            "SELECT post.id AS `id` FROM `post` " .
+            "JOIN (SELECT post.id AS `id` FROM `post` WHERE post.deleted = 0) AS `a` ON a.id = post.id " .
+            "WHERE post.deleted = 0";
+
+        $select = SelectBuilder::create()
+            ->select('id')
+            ->from('Post')
+            ->join(
+                Join
+                    ::createWithSubQuery(
+                        SelectBuilder::create()
+                            ->select('id')
+                            ->from('Post')
+                            ->build(),
+                        'a'
+                    )
+                    ->withConditions(
+                        WhereClause::create(
+                            Condition::equal(
+                                Expression::column('a.id'),
+                                Expression::column('post.id')
+                            )
+                        )
+                    )
+            )
+            ->build();
+
+        $this->assertEquals(
+            $sql,
+            $this->query->composeSelect($select)
+        );
+    }
+
+    public function testJoinSubQuery2(): void
+    {
+        $sql =
+            "SELECT post.id AS `id` FROM `post` " .
+            "JOIN (SELECT post.id AS `id` FROM `post` WHERE post.deleted = 0) AS `a` ON a.id = post.id " .
+            "WHERE post.deleted = 0";
+
+        $select = SelectBuilder::create()
+            ->select('id')
+            ->from('Post')
+            ->join(
+                SelectBuilder::create()
+                    ->select('id')
+                    ->from('Post')
+                    ->build(),
+                'a',
+                Condition::equal(
+                    Expression::column('a.id'),
+                    Expression::column('post.id')
+                )
+            )
+            ->build();
+
+        $this->assertEquals(
+            $sql,
+            $this->query->composeSelect($select)
+        );
+    }
+
+    public function testJoinSubQueryException1(): void
+    {
+        $this->expectException(LogicException::class);
+
+        $select = SelectBuilder::create()
+            ->select('id')
+            ->from('Post')
+            ->join(
+                SelectBuilder::create()
+                    ->select('id')
+                    ->from('Post')
+                    ->build(),
+            )
+            ->build();
+
+        $this->query->composeSelect($select);
+    }
+
     public function testWhereNotValue1()
     {
         $sql = $this->query->compose(Select::fromRaw([
diff --git a/tests/unit/Espo/ORM/Query/Part/JoinTest.php b/tests/unit/Espo/ORM/Query/Part/JoinTest.php
index 71f558bfd3..99833e6748 100644
--- a/tests/unit/Espo/ORM/Query/Part/JoinTest.php
+++ b/tests/unit/Espo/ORM/Query/Part/JoinTest.php
@@ -29,8 +29,10 @@
 
 namespace tests\unit\Espo\ORM\Query\Part;
 
+use Espo\ORM\Query\Part\Expression;
 use Espo\ORM\Query\Part\Join;
 use Espo\ORM\Query\Part\Expression as Expr;
+use Espo\ORM\Query\SelectBuilder;
 
 class JoinTest extends \PHPUnit\Framework\TestCase
 {
@@ -50,6 +52,8 @@ class JoinTest extends \PHPUnit\Framework\TestCase
 
         $this->assertTrue($join->isRelation());
         $this->assertFalse($join->isTable());
+
+        $this->assertEquals(Join::TYPE_RELATION, $join->getType());
     }
 
     public function testCreate2(): void
@@ -68,6 +72,8 @@ class JoinTest extends \PHPUnit\Framework\TestCase
 
         $this->assertTrue($join->isTable());
         $this->assertFalse($join->isRelation());
+
+        $this->assertEquals(Join::TYPE_TABLE, $join->getType());
     }
 
     public function testCreate3(): void
@@ -77,4 +83,18 @@ class JoinTest extends \PHPUnit\Framework\TestCase
         $this->assertEquals('Test', $join->getTarget());
         $this->assertEquals('testAlias', $join->getAlias());
     }
+
+    public function testCreate4(): void
+    {
+        $join = Join::createWithSubQuery(
+            SelectBuilder::create()
+                ->select(Expression::value(true))
+                ->build()
+            ,
+            'a'
+        );
+
+        $this->assertTrue($join->isSubQuery());
+        $this->assertEquals(Join::TYPE_SUB_QUERY, $join->getType());
+    }
 }