diff --git a/backend/src/main/java/org/raddatz/familienarchiv/repository/TagRepository.java b/backend/src/main/java/org/raddatz/familienarchiv/repository/TagRepository.java
index 7e43d52d..c3e7bbef 100644
--- a/backend/src/main/java/org/raddatz/familienarchiv/repository/TagRepository.java
+++ b/backend/src/main/java/org/raddatz/familienarchiv/repository/TagRepository.java
@@ -1,11 +1,13 @@
 package org.raddatz.familienarchiv.repository;
 
+import java.util.Collection;
 import java.util.List;
 import java.util.Optional;
 import java.util.UUID;
 
 import org.raddatz.familienarchiv.model.Tag;
 import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Modifying;
 import org.springframework.data.jpa.repository.Query;
 import org.springframework.data.repository.query.Param;
 
@@ -51,4 +53,67 @@ public interface TagRepository extends JpaRepository<Tag, UUID> {
             SELECT id FROM descendants
             """, nativeQuery = true)
     List<UUID> findDescendantIdsByName(@Param("name") String name);
+
+    /**
+     * Returns the IDs of the tag with the given ID AND all of its descendants
+     * via a recursive CTE. Used for merge validation and subtree delete.
+     * Includes a depth guard of 50 levels to prevent runaway queries.
+     */
+    @Query(value = """
+            WITH RECURSIVE descendants AS (
+                SELECT id, 0 AS depth FROM tag WHERE id = :tagId
+                UNION ALL
+                SELECT t.id, d.depth + 1 FROM tag t
+                JOIN descendants d ON t.parent_id = d.id
+                WHERE d.depth < 50
+            )
+            SELECT id FROM descendants
+            """, nativeQuery = true)
+    List<UUID> findDescendantIds(@Param("tagId") UUID tagId);
+
+    /**
+     * Reassigns document_tags rows from source to target, skipping rows where
+     * the target tag is already present (to avoid PK conflicts).
+     */
+    @Modifying(clearAutomatically = true)
+    @Query(value = """
+            UPDATE document_tags
+            SET tag_id = :targetId
+            WHERE tag_id = :sourceId
+              AND NOT EXISTS (
+                SELECT 1 FROM document_tags d2
+                WHERE d2.document_id = document_tags.document_id
+                  AND d2.tag_id = :targetId
+              )
+            """, nativeQuery = true)
+    void reassignDocumentTags(@Param("sourceId") UUID sourceId, @Param("targetId") UUID targetId);
+
+    /**
+     * Removes all document_tags rows for the given tag.
+     */
+    @Modifying(clearAutomatically = true)
+    @Query(value = "DELETE FROM document_tags WHERE tag_id = :tagId", nativeQuery = true)
+    void deleteDocumentTagsByTagId(@Param("tagId") UUID tagId);
+
+    /**
+     * Removes all document_tags rows for the given collection of tag IDs.
+     * Caller must guard against an empty collection — PostgreSQL rejects IN ().
+     */
+    @Modifying(clearAutomatically = true)
+    @Query(value = "DELETE FROM document_tags WHERE tag_id IN :ids", nativeQuery = true)
+    void deleteDocumentTagsByTagIds(@Param("ids") Collection<UUID> ids);
+
+    /**
+     * Re-parents all direct children of sourceId to targetId.
+     */
+    @Modifying(clearAutomatically = true)
+    @Query(value = "UPDATE tag SET parent_id = :targetId WHERE parent_id = :sourceId", nativeQuery = true)
+    void reparentChildren(@Param("sourceId") UUID sourceId, @Param("targetId") UUID targetId);
+
+    /**
+     * Returns (tag_id, count) pairs for all tags that appear in document_tags.
+     * Used to populate documentCount in the tag tree without N+1 queries.
+     */
+    @Query(value = "SELECT tag_id, COUNT(*) FROM document_tags GROUP BY tag_id", nativeQuery = true)
+    List<Object[]> findDocumentCountsPerTag();
 }