# Phase 67 — Safe missing-row synchronisation preview

From **Compare Table Data…**, choose source, target, and a shared base table, then select **Preview Missing-Row Sync…**.

The generated `INSERT INTO ... SELECT ... WHERE NOT EXISTS` statement only proposes insertion of rows whose primary key is absent in the target. It does not update differing records or delete target records. DBSyn does not execute the preview: the user must explicitly copy and review it.

## Safeguards and limitations

- Requires matching introspected column names, order, types, nullability, and primary key columns on both sides.
- SQL identifiers are backtick-escaped. Works only on the same MySQL server/connection.
- SQL may still fail due to foreign keys, triggers, generated columns, permissions, incompatible constraints, or concurrent writes. Inspect and adapt SQL before execution.
- The existing Phase 66 comparison remains limited to unordered 500-row samples and **must not** be treated as a complete diff.
- No automated conflict resolution, deletion, or update is implemented.
- Refresh introspection immediately before generating a preview; take a backup before applying any SQL.