Schema Diff — Compare ER Diagrams

Paste your old and new diagrams, click Compare, and instantly see every change.

Mermaid ER PlantUML Cross-format Added tables & columns Dropped columns Modified types FK changes Free — no sign-up
Old Diagram (baseline)
New Diagram (target)
or Ctrl+Enter

How Schema Diff Works

1

Paste your old diagram (the current state of your schema) on the left.

2

Paste your new diagram (the desired future state) on the right.

3

Click Compare to see every change, colour-coded by type and severity.

Both diagrams are parsed and normalised before comparison, so minor differences in whitespace or ordering never produce false positives. Green entries are safe additions; red entries are deletions that may cause data loss and are marked destructive; yellow entries are column modifications.

What changes are detected?

  • Added / dropped tables — new tables in the target or tables removed from the baseline
  • Added / dropped columns — new columns in a shared table, or columns that no longer appear
  • Modified columns — type changes, nullability flips, or default value changes on existing columns
  • Added / dropped indexes — new named indexes or removed indexes on shared tables
  • Added / dropped foreign keys — relationship additions and removals between tables

Destructive changes

A change is marked destructive when it may cause irreversible data loss on a live database: dropping a table, dropping a column, changing a column's data type, or adding a NOT NULL constraint to a column that could contain existing NULL values. Use these signals to decide whether to take a backup before running migrations.

Want the SQL migration script?

The Schema Diff page shows what changed in human-readable form. If you need the actual ALTER TABLE / CREATE TABLE SQL to apply those changes to a database, use the POST /v1/convert/migration endpoint (Developer tier) or the Schema Diff documentation for API usage.

Next steps

Learn how to write ER diagrams or automate diff in your CI pipeline.

📋
Schema Diff Docs API reference, response format, and curl examples
⚙️
Diagram to SQL Convert a single diagram to PostgreSQL, MySQL, SQLite, or Oracle DDL
🧜
Mermaid ER Syntax Tables, columns, PK/FK markers, relationships
🌱
PlantUML Syntax Entities, stereotypes, key differences from Mermaid

Frequently Asked Questions

What is a schema diff?
A schema diff compares two versions of a database schema — typically the current state and a proposed new state — and lists exactly what changed: which tables were added or removed, which columns were modified, and which relationships changed. It is the first step in planning a safe database migration.
Which diagram formats are supported?
Both Mermaid erDiagram and PlantUML entity syntax are supported. You can also diff a Mermaid baseline against a PlantUML target (or vice versa) — the tool parses each diagram independently before comparing the resulting schemas.
What does "destructive" mean?
A change is marked destructive when applying it to a live database could cause irreversible data loss: dropping a table, dropping a column, changing a column's data type (truncation risk), or adding a NOT NULL constraint without a default value (existing NULL rows would fail the constraint). Always take a backup before running destructive migrations.
Does this generate migration SQL?
The Schema Diff page shows human-readable change descriptions only — it does not generate SQL. To produce ALTER TABLE migration scripts for PostgreSQL, MySQL, SQLite, or Oracle, use the POST /v1/convert/migration API endpoint, which is available on the Developer plan.
Is the diff tool free?
Yes — the Schema Diff page and the POST /v1/diff API endpoint are completely free with no account required. The human-readable diff is always free; SQL migration generation requires a Developer plan.
Why does comparing identical diagrams produce changes?
Before diffing, both schemas are normalised: tables without a primary key get a synthetic id column injected. If one diagram defines an explicit PK and the other does not, the diff will report a column modification even if the original diagrams look similar. Define explicit PKs in both diagrams for a clean zero-change result.