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**Auto-increment (AUTO_INCREMENT in MySQL / SERIAL in PostgreSQL)** is a field property where **the value automatically increases by 1 as each new record is added**.

### The main points

1. **Used for primary keys**, so every record gets a unique identifier with no manual entry.
2. **Automatic increase**: when a new record is inserted, the DBMS assigns the next available number itself.
3. **A MySQL example:**

```sql
CREATE TABLE Employees (
    ID INT AUTO_INCREMENT PRIMARY KEY,
    Name VARCHAR(50) NOT NULL
);

INSERT INTO Employees (Name) VALUES ('Ivan');
INSERT INTO Employees (Name) VALUES ('Maria');
-- ID will automatically be 1 and 2
```

4. **A PostgreSQL example:**

```sql
CREATE TABLE Employees (
    ID SERIAL PRIMARY KEY,
    Name VARCHAR(50) NOT NULL
);
```

5. **Notes:**
  - It can start from any number (e.g. `AUTO_INCREMENT=100`).
  - The value **doesn't decrease when records are deleted**, the next number keeps increasing.

> Put simply, auto-increment is a way to **automatically number records**, so every one is unique.

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