# `OVERLAY`

**Applies to:** ✅ Data 360 SQL ✅ Tableau Hyper API

Replaces a substring within a string at a specified position.

## Syntax

```sql
overlay(<string> placing <replacement> from <start> for <count>)
```

## Arguments

### Required

- `<string>`: The original string to modify.
- `<replacement>`: The substring to insert.
- `<start>`: The starting position (1-based).
- `<count>`: The number of characters to replace.

## Returns

Returns a text value with the specified substring replaced.

## Examples

### Replace Substring

Replace characters within a string.

```sql
SELECT overlay('Txxxas' placing 'hom' from 2 for 4) AS result;
```

Returns `'Thomas'` by replacing 4 characters starting at position 2 with 'hom'.

## Related Documentation

- [String Types](/docs/data/data-cloud-query-guide/references/dc-sql-reference/string.md)