docs/sql-manual/sql-functions/scalar-functions/string-functions/overview.md

---
{
    "title": "String Functions Overview",
    "language": "en",
    "description": "String functions are built-in functions used to process and manipulate string data. They help perform various string operations,"
}
---

# String Functions Overview

String functions are built-in functions used to process and manipulate string data. They help perform various string operations, such as concatenation, splitting, replacement, searching, etc.

For URL parsing, domain extraction, and URL encoding or decoding, see [URL Functions](../url-functions/overview.md).

## UTF-8 Encoding Support

UTF-8 encoding is a variable-length character encoding that can represent almost all characters in the world, including Cyrillic, Greek, Chinese characters, emojis, and more.

In Doris string functions, UTF-8 encoding is supported unless specifically noted otherwise.

For example, the `substring` function can correctly handle UTF-8 encoded strings:

### ASCII Characters

```sql
mysql> SELECT substring('abc1', 2);
+----------------------+
| substring('abc1', 2) |
+----------------------+
| bc1                  |
+----------------------+
```

### Greek Letters

```sql
mysql> SELECT substring('αλφαβητον', 2, 4);
+---------------------------------------+
| substring('αλφαβητον', 2, 4)          |
+---------------------------------------+
| λφαβ                                  |
+---------------------------------------+
1 row in set (0.01 sec)
```

### Chinese Characters

```sql
mysql> SELECT substring('你好，世界', 2, 2);
+------------------------------------+
| substring('你好，世界', 2, 2)      |
+------------------------------------+
| 好，                               |
+------------------------------------+
```

### Emojis

```sql
mysql> SELECT substring('😊😊a😊 World!', 2, 3);
+-----------------------------------------+
| substring('😊😊a😊 World!', 2, 3)     |
+-----------------------------------------+
| 😊a😊                                  |
+-----------------------------------------+
```

## Performance Considerations

Since UTF-8 encoded characters have variable lengths, there may be performance impacts. Some functions provide both ASCII and UTF-8 versions for selection.

For example:
- The `length` function returns the byte length of a string
- The `char_length` function returns the character count of a string

```sql
mysql> select length('你好');
+------------------+
| length('你好')   |
+------------------+
|                6 |
+------------------+

mysql> select length('αλφαβητον');
+------------------------------+
| length('αλφαβητον')          |
+------------------------------+
|                           18 |
+------------------------------+

mysql> select char_length('你好');
+-----------------------+
| char_length('你好')   |
+-----------------------+
|                     2 |
+-----------------------+

mysql> select char_length('αλφαβητον');
+-----------------------------------+
| char_length('αλφαβητον')          |
+-----------------------------------+
|                                 9 |
+-----------------------------------+
```

## Special Notes

Some string functions that don't support UTF-8 encoding will be specifically mentioned in the documentation. For example, the `NGRAM_SEARCH` function only supports ASCII-encoded strings.

```sql
mysql> select ngram_search('abcab' , 'ab' , 2);
+----------------------------------+
| ngram_search('abcab' , 'ab' , 2) |
+----------------------------------+
|                              0.5 |
+----------------------------------+
```

For non-ASCII characters, `NGRAM_SEARCH` will still execute, but the results may not be as expected.

```sql
mysql> select ngram_search('αβγαβ' , 'αβ' , 2);
+-----------------------------------------+
| ngram_search('αβγαβ' , 'αβ' , 2)        |
+-----------------------------------------+
|                      0.6666666666666666 |
+-----------------------------------------+
```