# Hash Functions

Hash (md5, sha1 and sha256) SQL functions reference documentation and explanation.

Hash functions generate fixed-size string outputs from variable-length inputs. 

These functions are useful for data integrity verification, checksums, and data anonymization.

## Supported functions

- [`md5()`](#md5) – Generates a 128-bit (32 character) hash value
- [`sha1()`](#sha1) – Generates a 160-bit (40 character) hash value
- [`sha256()`](#sha256) – Generates a 256-bit (64 character) hash value

## Function reference

### md5()

Calculates an MD5 hash of the input value and returns it as a hexadecimal string.

**Arguments:**
- String, varchar, or binary value

**Return value:**
- A 32-character hexadecimal string representing the MD5 hash
- NULL if the input is NULL

**Examples:**
```questdb-sql title="md5() with string input" demo
SELECT md5('abc');
-- Returns: '900150983cd24fb0d6963f7d28e17f72'

SELECT md5('');
-- Returns: 'd41d8cd98f00b204e9800998ecf8427e'
```

```questdb-sql title="md5() with UTF-8 input" demo
SELECT md5('Hello, world!');
-- Returns: '6cd3556deb0da54bca060b4c39479839'
```

### sha1()

Calculates a SHA1 hash of the input value and returns it as a hexadecimal string.

**Arguments:**
- String, varchar, or binary value

**Return value:**
- A 40-character hexadecimal string representing the SHA1 hash
- NULL if the input is NULL

**Examples:**
```questdb-sql title="sha1() with string input" demo
SELECT sha1('abc');
-- Returns: 'a9993e364706816aba3e25717850c26c9cd0d89d'

SELECT sha1('');
-- Returns: 'da39a3ee5e6b4b0d3255bfef95601890afd80709'
```

```questdb-sql title="sha1() with UTF-8 input" demo
SELECT sha1('Hello, world!');
-- Returns: '943a702d06f34599aee1f8da8ef9f7296031d699'
```

### sha256()

Calculates a SHA256 hash of the input value and returns it as a hexadecimal string.

**Arguments:**
- String, varchar, or binary value

**Return value:**
- A 64-character hexadecimal string representing the SHA256 hash
- NULL if the input is NULL

**Examples:**
```questdb-sql title="sha256() with string input" demo
SELECT sha256('abc');
-- Returns: 'ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad'

SELECT sha256('');
-- Returns: 'e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855'
```

```questdb-sql title="sha256() with UTF-8 input" demo
SELECT sha256('Hello, world!');
-- Returns: '315f5bdb76d078c43b8ac0064e4a0164612b1fce77c869345bfc94c75894edd3'
```

## Notes and restrictions

### Input handling
- All hash functions support string, varchar, and binary inputs
- Empty strings produce a valid hash value
- NULL inputs always return NULL outputs
- UTF-8 strings are fully supported

### Thread safety
- Hash functions are not thread-safe
- Each function instance maintains its own internal state

### Output characteristics
- Output is always lowercase hexadecimal
- Output length is fixed regardless of input size:
  - MD5: 32 characters
  - SHA1: 40 characters
  - SHA256: 64 characters

### Implementation details
- Uses Java's built-in MessageDigest implementations
- Supported algorithms are guaranteed to be available on all Java platforms
- Processes input in a single pass

### Common use cases

#### Data integrity verification

```questdb-sql
SELECT 
    filename,
    sha256(content) = expected_hash as is_valid
FROM files;
```

#### Anonymizing sensitive data

```questdb-sql
SELECT 
    md5(email) as hashed_email,
    count(*) as user_count
FROM users
GROUP BY hashed_email;
```

#### Binary data hashing

```questdb-sql
SELECT 
    file_id,
    sha1(binary_content) as content_hash
FROM binary_files;
```