SQL provides several string functions that can be used to manipulate and perform operations on string data. Here are some of the most commonly used string functions in SQL:

1. LENGTH: This function returns the length of a string in characters. Here is an example:

SELECT LENGTH('Hello World');

The result of this query will be 11, which is the number of characters in the string 'Hello World'.

2. UPPER and LOWER: These functions convert a string to uppercase or lowercase, respectively. Here is an example:

SELECT UPPER('Hello World');
SELECT LOWER('Hello World');

The first query will return 'HELLO WORLD' and the second query will return 'hello world'.

3. SUBSTRING: This function returns a portion of a string, specified by the starting position and the number of characters to return. The syntax for the SUBSTRING function is:

SUBSTRING(string, start, length)

Here is an example:

SELECT SUBSTRING('Hello World', 1, 5);

The result of this query will be 'Hello', which is the first five characters of the string 'Hello World'.

4. TRIM: This function removes leading and trailing spaces from a string. Here is an example:

SELECT TRIM(' Hello World ');

The result of this query will be 'Hello World', with the leading and trailing spaces removed.

5. REPLACE: This function replaces occurrences of a string within another string. The syntax for the REPLACE function is:

REPLACE(string, search_string, replace_string)

Here is an example:

SELECT REPLACE('Hello World', 'Hello', 'Goodbye');

The result of this query will be 'Goodbye World'.

In conclusion, SQL provides several string functions that can be used to manipulate string data. These functions can be used to perform operations such as converting case, extracting substrings, removing spaces, and replacing text. These functions are useful when working with string data in SQL.

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