MySQL String Functions

Reference guide for MySQL string functions: CONCAT, SUBSTRING, TRIM, REPLACE, UPPER, LOWER, LENGTH, LIKE, REGEXP, FORMAT, and common string manipulation patterns.

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MySQL’s string functions cover splitting, joining, searching, formatting, and comparing text. Most follow the same conventions: 1-based indexing, NULL propagation (any NULL argument returns NULL), and case-insensitive comparisons by default.

Combining and Splitting

CONCAT — join strings together:

SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM users;

-- CONCAT_WS: join with a separator, skipping NULLs
SELECT CONCAT_WS(', ', city, state, country) AS address FROM locations;

CONCAT with a NULL argument returns NULL. CONCAT_WS skips NULL values but keeps the separator for non-NULL ones.

SUBSTRING / SUBSTR / MID — extract part of a string:

-- SUBSTRING(string, start, length)  — 1-based indexing
SELECT SUBSTRING('Hello, World!', 1, 5);   -- 'Hello'
SELECT SUBSTRING('Hello, World!', 8);      -- 'World!'  (to end of string)
SELECT SUBSTRING('Hello, World!', -6);     -- 'World!'  (from the right)

SUBSTRING_INDEX — split on a delimiter:

-- Get everything before the second comma
SELECT SUBSTRING_INDEX('a,b,c,d', ',', 2);   -- 'a,b'

-- Get everything after the last dot (domain from email)
SELECT SUBSTRING_INDEX('[e-mail]', '@', -1);  -- 'example.com'

Searching Within Strings

LOCATE / POSITION / INSTR — find a substring:

SELECT LOCATE('World', 'Hello, World!');       -- 8  (1-based position)
SELECT LOCATE('world', 'Hello, World!');       -- 8  (case-insensitive by default)
SELECT LOCATE('xyz', 'Hello, World!');         -- 0  (not found)

-- LOCATE with start position
SELECT LOCATE('o', 'Hello, World!', 6);        -- 9  (skips first 'o')

LIKE — pattern matching with wildcards:

-- % matches zero or more characters, _ matches exactly one
SELECT * FROM products WHERE name LIKE 'Widget%';       -- starts with Widget
SELECT * FROM products WHERE name LIKE '%Pro%';         -- contains Pro
SELECT * FROM products WHERE code LIKE 'SKU-___-2024';  -- three chars in middle

REGEXP / RLIKE — regular expression matching:

SELECT * FROM users WHERE email REGEXP '^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}$';
SELECT * FROM products WHERE name REGEXP 'Pro|Plus|Max';

REGEXP uses POSIX regex syntax. Use REGEXP_LIKE() (MySQL 8.0+) for the function form.

Modifying Strings

REPLACE — replace all occurrences of a substring:

SELECT REPLACE('Hello, World!', 'World', 'MySQL');  -- 'Hello, MySQL!'
UPDATE articles SET content = REPLACE(content, 'old_term', 'new_term');

TRIM / LTRIM / RTRIM — remove whitespace or specific characters:

SELECT TRIM('  hello  ');                    -- 'hello'
SELECT TRIM(LEADING '0' FROM '007');         -- '7'
SELECT TRIM(BOTH 'x' FROM 'xxhelloxx');      -- 'hello'

UPPER / LOWER — change case:

SELECT UPPER('hello');   -- 'HELLO'
SELECT LOWER('HELLO');   -- 'hello'

LPAD / RPAD — pad to a fixed length:

SELECT LPAD('42', 6, '0');    -- '000042'
SELECT RPAD('hello', 10, '.');  -- 'hello.....'

REPEAT — repeat a string N times:

SELECT REPEAT('ab', 3);   -- 'ababab'

REVERSE — reverse a string:

SELECT REVERSE('hello');  -- 'olleh'

Length and Comparison

MySQL has two length functions that produce different results for multibyte characters:

Function	What it measures
LENGTH(s)	Byte length
CHAR_LENGTH(s) / CHARACTER_LENGTH(s)	Character length

SELECT LENGTH('café');      -- 5  (the é is 2 bytes in UTF-8)
SELECT CHAR_LENGTH('café'); -- 4  (4 characters)

Always use CHAR_LENGTH when you care about the number of characters, not bytes.

STRCMP — compare two strings, returns -1, 0, or 1:

SELECT STRCMP('abc', 'abc');   -- 0
SELECT STRCMP('abc', 'abd');   -- -1
SELECT STRCMP('abd', 'abc');   -- 1

Formatting

FORMAT — format a number with thousands separators:

SELECT FORMAT(1234567.891, 2);    -- '1,234,567.89'
SELECT FORMAT(1234567.891, 2, 'de_DE');  -- '1.234.567,89'  (German locale)

LEFT / RIGHT — take N characters from start or end:

SELECT LEFT('Hello, World!', 5);   -- 'Hello'
SELECT RIGHT('Hello, World!', 6);  -- 'World!'

Common Patterns

Extract the domain from an email address:

SELECT SUBSTRING_INDEX(email, '@', -1) AS domain FROM users;

Truncate long strings for display:

SELECT CASE
    WHEN CHAR_LENGTH(description) > 100
    THEN CONCAT(LEFT(description, 97), '...')
    ELSE description
END AS short_description
FROM products;

Normalize whitespace:

UPDATE users SET name = TRIM(REGEXP_REPLACE(name, '\\s+', ' '));

Frequently Asked Questions

Are MySQL string comparisons case-sensitive?

By default, no — string comparisons use the column’s collation, and most common collations are case-insensitive (e.g., utf8mb4_general_ci, where ci = case-insensitive). For case-sensitive comparisons, use a _cs collation or the BINARY operator:

SELECT * FROM users WHERE BINARY username = 'Alice';
-- or: WHERE username COLLATE utf8mb4_bin = 'Alice'

What’s the difference between CHAR_LENGTH and LENGTH?

LENGTH returns byte count; CHAR_LENGTH returns character count. For ASCII strings they’re the same. For UTF-8 strings with multibyte characters (accented letters, emoji, CJK), they differ. Use CHAR_LENGTH for user-visible string length limits.

Troubleshooting

Problem	Solution
CONCAT returns NULL	One argument is NULL — use COALESCE(col, '') to replace NULLs before concatenating, or use CONCAT_WS
LIKE '%pattern%' is slow	Leading wildcard prevents index use — consider FULLTEXT index for text search at scale
String comparison not matching expected rows	Case sensitivity or collation mismatch — check the column’s collation with SHOW CREATE TABLE
REPLACE updates more rows than expected	REPLACE is case-insensitive by default — it replaces all case variants of the target string
SUBSTRING returns empty string, not NULL	SUBSTRING of an out-of-range position returns an empty string, not NULL

See also

• MySQL Character Sets and Collations — string functions behave differently across collations
• Full-Text Search in MySQL — searching strings at scale beyond LIKE