mysql中字符串相加

CONCAT(str1, str2, ..., strN)

CREATE TABLE users (
    id INT PRIMARY KEY,
    first_name VARCHAR(50),
    last_name VARCHAR(50)
);

INSERT INTO users (id, first_name, last_name) VALUES
(1, 'John', 'Doe'),
(2, 'Jane', 'Smith');

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

+-----------+
| full_name |
+-----------+
| John Doe  |
| Jane Smith|
+-----------+

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

SELECT 
    CASE 
        WHEN LENGTH(CONCAT(first_name, ' ', last_name)) <= 100 
        THEN CONCAT(first_name, ' ', last_name) 
        ELSE SUBSTRING(CONCAT(first_name, ' ', last_name), 1, 100) 
    END AS full_name 
FROM users;