MySQL中的$符號有許多用途。下面將介紹一些常見的用途。

1. 字符串拼接
SELECT CONCAT('Hello',' ','World','!'); ->Hello World!
2. 變量
SET @num = 1;
SELECT @num; ->1
3. 用戶變量
SET @sum = 20;
SELECT @sum := @sum + 10 as sum, @sum as total_sum; ->sum: 30, total_sum: 30
4. 參數
CREATE PROCEDURE demo(IN id INT, OUT name VARCHAR(20))
BEGIN
SELECT CONCAT('Welcome, ', name) INTO name FROM users WHERE id = id;
END;
CALL demo(1, @name);
SELECT @name; ->Welcome, Jack
5. 環境變量
SELECT @@global.max_connections, @@session.max_connections;