mysql nextval函数

-- 创建一个序列
CREATE TABLE my_sequence (
    seq_name VARCHAR(50) NOT NULL,
    next_val BIGINT NOT NULL DEFAULT 1,
    PRIMARY KEY (seq_name)
);

-- 插入初始值
INSERT INTO my_sequence (seq_name, next_val) VALUES ('my_seq', 1);

-- 自定义 NEXTVAL 函数
DELIMITER $$
CREATE FUNCTION NEXTVAL(seq_name VARCHAR(50)) RETURNS BIGINT
BEGIN
    DECLARE next_val BIGINT;
    UPDATE my_sequence SET next_val = next_val + 1 WHERE seq_name = seq_name;
    SELECT next_val INTO next_val FROM my_sequence WHERE seq_name = seq_name;
    RETURN next_val;
END$$
DELIMITER ;

-- 使用 NEXTVAL 函数
SELECT NEXTVAL('my_seq'); -- 返回 2
SELECT NEXTVAL('my_seq'); -- 返回 3