msql 实现sequence功能增强

create table sequence (  
    seq_name        VARCHAR(50)  NOT NULL COMMENT '序列名称',
    min_val         INT  UNSIGNED         NOT NULL COMMENT '最小值',
    max_val         INT  UNSIGNED         NOT NULL COMMENT '最大值',
    if_cycle        INT  UNSIGNED         NOT NULL COMMENT '是否循环',
    if_use          INT  UNSIGNED         NOT NULL COMMENT '是否使用中：0：未使用，1：已使用',
    current_val     INT  UNSIGNED         NOT NULL COMMENT '当前值',
    increment_val   INT  UNSIGNED         NOT NULL DEFAULT 1 COMMENT '步长(跨度)',
    PRIMARY KEY (seq_name)
);

delimiter $$

create function currval(v_seq_name VARCHAR(50))  
returns integer
begin
    declare v_curr_val integer;  
    set v_curr_val := 0;  
    select current_val into v_curr_val  
    from sequence
    where seq_name = v_seq_name;  
    return v_curr_val;  
end$$

create function nextval(v_seq_name VARCHAR(50))
returns INTEGER
begin
    declare v_curr_val INTEGER;
    declare v_increment_val INTEGER;
    declare v_if_cycle INTEGER;
    declare v_next_val INTEGER;
    declare v_min_val INTEGER;
    declare v_max_val INTEGER;

update sequence set if_use = 1 where seq_name = v_seq_name;
    select current_val, min_val, max_val, increment_val, if_cycle
    into v_curr_val, v_min_val, v_max_val, v_increment_val, v_if_cycle from sequence where seq_name = v_seq_name;
    set v_next_val := v_curr_val + v_increment_val;
    if (v_next_val > v_max_val) and (v_if_cycle = 0) then
        set v_next_val := -1;
    else
        set v_next_val := mod(v_next_val, v_max_val + 1);
        IF v_next_val < v_min_val then
            set v_next_val := v_min_val;
        end if;
        update sequence set current_val = v_next_val where seq_name = v_seq_name;   
    end if;  
    update sequence set if_use = 0 where seq_name = v_seq_name;   
    return v_next_val;
end$$

create function setval(v_seq_name VARCHAR(50), v_new_val INTEGER)  
returns integer
begin
    declare v_min_val int;
    declare v_max_val int;
    update sequence set if_use = 1 where seq_name = v_seq_name;    
    select min_val, max_val into v_min_val, v_max_val from sequence where seq_name = v_seq_name;
    if (v_new_val > v_max_val) or (v_new_val < v_min_val) then
        set v_new_val := -1;
    else
        update sequence set current_val = v_new_val where seq_name = v_seq_name;
    end if;
    update sequence set if_use = 0 where seq_name = v_seq_name;
    return v_new_val;
end$$

delimiter ;