SQL SERVER 中：
create table b(id int)
insert into b select 1
union all
select 2
union all
select 3
union all
select 4
if not exists(select id from b where id=5)
insert into b(id) values(5)
===
MYSQL中没有简单的语句，只能用存储过程实现
MYSQL:
create table b(id int);
insert into b(id) values(1),(2),(3),(4);
DELIMITER $$
DROP PROCEDURE IF EXISTS `test`.`sp_b`$$
CREATE PROCEDURE `test`.`sp_b`(in i_id int)
BEGIN
declare cnt int;
select count(1) from b where id=i_id into cnt;
if cnt = 0 then
insert into b select i_id;
end if;
END$$
DELIMITER ;
