关于mysql:MySQL-union合并查询

​ union 和 union all

​ 区别：union 将合并的后果去重；union all 不去重

​ select ename from emp where empno=7900 union select ename from emp where empno>=7900;

​ select ename from emp where empno=7900 union all select ename from emp where empno>=7900;900;