查看“什么时候用 in ？什么时候用 exists ？”的源代码

=== 示例 ===
<syntaxhighlight lang="sql">
select name 
from employees
where empno in
    (select distinct empno from job_history);
</syntaxhighlight><syntaxhighlight lang="sql">
select name
from employees
where exists
    (select empno from job_history where employees.empno=job_history.empno);
</syntaxhighlight>如果 employees 中记录数大于 job_history 时，用 in 效率高。

如果 employees 中记录数小于 job_history 时，用 exists 效率高。