存在条件（EXISTS Conditions）用于测试子查询中是否存在指定的行。

语法

EXISTS (subquery)

如果子查询至少返回一行，则说明存在您想要的数据。

示例

SELECT department_id FROM departments d WHERE EXISTS (SELECT * FROM employees e 
WHERE d.department_id = e.department_id) ORDER BY department_id;