select后面的子查询使用,只能是标量查询
#案例：查询每个部门的员工个数
SELECT d.*,(
SELECT COUNT(*)
FROM employees
WHERE employees.`department_id`=d.department_id
)
FROM
departments d;
#案例：查询员工号=102的部门名
SELECT department_name
FROM
employees e
INNER JOIN
departments d
ON
e.`department_id`=d.`department_id`
WHERE
e.`employee_id`=102;
用子查询
SELECT (
SELECT department_name
FROM
employees e
INNER JOIN
departments d
ON
e.`department_id`=d.`department_id`
WHERE
e.`employee_id`=102
);
由 dawei
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