SQL计数（选择）查询

我会这样做，将您希望员工计数的项目详细信息分组：

SELECT
    pno,
    pname,
    COUNT(employee.snn) AS num_emp
FROM
    project
INNER JOIN
    works_on
    ON works_on.pno = project.pno
INNER JOIN
    employee
    ON employee.ssn = works_on.essn
GROUP BY
    pno,
    pname;

编辑：

实际上，如果您要列出未分配员工的项目，则可以通过执行以下操作使原始查询更正确：

SELECT
    pno,
    pname,
    (
        SELECT
            COUNT(fname)
        FROM
            employee
        INNER JOIN
            works_on
            ON employee.ssn = works_on.essn 
        WHERE
            works_on.pno = project.pno
    ) AS num_emp
FROM
    project;