我需要向特定经理展示总员工，但不知道如何减少这种情况

Question

SELECT manager.ename AS "Mgr Name", ( Select COUNT(*) 
                                        FROM emp worker
                                        WHERE worker.mgr = manager.empno
                                        ) AS "Total Emp"
FROM emp worker, emp manager
WHERE worker.mgr = manager.empno;

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Answer 1

这个怎么样？它连接在同一个表上，以匹配所有经理和员工，并返回直接报告的数量：

  SELECT 
      m.ename AS [Mgr Name], 
      COUNT(DISTINCT e.empno) AS [Total Emp]
   FROM emp e
   JOIN emp m ON m.empno = e.mgr 
   GROUP BY m.ename;

Answer 2

使用相关子查询或join与group by：

SELECT m.ename AS "Mgr Name",
       ( Select COUNT(*) FROM emp e WHERE e.mgr = m.empno
       ) AS num_employees
FROM emp m;