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如何将此左连接查询作为子查询来替代?将左连接写为子查询
SELECT d.department, COUNT(e.employeeid)
FROM department d
LEFT JOIN employee e
ON d.id = e.employeeid
GROUP BY d.department
ORDER BY d.department;
这是我试过的,但不起作用。
SELECT d.department
FROM department AS d
HAVING COUNT(e.employeeid) > 1
(
SELECT COUNT(e.employeeid)
FROM employee AS e
WHERE d.id = e.employeeid
);
你试图完成什么结果?子查询通常比等效连接效率低。如果您只想要拥有多名员工的部门,则可以在第一个查询中使用第二个查询中的HAVING。 – Uueerdo