Question

如何将此左连接查询替换为子查询？

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;

这是我尝试过的，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
);

Answer 1

将相关的子查询放入选择列表中。

SELECT d.department,
       (select COUNT(e.employeeid) from employee e where d.id = e.employeeid)
FROM department d
ORDER BY d.department

也许你需要做SELECT DISTINCT以避免重复的行。

编辑：这种加入条件似乎有点奇怪。不应该d.id = e.departmentid或类似吗？

将左连接写为子查询

1 个答案: