Question

我有一个声明，我试图用来检索完整的员工列表，然后是他们的假期。即使他们没有假期，我也想留住员工。这是我到目前为止的陈述;

SELECT emp.em_pplid, emp.em_name
FROM employs emp
LEFT JOIN people ppl ON emp.em_pplid = ppl.pp_id
LEFT JOIN empwork ew ON emp.em_pplid = ew.ew_pplid
WHERE emp.em_deptid = ?
AND ppl.pp_employee = ?
AND emp.em_type < ?
AND ew.ew_from > ?
ORDER BY emp.em_name"

AND ew.ew_from = ?删除了empwork表中没有假期的员工。即使在empwork表中没有假期，如何让员工表中的所有人都拥有我想要的部门ID？

Answer 1

您的where子句会将left join转换为inner join s。将条件放入联接的on子句中。

SELECT emp.em_pplid, emp.em_name
FROM employs emp
LEFT JOIN people ppl ON emp.em_pplid = ppl.pp_id                        
LEFT JOIN empwork ew ON emp.em_pplid = ew.ew_pplid
                    AND ew.ew_from > ?
WHERE emp.em_deptid = ?
AND emp.em_type < ?
AND (ppl.pp_employee IS NULL OR ppl.pp_employee = ?)
ORDER BY emp.em_name

Answer 2

只应在LEFT JOIN子句中指定ON的右表的条件。如果不是，则由于INNER JOIN比较，加入会自动转移到NULL：

SELECT emp.em_pplid, emp.em_name
FROM employs emp
LEFT JOIN people ppl
 ON emp.em_pplid = ppl.pp_id AND ppl.pp_employee = ?
LEFT JOIN empwork ew 
 ON emp.em_pplid = ew.ew_pplid AND ew.ew_from > ?
WHERE emp.em_deptid = ?
AND emp.em_type < ?
ORDER BY emp.em_name"

Answer 3

如果我理解这个问题，这可能有用：

SELECT emp.em_pplid, emp.em_name
FROM employs emp
LEFT JOIN people ppl ON emp.em_pplid = ppl.pp_id
LEFT JOIN empwork ew ON emp.em_pplid = ew.ew_pplid
WHERE emp.em_deptid = ?
AND ppl.pp_employee = ?
AND emp.em_type < ?
AND (ew.ew_from > ? OR ew.ew_from IS NULL)
ORDER BY emp.em_name"

条件使用LEFT JOIN

3 个答案: