向JOIN语句中添加WHERE子句

Question

我有2个表，员工和假期。

我想选择一个员工名单，他们允许的假期和剩余的假期后，看看今年输入的假期。

的表格看起来像这样（只显示重要行）

employees 
    id, 
    name, 
    allowed_holiday 

holiday 
    id, 
    employee, 
    num_days, 
    date_to 

我有一个发现这些值精细当前的查询，但我想添加一个WHERE子句只能算假期对员工如果DATE_TO > xxxx-09-01 AND < xxxx-09-01。

SELECT 
     e.id, 
     e.name, 
     e.allowed_holiday, 
     (e.allowed_holiday - SUM(h.num_days)) AS remaining 
    FROM 
     employees AS e 
    LEFT JOIN 
     holiday AS h 
    ON 
     h.employee = e.id 
    GROUP BY 
     e.name 
    ORDER BY 
     e.name ASC 

我不确定在哪里为此添加WHERE子句，但不删除没有假期列出的数据的员工。

Answer 1

上h.date_to添加条件到ON条款不WHERE避免把它放回了INNER JOIN

LEFT JOIN 
     holiday AS h 
    ON 
     h.employee = e.id AND h.date_to > .... 

Answer 2

当你正在做一个LEFT JOIN，过滤条件必须在ON语句。因此，在你的情况下，它会是这样的：

SELECT 
    e.id, 
    e.name, 
    e.allowed_holiday, 
    (e.allowed_holiday - SUM(h.num_days)) AS remaining 
FROM 
    employees AS e 
LEFT JOIN 
    holiday AS h 
ON 
    h.employee = e.id AND h.to_date between xxxx-09-01 and xxxx-09-01 
GROUP BY 
    e.name 
ORDER BY 
    e.name ASC