向JOIN語句中添加WHERE子句

Question

我有2個表，員工和假期。

我想選擇一個員工名單，他們允許的假期和剩餘的假期後，看看今年輸入的假期。

的表格看起來像這樣（只顯示重要行）

employees 
    id, 
    name, 
    allowed_holiday 

holiday 
    id, 
    employee, 
    num_days, 
    date_to 

我有一個發現這些值精細當前的查詢，但我想添加一個WHERE子句只能算假期對員工如果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 

我不確定在哪裏爲此添加WHERE子句，但不刪除沒有假期列出的數據的員工。

Answer 1

上h.date_to添加條件到ON條款不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

當你正在做一個LEFT JOIN，過濾條件必須在ON語句。因此，在你的情況下，它會是這樣的：

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