如何統計和連接兩個MySQL表？

Question

我想寫出租車的次數，我只是不能得到它的工作，我有兩張桌子。一個叫vehicle，另一個叫rental。

在租賃是所有的「租賃」去當你預訂一輛車，所有的車輛存儲在車輛。這是我所做的幾乎可行的查詢。

SELECT vehicle.id,COUNT(rental.vid) as rented,IFNULL(rental.vid,0) as nothing, vehicle.make as make, vehicle.model as model, vehicle.regnr as regnr, vehicle.color as color, vehicle.state as state, vehicle.imgurl as img, vehicle.description as description, vehicle.id 
FROM rental,vehicle 
WHERE vid = vehicle.id GROUP BY vid 

，並打印出這一點：

不具有一個值（從未被租用），其餘不存在，我已經嘗試了很多不同的方式與IFNULL但沒有得到任何地方。

Answer 1

Select從Vehicle表和Left Join到Rental表。這將包括從未被租用他們的計數（rental.vid）爲0的車輛：

SELECT vehicle.id 
,COUNT(rental.vid) as rented 
, vehicle.make as make 
, vehicle.model as model 
, vehicle.regnr as regnr 
, vehicle.color as color 
, vehicle.state as state 
, vehicle.imgurl as img 
, vehicle.description as description 
FROM vehicle 
left join rental on vid = vehicle.id 
GROUP BY vehicle.id 

Here is a simplified example

隱含的加入你在你的例子是相當於inner join。通過左連接，您可以從源表中選擇所需的所有行。如果與您要加入的表格匹配，它們也會顯示。這是將數據追加到源表的好方法。