我需要幫助使用計數作爲查詢

Question

我需要選擇一個列表，顯示每個已租用汽車的客戶的客戶編號，標題，名字和姓氏，按客戶姓氏的字母順序排列以及計數他們每個人的預訂都已放置。

我已經完成了第一部分，但不知道在哪裏放置預訂的數量。

下面是表是

create table customer 
(customer_id char(4) primary key not null, 
customer_sname varchar (30) not null, 
customer_fname varchar (30) not null, 
customer_title varchar (6) not null, 
customer_address1 varchar (35) not null, 
customer_address2 varchar (35) null, 
customer_postcode varchar (25) null, 
customer_phone varchar (30) null, 
customer_email varchar (40) null, 
customer_di varchar (40) not null) 
ENGINE=InnoDB; 

create table car_booking 
(booking_id INTEGER AUTO_INCREMENT primary key not null, 
car_id char (4) not null, 
customer_id char (4) not null, 
hire_sdate date not null, 
hire_edate date not null) 
engine=innodb 

我已經這樣做了

select customer_id, customer_title, Customer_fname, customer_sname 
from customer 
where customer_id in 
(select customer_id from car_booking) 
order by customer_sname asc 

感謝

Answer 1

在SQL Server中，我會使用：

select c.customer_id, 
     c.customer_title,   
     c.customer_fname, 
     c.customer_sname, 
     count (*) 
    from cutomer c, 
     car_booking cb 
where cb.customer_id = c.customer_id 
group by c.customer_id, 
      c.customer_title, 
      c.customer_fname, 
      c.customer_sname 

不密切熟悉MySQL，所以它可能會pl有點不同，但這是一般的想法。

Answer 2

這將需要使用聚集函數（COUNT），GROUP BY子句和LEFT的JOIN到CAR_BOOKING表：

SELECT c.customer_id, c.customer_title, c.customer_fname, c.customer_sname, 
      COALESCE(COUNT(*), 0) AS num_bookings 
    FROM CUSTOMER c 
LEFT JOIN CAR_BOOKING cb ON cb.customer_id = c.customer_id 
GROUP BY c.customer_id, c.customer_title, c.customer_fname, c.customer_sname 
ORDER BY c.customer_sname 

因爲有不包裹在像COUNT聚合函數列，那些列需要在GROUP BY子句中定義。

我對CAR_BOOKINGS表使用了一個左外部連接來返回沒有任何預訂的客戶 - 這些記錄將顯示爲0，作爲num_booking列中的值。您可以在查詢中省略LEFT關鍵字，以僅返回客戶預訂計數&。 COALESCE是將空值轉換爲所需值的標準函數 - 在這種情況下，計數爲空...

Answer 3

select customer.customer_id, customer.customer_title, customer.customer_fname, customer.customer_sname, count(*) as Bookings 
from customer JOIN car_booking ON customer.customer_id = car_booking.customer_id 
GROUP BY customer.customer_id, customer.customer_title, customer.Customer_fname, customer.customer_sname 
order by customer_sname asc