與case語句SELECT語句

Question

我有一個客戶下表：

+------------+-------------+------+-----+---------+----------------+ 
| Field  | Type  | Null | Key | Default | Extra   | 
+------------+-------------+------+-----+---------+----------------+ 
| id   | int(11)  | NO | PRI | NULL | auto_increment | 
| first  | varchar(45) | YES |  | NULL |    | 
| last  | varchar(45) | YES |  | NULL |    | 
| password | varchar(45) | NO |  | NULL |    | 
| contact_id | int(11)  | NO | PRI | NULL |    | 
| address_id | int(11)  | NO | PRI | NULL |    | 
+------------+-------------+------+-----+---------+----------------+ 

並任命下列結構：

+-------------+------------+------+-----+---------+----------------+ 
| Field  | Type  | Null | Key | Default | Extra   | 
+-------------+------------+------+-----+---------+----------------+ 
| id   | int(11) | NO | PRI | NULL | auto_increment | 
| time  | datetime | NO |  | NULL |    | 
| cancelled | tinyint(1) | YES |  | 0  |    | 
| confirmed | tinyint(1) | YES |  | 0  |    | 
| customer_id | int(11) | NO | PRI | NULL |    | 
+-------------+------------+------+-----+---------+----------------+ 

我想用一個查詢來獲取客戶信息，如果他們有預約信息，那麼它會查詢它，否則它不會。

我嘗試使用以下命令：

CASE 
    WHEN (SELECT count(a.id) FROM appointment 
      INNER JOIN customer c ON a.customer_id = c.id) 
     THEN (SELECT c.first, c.last, c.id, a.id FROM appointent 
       INNER JOIN customer c ON a.customer_id = c.id) 
    ELSE 
     (SELECT c.first, c.last, c.id FROM customer) 
END; 

你有什麼建議嗎？

Answer 1

怎麼樣

SELECT * FROM Customer c LEFT JOIN Appointment a ON a.CustomerId = c.Id 

Answer 2

你可以做兩個查詢和UNION他們。

SELECT c.first, c.last, c.id, a.id FROM appointent a 
       INNER JOIN customer c ON a.customer_id = c.id 
UNION 
SELECT c.first, c.last, c.id, null FROM customer c 

或外部連接，其中a.id將與空，如果有在參加比賽沒有被填充。

SELECT c.first, c.last, c.id, a.id FROM customer c 
       OUTER JOIN appointent a ON a.customer_id = c.id 

Answer 3

按我的茲德拉夫科的答案評論，你可以也用：

select * from customer where id in (select customer_id from appointment where cancelled = 0) 

這將允許你在一個不錯的方式進行篩選。

您還可以過濾茲德拉夫科的回答是這樣的：

SELECT * FROM Customer c LEFT JOIN Appointment a ON a.CustomerId = c.Id 
WHERE a.cancelled 0 and a.confirmed = 1