在 MySQL 中使用自定义字符串创建别名列

Question

我有一个这样的 MySQL 表：

##customer##
+-----------+----+---------+
|customer_id|name|telephone|
+-----------+----+---------+
|     1     |Andi|+62932011|
|     2     |Boby|+62928291|
|     3     |Jane|+62932212|
|     4     |John|+62999021|
|     5     |Beth|+62999021|
|     6     |Noel|+62999021|
+-----------+----+---------+

##plus_membership##
+-----------------+-----------+-------+------------+
|plus_membership_id|customer_id|status |requested_at|
+------------------+-----------+-------+------------+
|        1         |     1     |   1   | 2018-11-01 |
|        2         |     2     |   0   | 2018-11-03 |
|        3         |     4     |   2   | 2018-11-04 |
|        4         |     6     |   1   | 2018-11-05 |
+------------------+-----------+-------+------------+

上述结构中有两个表，第一个是customer，主键为customer_id，第二个是plus_membership，外键为customer_id，plus_membership表是表格显示客户请求成为 plus 会员的请求，状态 1 表示客户被批准成为 plus 会员。我需要选择客户表并添加别名列假设别名列名称为成员资格，仅显示regular 或plus，plus 表示plus_membership 状态的客户为1，如果客户不存在于 plus_membership 表中或状态不是 1 在成员资格表中。例如：

SELECT *, .... AS membership FROM customer;

+-----------+----+---------+----------+
|customer_id|name|telephone|membership|
+-----------+----+---------+----------+
|     1     |Andi|+62932011|   Plus   |
|     2     |Boby|+62928291|  Regular |
|     3     |Jane|+62932212|  Regular | 
|     4     |John|+62999021|  Regular |
|     5     |Beth|+62999021|  Regular |
|     6     |Noel|+62999021|   Plus   |
+-----------+----+---------+----------+

Answer 1

您可以在两个表之间使用Left Join，并使用Case .. When 条件表达式来相应地计算membership。

Left Join 将确保考虑customer 表中的所有客户，无论他们在plus_membership 表中是否有相应的匹配行。

SELECT
 c.customer_id, 
 c.name, 
 c.telephone, 
 (CASE WHEN pm.status = 1 THEN 'Plus' ELSE 'Regular' END) AS membership
FROM customer AS c
LEFT JOIN plus_membership AS pm 
  ON pm.customer_id = c.customer_id 

---

另一种方法是使用Correlated Subquery 和Exists()。一般来说，这种会比Left Join方法效率低。

SELECT 
  c.customer_id, 
  c.name, 
  c.telephone, 
  CASE WHEN EXISTS (SELECT 1 
                    FROM plus_membership AS pm 
                    WHERE pm.customer_id = c.customer_id AND 
                          pm.status = 1
                   )
       THEN 'Plus' 
       ELSE 'Regular' 
  END AS membership 
FROM customer AS c

Answer 2

我们使用EXISTS 或IN 在另一个表中查找数据。

select customer_id, name, telephone,
  case when customer_id in (select customer_id from plus_membership where status = 1)
       then 'Plus' else 'Regular' end as membership
from customer
order by customer_id;