Question

我根据公共ID映射了两个表，但我想将所有与特定公司相关联的电话号码放在一行而不是多行，分为三列“电话”，“传真”和“免费电话”。下面是数据输出的示例。

Dr. Dimes   5553456879  Telephone
Dr. Dimes   5553455600  Toll Free
Dr. Dimes   5553450123  Fax

SELECT 
  fleet.company_name, 
  phone_number.phone_number, 
  phone_type.name
FROM 
  fleetseek.phone_number, 
  fleetseek.phone_type, 
  fleetseek.fleet, 
  fleetseek.fleet_phone
WHERE 
  phone_number.phone_type_id = phone_type.phone_type_id AND
  fleet.fleet_id = fleet_phone.fleet_id AND
  fleet_phone.phone_number_id = phone_number.phone_number_id

Answer 1

执行此操作的最简单方法可能是使用聚合：

ConsoleReporter reporter = ConsoleReporter.forRegistry(metrics).convertRatesTo(TimeUnit.SECONDS).convertDurationsTo(TimeUnit.MILLISECONDS).build();
    reporter.start(10, TimeUnit.SECONDS);

Answer 2

当您使用同一个表三次时，您可以通过连接和别名来完成获取一行中的所有列：

    SELECT
      fleet.company_name,
      telephone.phone_number AS Telephone,
      toll_free.phone_number AS Toll_Free,
      fax.phone_number AS Fax
    FROM 
      fleetseek.fleet
    JOIN 
      fleet_phone
    ON 
      fleet.fleet_id = fleet_phone.fleet_id
    LEFT OUTER JOIN (SELECT 
            phone_number
          FROM 
            fleetseek.phone_number, 
            fleetseek.phone_type
          WHERE 
            phone_number.phone_type_id = phone_type.phone_type_id AND
            phone_type.name = 'Telephone') telephone
    ON  
      fleet_phone.phone_number_id = telephone.phone_number_id
    LEFT OUTER JOIN 
      (SELECT 
         phone_number
       FROM 
         fleetseek.phone_number, 
         fleetseek.phone_type
       WHERE 
         phone_number.phone_type_id = phone_type.phone_type_id AND
         phone_type.name = 'Toll Free') toll_free
    ON  
      fleet_phone.phone_number_id = toll_free.phone_number_id
    LEFT OUTER JOIN 
      (SELECT 
         phone_number
       FROM 
         fleetseek.phone_number, 
         fleetseek.phone_type
       WHERE 
         phone_number.phone_type_id = phone_type.phone_type_id AND
         phone_type.name = 'Fax') fax
    ON  
      fleet_phone.phone_number_id = fax.phone_number_id;

一般设计选择建议：如果信息非常重要，请将您的手机类型与电话号码放在同一个表格中。必须添加连接会影响性能（同时影响空间）。通常，性能比空间更昂贵。

如何将多行合并为一行？ SQL PostgreSQL管理员

2 个答案: