sql - 使用 IF EXISTS 连接 SQL 选择语句

Question

如果这是一个愚蠢的问题，我深表歉意，但我无法理解这个问题。如果存在，我只想选择一个带冒号的值，这个值是“县”。但是，我不确定我是否以正确的方式进行此操作？任何帮助将不胜感激。

SELECT pe.personname AS "Person Name", pe.street||','||pe.town||','||
    IF NOT EXISTS(SELECT pe.county FROM persons pe WHERE pe.county = '')
    THEN pe.county||','|| 
    ELSE
    END IF
 pe.postcode as "Persons Address"
 FROM persons pe

---

更新版本，如果没有县，则输出多个逗号。

SELECT personname as "Person Name"
street||','||town||
   CASE county
   WHEN '' THEN ''
   ELSE ','||county
   END
||','||postcode as "Person Address"
from Suppliers;

Answer 1

我已经这样做了，但遇到了缺少关键字错误:-

 SELECT personname AS "Person Name", 
   street || ',' || town || ',' ||
     CASE WHEN county > ''
     THEN county || ','
     ELSE ''
     END 
   || postcode AS "Person Address"
 FROM Persons