IFNULL在嵌套select語句中mysql

Question

我想返回一條消息，如nested SELECT這樣的語句中沒有找到任何行。在這種情況下，當連續沒有Lake時，返回消息是'找不到湖'。我試過IFNULL，但是當沒有湖時它什麼都不會返回。

SELECT GROUP_CONCAT(concat_ws(', ', Lake_name, 
            IFNULL(Area, 'not available'),        
            IFNULL(Altitude, 'not available'), 
            IFNULL(Depth, 'not available'), 
            IFNULL(River_outflow, 'not available') 
          ) SEPARATOR '; ') 
From LAKE WHERE IFNULL(Lake_name, 'no lakes found') IN 
         (SELECT Lake FROM LOCATION where County IN 
         (SELECT County FROM CITY WHERE City_name='$userinput')) GROUP BY NULL;"; 

Answer 1

我想你應該選擇這種方式

SELECT CASE when Lake_name is null then 'no lakes found' else 
     GROUP_CONCAT(concat_ws(', ', Lake_name, 
             IFNULL(Area, 'not available'),        
             IFNULL(Altitude, 'not available'), 
             IFNULL(Depth, 'not available'), 
             IFNULL(River_outflow, 'not available') 
           ) SEPARATOR '; ') END my_lake 
    From LAKE 
    WHERE LAKE_name IN (SELECT Lake_name FROM LOCATION where County IN 
         (SELECT County FROM CITY WHERE City_name='$userinput')) 
    OR LAKE_name is null ;