SQL - SELECT with inner IF or CASE

Question

試圖找到一行，根據找到的值顯示'YES'或'NO'（如果樹已在參數中給出的日期之前處理過，請說YES否）。

這裏是我的功能：

CREATE OR REPLACE FUNCTION tree_care(care_date DATE) 
RETURNS TABLE(name VARCHAR(32), type VARCHAR(32), treated TEXT) AS 
$$ 
BEGIN 
RETURN QUERY 
SELECT tree.name, 
     tree.type, 
     IF EXISTS (SELECT * FROM treatment 
        JOIN tree ON tree.name = treatment.tree_name 
        WHERE treatment.date < care_date) THEN 
      'YES'::text 
     ELSE 
      'NO'::text 
     END IF 
FROM tree; 
END; 
$$ 

而且我得到以下錯誤：

ERROR: syntax error at or near "EXISTS" 
LINE 8:   IF EXISTS (SELECT * FROM treatment 

怎樣才能實現一個SELECT內的IF語句？

PS：使用PostgreSQL 9.4

Answer 1

IF是控制流程。使用CASE因爲這是一個SELECT語句中：

SELECT tree.name, 
     tree.type, 
     (CASE WHEN EXISTS (SELECT 1 
          FROM treatment 
          WHERE tree.name = treatment.tree_name AND 
           treatment.date < care_date 
         ) THEN 
      THEN 'YES'::text 
      ELSE 'NO'::text 
     END) as Flag 
FROM tree; 

我也猜你打算相關子查詢，而不是一個獨立的子查詢。