PostgreSQL（SQL）如果否則那麼需要語法

Question

我想創建以下函數來檢查是否存在某些東西。它應該返回true或false。我創建了一個觸發器來執行該功能。在任何地方都不起作用/語法錯誤。請幫助！

CREATE FUNCTION checkForGPU() RETURNS BOOLEAN AS $approved$ 
    BEGIN 
     IF EXISTS (
      SELECT C.ID 
      FROM Components C, ComputerComponents CC, ComponentType T, Computer PC 
      WHERE CC.ComputerID = C.ID 
      AND CC.ComputerID = PC.ID 
      AND C.Type = T.ID 
      AND T.Name = 'GPU') 
     THEN 
      RETURN TRUE 
     ELSE THEN 
     RETURN FALSE 

    END;  
    $approved$ LANGUAGE plpgsql; 


CREATE TRIGGER checkGPU 
AFTER INSERT OR UPDATE OR DELETE ON ComputerComponents 
EXECUTE PROCEDURE checkForGPU(); 

Answer 1

你的函數可以rewrited更簡單，無需IF-THEN-ELSE語句

CREATE FUNCTION checkForGPU() RETURNS BOOLEAN AS $approved$ 
    BEGIN 
     RETURN EXISTS(
      SELECT C.ID 
      FROM Components AS C, ComputerComponents AS CC, ComponentType AS T, Computer AS PC 
      WHERE CC.ComputerID = C.ID 
      AND CC.ComputerID = PC.ID 
      AND C.Type = T.ID 
      AND T.Name = 'GPU' 
     ); 
    END;  
    $approved$ LANGUAGE plpgsql;