16. 通过内部函数提高SQL效率.
SELECT H.EMPNO,E.ENAME,H.HIST_TYPE,T.TYPE_DESC,COUNT(*) FROM HISTORY_TYPE T,EMP E,EMP_HISTORY H WHERE H.EMPNO = E.EMPNOAND H.HIST_TYPE = T.HIST_TYPEGROUP BY H.EMPNO,E.ENAME,H.HIST_TYPE,T.TYPE_DESC;
通过调用下面的函数可以提高效率.
FUNCTION LOOKUP_HIST_TYPE(TYP IN NUMBER) RETURN VARCHAR2AS TDESC VARCHAR2(30); CURSOR C1 IS SELECT TYPE_DESC FROM HISTORY_TYPE WHERE HIST_TYPE = TYP;BEGIN OPEN C1; FETCH C1 INTO TDESC; CLOSE C1; RETURN (NVL(TDESC,’?’));END;FUNCTION LOOKUP_EMP(EMP IN NUMBER) RETURN VARCHAR2
AS
ENAME VARCHAR2(30); CURSOR C1 IS SELECT ENAME FROM EMP WHERE EMPNO=EMP;BEGIN OPEN C1; FETCH C1 INTO ENAME; CLOSE C1; RETURN (NVL(ENAME,’?’));END;
SELECT H.EMPNO,LOOKUP_EMP(H.EMPNO),
H.HIST_TYPE,LOOKUP_HIST_TYPE(H.HIST_TYPE),COUNT(*)
FROM EMP_HISTORY HGROUP BY H.EMPNO , H.HIST_TYPE;(译者按: 经常在论坛中看到如 ’能不能用一个SQL写出….’ 的贴子, 殊不知复杂的SQL往往牺牲了执行效率. 能够把握上面的运用函数解决问题的方法在实际工作中是非常有意义的)
