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NVL?NVL2?COALESCE??????????????????????NULL???????????????????????

???NVL???NON-VALUE LOOKUP??Oracle?????????????????????????????????????NVL??????NVL(expr1, expr2)???expr1?????expr2????????NVL(comm, 0)????????comm???comm??????0?

NVL2????NVL?????????????????????????????????????NVL2????NVL2(expr1, expr2, expr3)????????NVL2??????????????????NVL2(comm, 1000, 0)????????comm???comm??????1000?????0?

???COALESCE?????????????????????????????COALESCE????NVL???????????????COALESCE?????????????NULL?????COALESCE(c1, c2, c3, c4)???c1?c2?c3?c4??????NULL??

???????COALESCE????????????????V_VIEW???????????????????NULL????????????COALESCE??????NULL??

CREATE OR REPLACE VIEW V_VIEW ASSELECT NULL AS C1, NULL AS C2, '1' AS C3, NULL AS C4 FROM DUALUNION ALLSELECT NULL AS C1,'2' AS C2,NULL AS C3,'4'AS C4 FROM DUAL;SELECT * FROM V_VIEW;

??????????????COALESCE??????

SELECT COALESCE(C1,C2,C3,C4) AS RE FROM V_VIEW;

???????????????????NULL??????????????????????????????????????????