将空值转换为实际值

CREATE OR REPLACE VIEW v AS
SELECT NULL AS c1, NULL AS c2, '1' AS c3, NULL AS c4, '2' AS c5 FROM dual
UNION ALL
SELECT NULL AS c1, NULL AS c2, NULL AS c3, '3' AS c4, '2' AS c5 FROM dual;
SELECT * FROM v;

-- 使用nvl函数
SELECT NVL(c3, '0') c3 FROM v;

-- 使用coalesce函数
SELECT COALESCE(c3, '0') c3 FROM v;

-- 方法一
SELECT NVL(NVL(NVL(NVL(c1, c2), c3), c4), c5) AS c from v;

-- 方法二
SELECT COALESCE(c1, c2, c3, c4, c5) AS c FROM v;