Oracle DECODE vs. CASE。查询返回不同的结果集

Question

我有以下两个版本的SQL语句。一个使用DECODE另一个情况。其他一切都一样。 CASE提供不同的输出（基本上是具有相应访问权限的组）时，DECODE可以按预期工作。是什么原因？

两者都可以在客户端中运行，不依赖于数据库模式。只有使用DECODE或CASE的子查询不同，其他所有部分都相同。

欢迎任何建议！

输出解码：

输出案例：

解码（短码）

...
SELECT    principal
         || DECODE (MAX (DECODE ("ON", 'N', permission)),
                    NULL, MAX (DECODE ("ON", 'O', permission)),
                    MAX (DECODE ("ON", 'N', permission)))    latest_permission,
            principal
         || DECODE (MAX (DECODE ("ON", 'O', permission)),
                    NULL, MAX (DECODE ("ON", 'N', permission)),
                    MAX (DECODE ("ON", 'O', permission)))    dont_overwrite_existing
    FROM
...

---

CASE缩写

---

...
SELECT    principal
     || CASE MAX (
                 CASE "ON" WHEN 'N' THEN permission END)
            WHEN NULL
            THEN
                MAX (
                    CASE "ON"
                        WHEN 'O' THEN permission
                    END)
            ELSE
                MAX (
                    CASE "ON"
                        WHEN 'N' THEN permission
                    END)
        END    latest_permission,
        principal
     || CASE MAX (
                 CASE "ON" WHEN 'O' THEN permission END)
            WHEN NULL
            THEN
                MAX (
                    CASE "ON"
                        WHEN 'N' THEN permission
                    END)
            ELSE
                MAX (
                    CASE "ON"
                        WHEN 'O' THEN permission
                    END)
        END    dont_overwrite_existing

FROM 
...

---

完整的解码器

---

WITH
    old
    AS
        (SELECT REGEXP_SUBSTR (acl, '[^\(]+')     principal,
                REGEXP_SUBSTR (acl, '\(.+')       permission
           FROM (    SELECT REGEXP_SUBSTR (
                                ':Group1(RWDA),:Group2(RWD),:Group3(RW),:Group4((R)',
                                '[^,]+',
                                1,
                                LEVEL)    acl
                       FROM DUAL
                      WHERE ':Group1(RWDA),:Group2(RWD),:Group3(RW),:Group4((R)'
                                IS NOT NULL
                 CONNECT BY REGEXP_SUBSTR (
                                ':Group1(RWDA),:Group2(RWD),:Group3(RW),:Group4((R)',
                                '[^,]+',
                                1,
                                LEVEL)
                                IS NOT NULL)),
    new
    AS
        (SELECT REGEXP_SUBSTR (acl, '[^\(]+')     principal,
                REGEXP_SUBSTR (acl, '\(.+')       permission
           FROM (    SELECT REGEXP_SUBSTR (':Group1(R),:Group1(RW),:GroupA(RWDA),:Group5(R)',
                                           '[^,]+',
                                           1,
                                           LEVEL)    acl
                       FROM DUAL
                      WHERE ':Group1(RWDA),:Group2(RWD),:Group3(RW),:Group4(R)'
                                IS NOT NULL
                 CONNECT BY REGEXP_SUBSTR (
                                ':Group1(R),:Group1(RW),:GroupA(RWDA),:Group5(R)',
                                '[^,]+',
                                1,
                                LEVEL)
                                IS NOT NULL)),
    principalsToDelete
    AS
        (SELECT DISTINCT REGEXP_SUBSTR (acl, '[^\(]+')     principal
           FROM (    SELECT REGEXP_SUBSTR (NULL,
                                           '[^,]+',
                                           1,
                                           LEVEL)    acl
                       FROM DUAL
                      WHERE NULL IS NOT NULL
                 CONNECT BY REGEXP_SUBSTR (NULL,
                                           '[^,]+',
                                           1,
                                           LEVEL)
                                IS NOT NULL))
  SELECT    principal
         || DECODE (MAX (DECODE ("ON", 'N', permission)),
                    NULL, MAX (DECODE ("ON", 'O', permission)),
                    MAX (DECODE ("ON", 'N', permission)))    latest_permission,
            principal
         || DECODE (MAX (DECODE ("ON", 'O', permission)),
                    NULL, MAX (DECODE ("ON", 'N', permission)),
                    MAX (DECODE ("ON", 'O', permission)))    dont_overwrite_existing
    FROM (SELECT o.*, 'O' "ON"
            FROM old o
          UNION
          SELECT n.*, 'N' "ON"
            FROM new n)
   WHERE principal NOT IN (SELECT principal FROM principalsToDelete)
GROUP BY principal
ORDER BY principal;

---

CASE Full

---

WITH
    old
    AS
        (SELECT REGEXP_SUBSTR (acl, '[^\(]+')     principal,
                REGEXP_SUBSTR (acl, '\(.+')       permission
           FROM (    SELECT REGEXP_SUBSTR (
                                ':Group1(RWDA),:Group2(RWD),:Group3(RW),:Group4((R)',
                                '[^,]+',
                                1,
                                LEVEL)    acl
                       FROM DUAL
                      WHERE ':Group1(RWDA),:Group2(RWD),:Group3(RW),:Group4((R)'
                                IS NOT NULL
                 CONNECT BY REGEXP_SUBSTR (
                                ':Group1(RWDA),:Group2(RWD),:Group3(RW),:Group4((R)',
                                '[^,]+',
                                1,
                                LEVEL)
                                IS NOT NULL)),
    new
    AS
        (SELECT REGEXP_SUBSTR (acl, '[^\(]+')     principal,
                REGEXP_SUBSTR (acl, '\(.+')       permission
           FROM (    SELECT REGEXP_SUBSTR (':Group1(R),:Group1(RW),:GroupA(RWDA),:Group5(R)',
                                           '[^,]+',
                                           1,
                                           LEVEL)    acl
                       FROM DUAL
                      WHERE ':Group1(RWDA),:Group2(RWD),:Group3(RW),:Group4(R)'
                                IS NOT NULL
                 CONNECT BY REGEXP_SUBSTR (
                                ':Group1(R),:Group1(RW),:GroupA(RWDA),:Group5(R)',
                                '[^,]+',
                                1,
                                LEVEL)
                                IS NOT NULL)),
    principalsToDelete
    AS
        (SELECT DISTINCT REGEXP_SUBSTR (acl, '[^\(]+')     principal
           FROM (    SELECT REGEXP_SUBSTR (NULL,
                                           '[^,]+',
                                           1,
                                           LEVEL)    acl
                       FROM DUAL
                      WHERE NULL IS NOT NULL
                 CONNECT BY REGEXP_SUBSTR (NULL,
                                           '[^,]+',
                                           1,
                                           LEVEL)
                                IS NOT NULL))
  SELECT    principal
         || CASE MAX (
                     CASE "ON" WHEN 'N' THEN permission END)
                WHEN NULL
                THEN
                    MAX (
                        CASE "ON"
                            WHEN 'O' THEN permission
                        END)
                ELSE
                    MAX (
                        CASE "ON"
                            WHEN 'N' THEN permission
                        END)
            END    latest_permission,
            principal
         || CASE MAX (
                     CASE "ON" WHEN 'O' THEN permission END)
                WHEN NULL
                THEN
                    MAX (
                        CASE "ON"
                            WHEN 'N' THEN permission
                        END)
                ELSE
                    MAX (
                        CASE "ON"
                            WHEN 'O' THEN permission
                        END)
            END    dont_overwrite_existing

    FROM (SELECT o.*, 'O' "ON"
            FROM old o
          UNION
          SELECT n.*, 'N' "ON"
            FROM new n)
   WHERE principal NOT IN (SELECT principal FROM principalsToDelete)
GROUP BY principal
ORDER BY principal;

Answer 1

差异来自与null相比的行为差异：

SQL> select decode(null,null,1) from dual;

DECODE(NULL,NULL,1)
-------------------
                  1

SQL> select case null when null then 1 end from dual;

CASENULLWHENNULLTHEN1END
------------------------

第二个查询返回NULL而不是1，因此如果x为NULL，'case x when null then 1 end'不会返回1，但这是可行的：

SQL> select case when null is null then 1 end from dual;

CASEWHENNULLISNULLTHEN1END
--------------------------
                         1