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前言 MySQL应用常使用FROM_UNIXTIME和UNIX_TIMESTAMP函数，但是无法在YashanDB直接执行，需要改写。本文探讨如何利用YashanDB高度兼容Oracle的特点快速完成函数改写的方案。
问题 MySQL示例使用FROM_UNIXTIME和UNIX_TIMESTAMP，执行成功
mysql> select FROM_UNIXTIME( 8640000 + UNIX_TIMESTAMP('2024-01-01 00:00:00')) from dual;
+-------------------------------------------------------------------+
| FROM_UNIXTIME( 8640000 + UNIX_TIMESTAMP('2024-01-01 00:00:00')) |
+-------------------------------------------------------------------+
| 2024-04-10 00:00:00 |
+-------------------------------------------------------------------+
1 row in set (0.00 sec)
YashanDB报错
SQL> select FROM_UNIXTIME( 8640000 + UNIX_TIMESTAMP('2024-01-01 00:00:00')) from dual;
[1:8]YAS-04243 invalid identifier "FROM_UNIXTIME"
解决方法 思考 MySQL的常用函数FROM_UNIXTIME和UNIX_TIMESTAMP，没有在YashanDB实现。但是，由于YashanDB高度兼容Oracle，而应用从MySQL数据库切换到Oracle数据库，也是常见的数据库切换。因此，可以先借鉴Oracle如何兼容MySQL的常用函数FROM_UNIXTIME和UNIX_TIMESTAMP。
改写方案 通过搜索，找到Oracle的改写方法：forums.oracle.com/ords/apexds…
CREATE OR REPLACE FUNCTION SYS.FROM_UNIXTIME
(
TIMESTAMP_ IN NUMBER
) RETURN DATE AS
SECONDS_PER_DAY NUMBER := 86400;
MASK VARCHAR2(50 CHAR) := 'YYYY-MM-DD HH24:MI:SS';
REF_DATE DATE;
BEGIN
REF_DATE := TO_DATE('1970-01-01 00:00:00', MASK);
RETURN REF_DATE + (TIMESTAMP_/SECONDS_PER_DAY);
END FROM_UNIXTIME;
/
CREATE OR REPLACE FUNCTION SYS.UNIX_TIMESTAMP
(
DATE_ IN DATE
) RETURN NUMBER AS
SECONDS_PER_DAY NUMBER := 86400;
DIFF_BETWEEN_DATES NUMBER;
MASK VARCHAR2(50 CHAR) := 'YYYY-MM-DD HH24:MI:SS';
REF_DATE DATE;
BEGIN
REF_DATE := TO_DATE('1970-01-01 00:00:00', MASK);
DIFF_BETWEEN_DATES := DATE_ - REF_DATE;
RETURN DIFF_BETWEEN_DATES * SECONDS_PER_DAY;
END UNIX_TIMESTAMP;
/
CREATE PUBLIC SYNONYM FROM_UNIXTIME FOR SYS.FROM_UNIXTIME;
CREATE PUBLIC SYNONYM UNIX_TIMESTAMP FOR SYS.UNIX_TIMESTAMP;
成果 实现YashanDB的FROM_UNIXTIME和UNIX_TIMESTAMP函数，YashanDB报错的问题得到解决
SQL> select FROM_UNIXTIME( 8640000 + UNIX_TIMESTAMP('2024-01-01 00:00:00')) from dual;
FROM_UNIXTIME(864000
------------------------------------------------
2024-04-10 00:00:00
1 row fetched.
