函数说明
CONVERT_TZ()将datetime 值dt 从FROM_tz 给定的时区转化为to_tz，
并返回结
果值。如果参数是不合法的，该函数返回NULL。
如果在从FROM_tz 转化到UTC 时值超出了TIMESTAMP 类型支持的范围，
就不
会进行转化。关于TIMESTAMP 的取值范围，在TIMESTAMP 章节中有描述。
要使用诸如“MET”或“Europe/Moscow”命名时区，必须适当的设置时区表。
示例
示例1：from_tz 为'+00:00', to_tz 为'-07:00'，进行时区转换。
gbase> SELECT CONVERT_TZ('2020-01-01 12:00:00','+00:00','-07:00')
FROM dual;
+-----------------------------------------------------+
| CONVERT_TZ('2020-01-01 12:00:00','+00:00','-07:00') |
+-----------------------------------------------------+
| 2020-01-01 05:00:00
|
+-----------------------------------------------------+
1 row in set
示例2：CONVERT_TZ()函数支持夏令时的时区内转换。
gbase> SELECT CONVERT_TZ('2020-03-11
2:00:00','US/Eastern','US/Central') FROM dual;
+------------------------------------------------------------+
| CONVERT_TZ('2020-03-11 2:00:00','US/Eastern','US/Central') |
+------------------------------------------------------------+
GBase 8a MPP Cluster 产品手册
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文档版本953（2022-04-10）
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| 2020-03-11 01:00:00
|
+------------------------------------------------------------+
1 row in set (Elapsed: 00:00:00.02)
gbase> SELECT CONVERT_TZ('2020-03-11
3:00:00','US/Eastern','US/Central') FROM dual;
+------------------------------------------------------------+
| CONVERT_TZ('2020-03-11 3:00:00','US/Eastern','US/Central') |
+------------------------------------------------------------+
| 2020-03-11 02:00:00
|
+------------------------------------------------------------+
1 row in set (Elapsed: 00:00:00.02)
注：以下方式不支持夏令时：
gbase> SELECT CONVERT_TZ('2020-03-11 2:00:00','-6:00','-7:00') FROM
dual;
+--------------------------------------------------+
| CONVERT_TZ('2020-03-11 2:00:00','-6:00','-7:00') |
+--------------------------------------------------+
| 2020-03-11 01:00:00
|
+--------------------------------------------------+
1 row in set (Elapsed: 00:00:00.02)
gbase> SELECT CONVERT_TZ('2020-03-11 3:00:00','-6:00','-7:00') FROM
dual;
+--------------------------------------------------+
| CONVERT_TZ('2020-03-11 3:00:00','-6:00','-7:00') |
+--------------------------------------------------+
| 2020-03-11 02:00:00
|
+--------------------------------------------------+
1 row in set (Elapsed: 00:00:00.00)