§ SQL兼容性 - TO_DATE()函数

§ 1. 语法

TO_DATE(string , fmt)

§ 2. 定义和用法

TO_DATE()函数用于将字符串类型数据 string, 转换为日期(时间)格式类型，fmt 用于指定 string 的日期模型。

当前 fmt 日期模型支持说明：

格式	描述	示例或说明
YYYY/RRRR	4-digit year	2023
YYY	3-digit year	023
YY	2-digit year	23
RR	2-digit year	与YY类似，但会因指定的年号与当前年份的后两位数字返回不同的值： - 当前年份后两位为[00,49]: 'RR'对应年号在[00,49], 返回年号前两位数值与当前年份相同 'RR'对应年号在[50,99], 返回年号前两位数值比当前年份小1 - 当前年份后两位为[50,99]: 'RR'对应年号在[00,49], 返回年号前两位数值比当前年份大1 'RR'对应年号在[50,99], 返回年号前两位数值与当前年份相同
Y	1-digit year	1
DD	Day of month (1-31)
HH, HH12	Hour of day (1-12).
HH24	Hour of day (0-23).
MI	Minute that ranges from 0 to 59
MM	Month that ranges from 01 through 12, where January is 01.
MON	Abbreviated name of the month.	JAN,FEB
MONTH	Name of the month.	JANUARY
SS	Second (0-59).
AM/A.M.	Meridian indicator with or without periods.	格式与PM等价，最后的date值取决于第一个参数字串中对应位置值
PM/P.M.	Meridian indicator with or without periods.	格式与AM等价，最后的date值取决于第一个参数字串中对应位置值
D	Day of week (1-7). This element depends on the NLS territory of the session.
DDD	Day of year (1-366).
`-` `/` `,` `.` `;` `:`	Punctuation is reproduced in the result.	Any non-alphanumeric character is allowed to match the punctuation characters in the format model.
SSSSS	Seconds past midnight (0-86399).
J	Julian day; the number of days since January 1, 4712 BC. Number specified with J must be integers.	GreatSQL 只支持 0001-01-01 ~ 9999-12-31 之间的 Julian day，即范围为：[1721424, 5373484]
X	Local radix character.	'HH:MI:SSXFF'
Y,YYY	Year with comma in this position.	2,023
RM	Roman numeral month (I-XII; January = I).
TH	Ordinal Number	DDTH
FM	Fill mode.
FX	Format exact.

输出说明：

当fmt 同时指定日期或时间格式，string 被成功解析后，将返回一个 DATETIME值(日期+时间，如: 2023-01-01 00:00:00)
未被指定的日期或时间部分，TO_DATE() 会为返回的DATETIME值对应部分赋予默认值。

日期或时间部分	默认值
year	本年(即'select now()' 所在年份)
month	本月(即'select now()' 所在月份)
day	每月1日
hour	0
minute	0
second	0

示例：

TO_DATE()输入	返回值
TO_DATE('2023', 'YYYY')	2023-05-01 00:00:00
TO_DATE('202310', 'YYYYMM')	2023-10-01 00:00:00
TO_DATE('11', 'HH')	2023-05-01 11:00:00

§ 3. SQL兼容说明

对指定格式包含HH或HH12时，GreatSQL与Oracle在显示12点时，返回值不同。例如：

TO_DATE()输入	Oracle返回	GreatSQL返回
TO_DATE('12','HH12')	2023-05-01 12:00:00	2023-05-01 00:00:00
TO_DATE('12','HH')	2023-05-01 12:00:00	2023-05-01 00:00:00

在Oracle中，YY/RR 格式可以读取 2位/3位/4位年份数字，而GreatSQL只能读取2位年份数字。

TO_DATE()输入	Oracle返回	GreatSQL返回
TO_DATE('20121018', 'YYMMDD')	2012-10-18	NULL
TO_DATE('2012-10-18','YY-MM-DD')	2012-10-18	NULL
TO_DATE('20121018', 'RRMMDD')	2012-10-18	NULL
TO_DATE('012-10-18','RR-MM-DD')	2012-10-18	NULL
TO_DATE('12-10-18','YY-MM-DD')	2012-10-18	2012-10-18 00:00:00

§ 4. 示例

greatsql> select to_date('2003-01-02 10:11:12 PM', 'YYYY-MM-DD HH12:MI:SS PM') from dual;
+---------------------------------------------------------------+
| to_date('2003-01-02 10:11:12 PM', 'YYYY-MM-DD HH12:MI:SS PM') |
+---------------------------------------------------------------+
| 2003-01-02 22:11:12                                           |
+---------------------------------------------------------------+

greatsql> select to_date('03-01-02 8:11:2.123456', 'YY-MM-DD HH24:MI:SS') from dual;
+----------------------------------------------------------+
| to_date('03-01-02 8:11:2.123456', 'YY-MM-DD HH24:MI:SS') |
+----------------------------------------------------------+
| 2003-01-02 08:11:02                                      |
+----------------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

greatsql> show warnings;
+---------+------+--------------------------------------------------------------+
| Level   | Code | Message                                                      |
+---------+------+--------------------------------------------------------------+
| Warning | 1292 | Truncated incorrect datetime value: '03-01-02 8:11:2.123456' |
+---------+------+--------------------------------------------------------------+

greatsql> SELECT TO_DATE('23:45:33','hh24:mi:ss');
+----------------------------------+
| TO_DATE('23:45:33','hh24:mi:ss') |
+----------------------------------+
| 2023-05-01 23:45:33              |
+----------------------------------+

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