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GREATEST and COALESCE return different data types
for a combination of TIME and DATE parameters:
mysql> SELECT GREATEST(CURRENT_TIME, CURRENT_DATE), COALESCE(CURRENT_TIME, CURRENT_DATE);
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+--------------------------------------+--------------------------------------+
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| GREATEST(CURRENT_TIME, CURRENT_DATE) | COALESCE(CURRENT_TIME, CURRENT_DATE) |
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+--------------------------------------+--------------------------------------+
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| 838:59:59 | 0000-00-00 16:24:54 |
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+--------------------------------------+--------------------------------------+
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1 row in set, 1 warning (0.00 sec)
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mysql> show warnings;
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+---------+------+----------------------------------------------+
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| Level | Code | Message |
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+---------+------+----------------------------------------------+
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| Warning | 1292 | Truncated incorrect time value: '1944:00:00' |
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+---------+------+----------------------------------------------+
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1 row in set (0.00 sec)
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GREATEST returns TIME, which looks wrong.
COALESCE return DATETIME, which looks more correct.
(DATETIME is a minumal common type for DATE and TIME).
GREATEST should be fixed to return DATETIME as well.
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