5.29 Treating Dates or Times as Numbers
5.29.1 Problem
You want to treat a temporal
string as a number.
5.29.2 Solution
Perform a string-to-number conversion.
5.29.3 Discussion
In many cases, it is possible in MySQL to treat date and time values
as numbers. This can sometimes be useful if you want to perform an
arithmetic operation on the value. To force conversion of a temporal
value to numeric form, add zero or use it in a
numeric context:
mysql> SELECT t1,
-> t1+0 AS 't1 as number',
-> FLOOR(t1) AS 't1 as number',
-> FLOOR(t1/10000) AS 'hour part'
-> FROM time_val;
+----------+--------------+--------------+-----------+
| t1 | t1 as number | t1 as number | hour part |
+----------+--------------+--------------+-----------+
| 15:00:00 | 150000 | 150000 | 15 |
| 05:01:30 | 50130 | 50130 | 5 |
| 12:30:20 | 123020 | 123020 | 12 |
+----------+--------------+--------------+-----------+
The same kind of conversion can be performed for date or
date-and-time values:
mysql> SELECT d, d+0 FROM date_val;
+------------+----------+
| d | d+0 |
+------------+----------+
| 1864-02-28 | 18640228 |
| 1900-01-15 | 19000115 |
| 1987-03-05 | 19870305 |
| 1999-12-31 | 19991231 |
| 2000-06-04 | 20000604 |
+------------+----------+
mysql> SELECT dt, dt+0 FROM datetime_val;
+---------------------+----------------+
| dt | dt+0 |
+---------------------+----------------+
| 1970-01-01 00:00:00 | 19700101000000 |
| 1987-03-05 12:30:15 | 19870305123015 |
| 1999-12-31 09:00:00 | 19991231090000 |
| 2000-06-04 15:45:30 | 20000604154530 |
+---------------------+----------------+
A value produced by adding zero is not the same as that produced by
conversion into basic units like seconds or days. The result is
essentially what you get by removing all the delimiters from the
string representation of the original value. Also, conversion to
numeric form works only for values that MySQL interprets temporally.
If you try converting a literal string to a number by adding zero,
you'll just get the first component:
mysql> SELECT '1999-01-01'+0, '1999-01-01 12:30:45'+0, '12:30:45'+0;
+----------------+-------------------------+--------------+
| '1999-01-01'+0 | '1999-01-01 12:30:45'+0 | '12:30:45'+0 |
+----------------+-------------------------+--------------+
| 1999 | 1999 | 12 |
+----------------+-------------------------+--------------+
This same thing happens with functions such as DATE_FORMAT(
) and TIME_FORMAT( ), or if you pull out
parts of DATETIME or TIMESTAMP
values with LEFT( ) or RIGHT(
). In +0 context, the results of these
functions are treated as strings, not temporal types.
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