# Date and Time Operations

# Date arithmetic

NOW() + INTERVAL 1 DAY  -- This time tomorrow

CURDATE() - INTERVAL 4 DAY -- Midnight 4 mornings ago

Show the mysql questions stored that were asked 3 to 10 hours ago (180 to 600 minutes ago):

SELECT qId,askDate,minuteDiff  
FROM 
(   SELECT qId,askDate,
    TIMESTAMPDIFF(MINUTE,askDate,now()) as minuteDiff 
    FROM questions_mysql 
) xDerived 
WHERE minuteDiff BETWEEN 180 AND 600 
ORDER BY qId DESC 
LIMIT 50;

+----------+---------------------+------------+
| qId      | askDate             | minuteDiff |
+----------+---------------------+------------+
| 38546828 | 2016-07-23 22:06:50 |        182 |
| 38546733 | 2016-07-23 21:53:26 |        195 |
| 38546707 | 2016-07-23 21:48:46 |        200 |
| 38546687 | 2016-07-23 21:45:26 |        203 |
| ...      |                     |            |
+----------+---------------------+------------+

MySQL manual pages for TIMESTAMPDIFF().

Beware Do not try to use expressions like CURDATE() + 1 for date arithmetic in MySQL. They don't return what you expect, especially if you're accustomed to the Oracle database product. Use CURDATE() + INTERVAL 1 DAY instead.

# SYSDATE(), NOW(), CURDATE()


 SELECT SYSDATE();

This function returns the current date and time as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context. It returns the date and time in the current time zone.


 SELECT NOW();

This function is a synonym for SYSDATE().


 SELECT CURDATE();

This function returns the current date, without any time, as a value in 'YYYY-MM-DD' or YYYYMMDD format, depending on whether the function is used in a string or numeric context. It returns the date in the current time zone.

# Now()

Select Now();

Shows the current server date and time.

Update `footable` set mydatefield = Now();

This will update the field mydatefield with current server date and time in server's configured timezone, e.g.

'2016-07-21 12:00:00'

# Testing against a date range

Although it is very tempting to use BETWEEN ... AND ... for a date range, it is problematical. Instead, this pattern avoids most problems:

WHERE x >= '2016-02-25'
  AND x  < '2016-02-25' + INTERVAL 5 DAY

Advantages:

  • BETWEEN is 'inclusive' thereby including the final date or second.
  • 23:59:59 is clumsy and wrong if you have microsecond resolution on a DATETIME.
  • This pattern avoid dealing with leap years and other data calculations.
  • It works whether x is DATE, DATETIME or TIMESTAMP.

# Extract Date from Given Date or DateTime Expression

SELECT DATE('2003-12-31 01:02:03');

The output will be:

2003-12-31

# Using an index for a date and time lookup

Many real-world database tables have many rows with DATETIME OR TIMESTAMP column values spanning a lot of time, including years or even decades. Often it's necessary to use a WHERE clause to retrieve some subset of that timespan. For example, we might want to retrieve rows for the date 1-September-2016 from a table.

An inefficient way to do that is this:


WHERE DATE(x) = '2016-09-01'   /* slow! */

It's inefficient because it applies a function -- DATE() -- to the values of a column. That means MySQL must examine each value of x, and an index cannot be used.

A better way to do the operation is this


WHERE x >= '2016-09-01'
   AND x <  '2016-09-01' + INTERVAL 1 DAY

This selects a range of values of x lying anywhere on the day in question, up until but not including (hence <) midnight on the next day.

If the table has an index on the x column, then the database server can perform a range scan on the index. That means it can quickly find the first relevant value of x, and then scan the index sequentially until it finds the last relevant value. An index range scan is much more efficient than the full table scan required by DATE(x) = '2016-09-01.

Don't be tempted to use this, even though it looks more efficient.


 WHERE x BETWEEN '2016-09-01' AND '2016-09-01' + INTERVAL 1 DAY  /*  wrong! */

It has the same efficiency as the range scan, but it will select rows with values of x falling exactly at midnight on 2-Sept-2016, which is not what you want.