Extract function for date/time

[vaijns]

Sometimes when getting a date from a database you might not care about the whole date but just a part of it.
For example, if you have a list of articles and want to give the user a filter option for the year in which the article released, you just care about the year.
At the moment you'd have to get the whole date and then in your application code extract the year from it. This isn't a big deal of course but in SQL you have the EXTRACT(YEAR FROM CURRENT_TIMESTAMP) function.

I think having this function in sqlpp would be useful.

Similarly to #610 there's multiple options for the design of such a function.

• Instead of extract you could have multiple functions year_of(date), month_of(date), ... like the functions MySQL has (YEAR(date), MONTH(date), ...).
• You could have a templated function extract<datepart::year_t>(date)
• You could have a parameter for the datepart and have the template parameter deduced extract(datepart::year, date)

Due to the EXTRACT(datepart FROM date) function being standardized I strongly prefer the latter two.

-Jonas

[rbock]

The extract(datepart::year, date variant sounds reasonable to me.

As in #610: If you want to implement this, please wait until the optional-no-dynamic branch is a bit more advanced and then add it there.