Question

I came across a function in sqlite that I need to convert to postgres syntax. It’s date(date, '-'||strftime('%w',date)||' days').

Can anyone help me convert this sqlite segment to postgres?

Answer 1

This SQLite:

date(date, '-' || strftime('%w', date) || ' days')

Is, AFAIK, subtracting the day-in-the-week number days (i.e. 0 for Sunday, 1 for Monday, …) from date and then converting the result back to a date; see the date function reference for details.

I think the equivalent for PostgreSQL would be:

d - extract(dow from d)::int

where d is your date; subtracting an integer from a date subtracts that number of days. If d is a timestamp, then you might need to add some casting. There is date_trunc('week', 'd') as well but that starts numbering the days from Monday so you’d be off by one with that.

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Here’s a quick breakdown of the SQLite with the date variable replaced by d to avoid confusion with the date() function:

date(d, '-' || strftime('%w', d) || ' days')

First of all, || is the standard SQL string concatenation operator. The strftime function is a general purpose date and time formatter that comes from POSIX; the %w format specifier means “day of the week as a number with Sunday being day zero”; so the strftime call gives you 0 for Sunday, 1 for Monday, and so on up to 6 for Saturday. If d is a Tuesday, then the strftime call will yield 2 and the whole thing ends up as:

date(d, '-2 days')

The modifiers for the SQLite date function have various forms but '-2 days' means just what you’d think: subtract two days from d. The overall result is that you get d truncated to the week (where Sunday is considered to be the first day of the week).

On the PostgreSQL side:

d - extract(dow from d)::int

we can start with extract; extract is used to extract specific parts of a date or time and dow means “day of the week as a number with Sunday being day zero”. Sound familiar? Then the ::int casts the DOW number to an integer and is needed because the DOW actually comes out as a double precision value and there is no operator defined for subtracting a double from a date in PostgreSQL; the cast can also be written in the standard form as cast(x as int). When you subtract an integer from a date in PostgreSQL, you subtract that many days; you can be more explicit by saying things like - interval '3 days' but that would just add more noise in this case so I opted for simplicity. If it is Tuesday, then our PostgreSQL version looks like:

d - 2

and that’s the same as:

d - interval '2 days'

And after the subtraction we’d be back on Sunday. There’s also date_trunc in PostgreSQL but that would truncate to Monday not Sunday.