Sqlite comes with a bundle of built-in functions including aggregate functions such as count()
, sum()
,^{1} or avg()
. No function to compute standard deviation, though. There are ways to compute the variance in a query directly, and its square root, but hopefully sqlite3 comes with a set of extensions that can be loaded directly into sqlite command. See also miscellaneous function.
The following assumes that you already downloaded the extension-functions.c
from the link above file and compiled it as shared library. On Linux, this is as easy as:
% gcc -g -fPIC -shared extension-functions.c -o extension-functions.so
Here is a toy example:
sqlite> .load ./extension-functions
sqlite> create table data(x real not null);
sqlite> insert into data select (abs(random() % 10)) as x from generate_series(1, 20);
sqlite> select round(stdev(x), 2) as sd from data;
sd
----
2.56
Note that generate_series()
is already loaded in the command-line shell.
How about writing a custom function, e.g. a percentile()
function which would return the percentile rank of an observation in a series of values? There’s a two-part series on this topic, which explains the basics: Writing a Custom SQLite Function (in C). Such functions are known as Run-Time Loadable Extensions, and there’s some boilerplate C code to get started.
If you are in a hurry, though, sqlean provides a bunch of domain-specific functions.^{2} In the case of statistical functions, you will find the median, various pre-defined percentile as well as a percentile rank function, Pearson correlation coefficient, as well as sample and population estimate of the variance or the standard deviation, and many more. Installation is quite simple since there are binaries available for each OS.
sqlite> .load ./stats
sqlite> create table data(x real not null);
sqlite> insert into data select (abs(random() % 10)) as x from generate_series(1, 20);
sqlite> select avg(x) as avg from data;
avg
----
4.55
sqlite> select percentile(x, 25) as p25, percentile(x, 50) as p50, percentile(x, 75) as p75 from data;
p25 p50 p75
--- --- ---
2.0 5.0 7.0
♪ Lauren Bush • In a Mellow Tone
There’s also a total()
function which behaves a little bit differently as it returns “0.0” in case the query returns only NULL values, while sum()
will return NULL like other SQL variants. ↩︎
See also SQLiteStudio which seems to provide a function editor. ↩︎