Strange query grouping in sqlite3 - sql

Question

I have a table like this:
CREATE TABLE prices2
(
id_price integer primary key,
date text,
value real
);
And let's say I have this values:
insert into prices2( '01/01/2017', 1 );
insert into prices2( '02/01/2017', 2 );
insert into prices2( '02/01/2017', 3 );
insert into prices2( '04/01/2017', 7 );
insert into prices2( '04/01/2017', 6 );
I wonder why this select is correct and returns some results, and what is returning:
select date,
count(*)
from prices2
where date
group by date like '%/01/2017';
Maybe "date like "%/01/2017" is a boolean expression that returns some kind of value???

Answer 1

date like '%/01/2017' indeed is a boolean expression; in SQLite, it returns either 0 or 1:
> select date, date like '%/01/2017' from prices2;
01/01/2017|1
02/01/2017|1
02/01/2017|1
04/01/2017|1
04/01/2017|1
So when you use this expression in the GROUP BY clause, you get one group for 1 values, and one group each for 0 and NULL values (but there are none in this example).
As for where date, the documentation says:
The SQL language features several contexts where an expression is evaluated and the result converted to a boolean (true or false) value. These contexts are:
the WHERE clause of a SELECT, UPDATE or DELETE statement,
[...]
To convert the results of an SQL expression to a boolean value, SQLite first casts the result to a NUMERIC value in the same way as a CAST expression. A numeric zero value (integer value 0 or real value 0.0) is considered to be false. A NULL value is still NULL. All other values are considered true.
The date values begin with a number that is not zero, so they are interpreted as true.