Shorthand CASE with additional conditions - sql

Question

Up until today I held as true the notion that shorthand CASE doesn't accept additional conditions, as in
CASE evaluate_this
WHEN TRUE AND another_field ... THEN ...
would never work.
Today, as I was distractingly writing some code, I violated this rule just to realise that one CASE expression still worked as intended, the other didn't.
Assuming the following data:
id
boolean_one
boolean_two
field_one
field_two
1
TRUE
TRUE
NULL
'a'
2
TRUE
FALSE
2
'a'
3
FALSE
TRUE
8
NULL
4
FALSE
FALSE
NULL
NULL
The one that did not work looked like this:
CASE boolean_one
WHEN TRUE AND field_one IS NOT NULL THEN field_one * UNIFORM(1, 10, RANDOM())
ELSE field_one
END AS new_field_one
The idea is that in most circumstances the value for new_field_one should match field_one, the only exception being when boolean_one = TRUE AND field_one IS NOT NULL in which case it should be assigned the product of field_one and a random integer.
Below the desired and actual values for new_field_one based on the above
id
desired
actual
1
NULL
NULL
2
20
20
3
8
8
4
NULL
5
So it applies the randomisation also when boolean_one = FALSE AND field_one IS NULL.
Fixing the expression to
CASE
WHEN boolean_one = TRUE AND field_one IS NOT NULL THEN...
solves the issue. What I find more interesting, and confusing, is that the second expression still did what it was meant to do:
CASE boolean_two
WHEN TRUE AND field_two = 'b' THEN 'a'
WHEN TRUE AND field_two = 'a' THEN 'b'
ELSE field_two
END AS new_field_two
which in this case means that new_field_two = 'b' for the row with ID = 1, and all other remain equal to field_two.
So, this doesn't seem to be an issue of adding an extra condition to a shorthand CASE per se, I would guess it must have something to do specifically with the fact that the second condition is a NULL check. Am I on the right track?

Answer 1

With the shortcut syntax the "when expression" is evaluated as a whole and then that value is compared against the "case expression". It's not matching the value as true and then running an extra check
In fact your true and is essentially ignored because true and X is just equivalent to X in Boolean logic. The test then is whether boolean_one matches the non-null state of field_one. The second and fourth rows correspond to true = true and false = false which is when you're seeing the random result values.
You could certainly nest case if there were a real benefit to legibility:
case boolean_one
when true then
case field_one is not null ... end
else field_one
end

Answer 2

Shawnt00 answer is good,
I like to think of the two forms as
chained If expression
CASE
WHEN check1 THEN r1
WHEN check2 THEN r2
WHEN check3 THEN r3
END
if the same as
IF(check1)
r1
ELSE IF (check2)
r2
...
where-as this "shortcut" version is the Switch statement form:
CASE expression
WHEN val1 THEN r1
WHEN val2 THEN r2
WHEN val3 THEN r2
END
is the same as
SWITCH(expression){
CASE val1: r1
CASE val2: r2
CASE val3: r3
}
which is to say the VAL things need to evaluate down, thus for you stumbled upon cases, you are writing extra boolean logic, which resolves down to a boolean answer. And as the doc's note, for a expression to match a value they need to be of the same type of can be auto converted.
thus this gives a SQL error:
select
case false
when true then 1
when 'bob' then 2
end;
Boolean value 'bob' is not recognized
because the second value cannot be auto converted to the same type as the case expression, which is a boolean value.