MySQL CASE statement returning null - mysql

Question

I have two 'identical' operations
(1)
SELECT ticket_type_name
, amount as 'original_amount'
, CASE ticket_type_name
WHEN ticket_type_name = 'General Admission - Friday'
and amount > 0 THEN amount / 400
END AS 'NewAmount'
from analytics.deleteme
(2)
select distinct ticket_type_name, amount, amount/400 AS NewAmount
from analytics.deleteme
where amount > 0
and ticket_type_name = 'General Admission - Friday'
The first statement generates a value of NULL
The second statement generates a correct value of 1 (400/400)
I have added an ELSE statement at the end:
SELECT DISTINCT ticket_type_name
, amount as 'original_amount'
, CASE ticket_type_name
WHEN ticket_type_name = 'General Admission - Friday' and amount > 0 THEN amount / 400
else amount/575
END AS 'NewAmount'
from analytics.deleteme
where amount > 0
Results are the reverse of what they should be!
GA Friday should be 400/400 = 1
Everything else, eg GA Weekend should be divided by 575 (=0.82, not 1.18!)
ticket_type_name, amount, NewAmount
General Admission - Friday 400.0 0.69
General Admission - Weekend 475.0 1.18
General Admission - Saturday 375.0 0.93
Children (12 to 14 Years) - Weekend 100.0 0.25
Children (12 to 14 Years) - Saturday 50.0 0.125

Answer 1

I could replicate your bug.
Just remove ticket_type_name after CASE and it works:
SELECT
DISTINCT ticket_type_name,
amount AS 'original_amount',
CASE
WHEN
ticket_type_name = 'General Admission - Friday'
AND amount > 0
THEN
amount / 400
ELSE
amount / 575
END AS 'NewAmount'
FROM analytics.deleteme
WHERE amount > 0;
You don't need to put anything between CASE and WHEN but it's funny how this broke the query. The first comment below perfectly explains why.
As an alternative, you may move ticket_type_name after CASE and put the comparison value after WHEN. Like this:
SELECT
DISTINCT ticket_type_name,
amount AS 'original_amount',
CASE ticket_type_name
WHEN
'General Admission - Friday'
THEN
amount / 400
ELSE
amount / 575
END AS 'NewAmount'
FROM analytics.deleteme
WHERE amount > 0;
Long story short: either use CASE field WHEN value… or CASE WHEN condition…. You don't want to use CASE field WHEN condition….