is it possible to use case statement + round() in database - mysql

Question

i have table called 'test' and want to calculate based on different codes, most of them should be saved in 5 decimals except certain code, like containing jpy in 3 decimals, and xua in 2 decimals
create table test(
id int, ymd date,
code varchar(10),
price int
)
insert into test(id, ymd, code, price) values
(1, '2019-01-01', 'auus', 75125),
(2, '2019-01-02', 'nzus', 68541),
(3, '2019-01-03', 'xuaus', 131485),
(4, '2019-01-04', 'aujp', 77852),
(5, '2019-01-05', 'usjp', 110852),
(6, '2019-01-06', 'xuaus', 131091)
So my execute code is:
select id, ymd, code, price,
case
when code like '%xua%' then round(price/100,2)
when code like '%jp%' then round(price/1000,3)
else round(price/100000,5)
end as t
from test
ideal result:
id ymd code price t
1 2019-01-01 auus 75125 0.75125
2 2019-01-02 nzus 68541 0.68541
3 2019-01-03 xuaus 131485 1314.85
4 2019-01-04 aujp 77852 77.852
5 2019-01-05 usjp 110852 110.852
6 2019-01-06 xuaus 131091 1310.91
intersting, above sql works well with Mysql, but i am using mariadb, and just can't get results as same as mysql, spent 2 days to fix problem, but still don't know, please help

Answer 1

mysql> select id, ymd, code, price,
-> case
-> when code like '%xua%' then round(price/100,2)
-> when code like '%jp%' then round(price/1000,3)
-> else round(price/100000,5)
-> end as t
-> from test ;
+------+------------+-------+--------+------------+
| id | ymd | code | price | t |
+------+------------+-------+--------+------------+
| 1 | 2019-01-01 | auus | 75125 | 0.75125 |
| 2 | 2019-01-02 | nzus | 68541 | 0.68541 |
| 3 | 2019-01-03 | xuaus | 131485 | 1314.85000 |
| 4 | 2019-01-04 | aujp | 77852 | 77.85200 |
| 5 | 2019-01-05 | usjp | 110852 | 110.85200 |
| 6 | 2019-01-06 | xuaus | 131091 | 1310.91000 |
+------+------------+-------+--------+------------+
6 rows in set (0.04 sec)
Using FORMAT instead of ROUND:
mysql> select id, ymd, code, price,
case when code like '%xua%' then format(price/100,2)
when code like '%jp%' then format(price/1000,3)
else format(price/100000,5) end as t from test;
+------+------------+-------+--------+----------+
| id | ymd | code | price | t |
+------+------------+-------+--------+----------+
| 1 | 2019-01-01 | auus | 75125 | 0.75125 |
| 2 | 2019-01-02 | nzus | 68541 | 0.68541 |
| 3 | 2019-01-03 | xuaus | 131485 | 1,314.85 |
| 4 | 2019-01-04 | aujp | 77852 | 77.852 |
| 5 | 2019-01-05 | usjp | 110852 | 110.852 |
| 6 | 2019-01-06 | xuaus | 131091 | 1,310.91 |
+------+------------+-------+--------+----------+
6 rows in set (0.00 sec)
mysql> select ##version;
+----------------------------------------+
| ##version |
+----------------------------------------+
| 10.3.11-MariaDB-1:10.3.11+maria~bionic |
+----------------------------------------+
1 row in set (0.00 sec)
Note that it includes a "thousands-separator" when appropriate. See the 3rd argument to FORMAT() or the Locale setting to change that.
I suspect it is the display process that is formatting the output differently. By switching from ROUND to FORMAT, I managed to get MariaDB's output to be nearly the same as MySQL's. The remaining difference is the added commas ("thousands separators"), which may show as '.' for some Locales.
In contrast, for MySQL 5.6.22:
+------+------------+-------+--------+---------+
| id | ymd | code | price | t |
+------+------------+-------+--------+---------+
| 1 | 2019-01-01 | auus | 75125 | 0.75125 |
| 2 | 2019-01-02 | nzus | 68541 | 0.68541 |
| 3 | 2019-01-03 | xuaus | 131485 | 1314.85 |
| 4 | 2019-01-04 | aujp | 77852 | 77.852 |
| 5 | 2019-01-05 | usjp | 110852 | 110.852 |
| 6 | 2019-01-06 | xuaus | 131091 | 1310.91 |
+------+------------+-------+--------+---------+
The numeric values are the same, but the display is different. The difference seems to come from the commandline tool mysql, not from ROUND, itself. Note that t is right-justified, implying that the values are seen as numeric.
If this offends someone enough, file a bug report with MariaDB.
77.75200000000001 -- This is representative of some intermediate computation using DOUBLE instead of all DECIMAL. MySQL (and MariaDB) do a reasonably good job of second-guessing where the number are headed. And usually they get away with whatever is done.
In DOUBLE, 77.75200000000001 is not exactly equal to the DECIMAL 77.752 because one is binary, one is decimal. For this reason, I often recommend not using FLOAT or DOUBLE for "money".
Assuming your real goal is to represent a monetary value as 77.7520000000000000000000000..., that is exactly '77.752', and, assuming you need at most 5 decimal places for the various values, I recommend you do this:
t DECIMAL(m, 5)
where m is a suitably large number for any values you may eventually have. For the numbers given, (9,5) will suffice, but I suspect you should do more like DECIMAL(14,5) to allow for a billion dollars/euros/yen/etc.
What I don't know is where in the processing DOUBLE crept in.
Latest 'advice'
Use DECIMAL(14,5) for all monetary values in your system, not INT.
14,5 lets you get up to a billion 'dollars'; change that as needed for your expected max value.
Ignore my comments about FORMAT(); it seems to be too confusing.
Get rid of the CASE clause, at least for that particular usage.
Most arithmetic among DECIMAL values will be exact, and not encounter 77.75200000000001. If it crops up again, start a new Question and include all the steps, datatypes, etc, involved in the computation.
The above notes refer to storing and computing. For displaying, please specify the requirements:
Plan A: 5 decimal places is OK.
Plan B: need to round to 3 or 2 decimals for some values.
Plan C: You have application code, not in SQL, that can deal with the issue.
Plan D:...