使用CASE WHEN语句（MySQL）

Question

Struggling with a case when statement (MySQL) and its probably something simple I am missing.

I have a fairly complex query which outputs some sensor values and low and high limits (that will eventually trip alarms), as part of that query I use -

case when sequence=0 then value END as lowlimit, 
case when sequence=1 then value END as highlimit

The output from the select is -

status  sensor  lowlimit    highlimit

163      49      NULL          24
163      49       15          NULL
128      25      NULL          8
128      25       4           NULL

but I need the results to be 'concatenated' (a single row for each unique status), thus -

status  sensor  lowlimit    highlimit

163       49      15           24
128       25      4             8

so probably need something adding to the case statements, but if I use

max(case when sequence=0 then value END) as lowlimit, 
max(case when sequence=1 then value END) as highlimit

then only 1 row is output -

 status sensor  lowlimit    highlimit

 163     49        15          24

any assistance appreciated.

Regards Active

Answer 1

If you need the result on the same row You should use a seleft join (not case)

select a.status, a.sensor, a.values as lowlimit, b.value as highlimit
from my_table a 
inner join (
    select status, sensor, value from my_table 
    where sequence = 1
  ) b on a.sensor = b.sensor and a.status = b.status
where a.sequence = 0