MySQL 5.7

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select version();


version()


5.7.39


CREATE TABLE `sample` (
  `property` varchar(50) DEFAULT NULL,
  `devloc` varchar(50) DEFAULT NULL,
  `sensortype` varchar(50) DEFAULT NULL,
  `timest` datetime DEFAULT NULL,
  `value` decimal(8,4) NOT NULL,
  KEY `property` (`property`),
  KEY `devloc` (`devloc`),
  KEY `sensortype` (`sensortype`),
  KEY `timest` (`timest`),
  KEY `value` (`value`),
  KEY `timest_2` (`timest`,`devloc`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
select x.timest,     
    max(case when x.devloc='outside' then x.value end) as outside,     
    max(case when x.devloc='hvac_main_return' then x.value end) as hvac_main_return,     
    max(case when x.devloc='hvac_main_supply' then x.value end) as hvac_main_supply,
    ((max(case when x.devloc='hvac_main_return' then x.value end)) -     
      (max(case when x.devloc='hvac_main_supply' then x.value end))) AS diff
from sample x where date(timest) = curdate() 
group by timest     
order by timest desc;


timest
outside
hvac_main_return
hvac_main_supply
diff


SELECT timest,
       outside,
       hvac_main_return,
       hvac_main_supply,
       hvac_main_supply - hvac_main_return AS hvac_main_net
  (SELECT x.timest,
          max(CASE WHEN x.devloc='outside' THEN x.value END) AS outside,
          max(CASE WHEN x.devloc='hvac_main_return' THEN x.value END) AS hvac_main_return,
          max(CASE WHEN x.devloc='hvac_main_supply' THEN x.value END) AS hvac_main_supply
   FROM sample x
   WHERE date(timest) = curdate()
   GROUP BY timest) AS y
ORDER BY timest DESC;

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '(SELECT x.timest,
          max(CASE WHEN x.devloc='outside' THEN x.value END) A' at line 6