表名指定两次，既作为更新的目标，又作为单独的数据源

Question

I am receiving an expected error from my code whenever I attempt to update my table using a select statement although I am trying different workarounds with no hopes to success. Any help with my issue would be highly appreciated. Thank you.

UPDATE `table`
SET Slope = CASE

when (SELECT ((SELECT (SELECT IntColumn FROM `table` ORDER by DateColumn desc limit 1) - (SELECT IntColumn FROM `table` ORDER by DateColumn ASC limit 1) FROM DUAL)) as x) > 0 THEN 'Uptrend' 

when (SELECT ((SELECT (SELECT IntColumn FROM `table` ORDER by DateColumn desc limit 1) - (SELECT IntColumn FROM `table` ORDER by DateColumn ASC limit 1) FROM DUAL)) AS x) < 0 THEN 'Downtrend' 

when (SELECT ((SELECT (SELECT IntColumn FROM `table` ORDER by DateColumn desc limit 1) - (SELECT dialcount FROM `table` ORDER by DateColumn ASC limit 1) FROM DUAL)) AS x) = 0 THEN 'Notrend' 

else 'unknown' END

UPDATE `table`
SET Slope = CASE
when ((SELECT (SELECT IntColumn FROM `table` ORDER by DateColumn desc limit 1) - (SELECT IntColumn FROM `table` ORDER by DateColumn ASC limit 1) FROM DUAL)) > 0 THEN 'Uptrend' 
when ((SELECT (SELECT IntColumn FROM `table` ORDER by DateColumn desc limit 1) - (SELECT IntColumn FROM `table` ORDER by DateColumn ASC limit 1) FROM DUAL)) < 0 THEN 'Downtrend' 
when ((SELECT (SELECT IntColumn FROM `table` ORDER by DateColumn desc limit 1) - (SELECT dialcount FROM `table` ORDER by DateColumn ASC limit 1) FROM DUAL)) = 0 THEN 'Notrend' 

else 'unknown' END

Answer 1

update table set Slope = case 
when ((SELECT IntColumn FROM `table` ORDER by DateColumn desc limit 1) - (SELECT IntColumn FROM `table` ORDER by DateColumn ASC limit 1))>0 then 'Uptrend'
when ((SELECT IntColumn FROM `table` ORDER by DateColumn desc limit 1) - (SELECT IntColumn FROM `table` ORDER by DateColumn ASC limit 1))<0 then 'Downtrend'
when ((SELECT IntColumn FROM `table` ORDER by DateColumn desc limit 1) - (SELECT IntColumn FROM `table` ORDER by DateColumn ASC limit 1))=0 then 'Notrend'
else 'unknown' end

Answer 2

I finally got it working as following:

UPDATE `table`
SET Slope = CASE
when ((SELECT(SELECT (SELECT intcolumn FROM (SELECT intcolumn FROM `table` ORDER by DateColumn desc limit 1) as lastdc) - (SELECT intcolumn FROM (SELECT intcolumn FROM `table` ORDER by DateColumn ASC limit 1) as firstdc) FROM DUAL) as dff)) > 0 THEN 'Uptrend' 
when ((SELECT(SELECT (SELECT intcolumn FROM (SELECT intcolumn FROM `table` ORDER by DateColumn desc limit 1) as lastdc) - (SELECT intcolumn FROM (SELECT intcolumn FROM `table` ORDER by DateColumn ASC limit 1) as firstdc) FROM DUAL) as dff)) < 0 THEN 'Downtrend' 
when ((SELECT(SELECT (SELECT intcolumn FROM (SELECT intcolumn FROM `table` ORDER by DateColumn desc limit 1) as lastdc) - (SELECT intcolumn FROM (SELECT intcolumn FROM `table` ORDER by DateColumn ASC limit 1) as firstdc) FROM DUAL) as dff)) = 0 THEN 'Notrend' 
else 'unknown' END

Answer 3

If you just want a query to calculate the slope, something like this should work:

SELECT CASE WHEN lastInt > firstInt THEN 'Uptrend'
    WHEN lastInt > firstInt THEN 'Downtrend'
    WHEN lastInt > firstInt THEN 'Notrend'
    END AS `slope`
FROM (
    SELECT (SELECT IntColumn FROM `table` ORDER BY DateColumn DESC LIMIT 1) AS lastInt
        , (SELECT IntColumn FROM `table` ORDER BYDateColumn ASC LIMIT 1) AS firstInt
    ) AS boundInts
;