calculate standard deviation inside pivot table

Question

I have a pivot query that looks like below, getting the count and AVG of rows that works just fine, but not standard deviation, STD .

How may I modify the SQL below to get the STD ?

SELECT mid                             as mID,
   round((x.qty_sum / x.qty_count), 5) as qtAVG,
   round(x.qty_stddev, 5)              as qtSTDDEV,
   x.qty_count                         as qtCOUNT,
   round((x.rel_sum / x.rel_count), 5) as relAVG,
   round(x.rel_stddev, 5)              as relSTDDEV,
   x.rel_count                         as relCOUNT,
FROM (SELECT mid,
         SUM(CASE WHEN (mt = "qt") THEN 1 ELSE 0 END)   as qty_count,
         SUM(CASE WHEN (mt = "qt") THEN rt ELSE 0 END)  as qty_sum,
         STD(CASE WHEN (mt = "qt") THEN rt ELSE 0 END)  as qty_stddev
         SUM(CASE WHEN (mt = "rel") THEN 1 ELSE 0 END)  as rel_count,
         SUM(CASE WHEN (mt = "rel") THEN rel ELSE 0 END) as rel_sum,
         STD(CASE WHEN (mt = "rel") THEN rel ELSE 0 END) as rel_stddev
  FROM t_r
  GROUP BY mid) x;

Answer 1

I think your only problem is the ELSE 0 . You simply want NULL values, because they will be ignored:

SELECT mid                                 as mID,
       round((x.qty_sum / x.qty_count), 5) as qtAVG,
       round(x.qty_stddev, 5)              as qtSTDDEV,
       x.qty_count                         as qtCOUNT,
       round((x.rel_sum / x.rel_count), 5) as relAVG,
       round(x.rel_stddev, 5)              as relSTDDEV,
       x.rel_count                         as relCOUNT,
FROM (SELECT mid,
             SUM( mt = 'qt' )   as qty_count,
             SUM(CASE WHEN mt = 'qt' THEN rt END)  as qty_sum,
             STD(CASE WHEN mt = 'qt' THEN rt END)  as qty_stddev,
             SUM( mt = 'rel' ) as rel_count,
             SUM(CASE WHEN mt = 'rel' THEN rel END) as rel_sum,
             STD(CASE WHEN mt = 'rel' THEN rel END) as rel_stddev
      FROM t_r
      GROUP BY mid
     ) x;

Note certain other changes:

• I simplified the logic for the counts, to remove the CASE expression. This uses a MySQL extension that treats booleans as numbers with 1 for true and 0 for false.
• I replaced the double quotes with single quotes. Single quotes are the standard delimiter for strings.
• I removed the ELSE clauses. Aggregation functions ignore NULL values, so this should fix your problem.

Answer 2

Seems like you are trying to outsmart mysql with that subquery. As far as concerns, you don't need this additional level of complexity, just use a simple aggregated query with a WHERE clause that filters records having mt = "qt".

SELECT 
    mid as mID,
    ROUND(AVG(rt), 5) as qtAVG,
    ROUND(STD(rt), 5) as qtSTDDEV,
    COUNT(*) as qtCOUNT,
FROM t_r
WHERE mt =  "qt"
GROUP BY mt