Re: MySQL Update query not giving expected result

[Peter Brawley]

To debug such an issue, create a select statement that lays out the values relevant to the update ...

select 
  a.Parsreels_Suiker, b.aangepas, a.datum,
  b.datumaangepas,  b.suiker_onder_aangepas,
  b.DatumAangepas2, b.Suiker_onder_Aangepas2, 
  suiker_onder,
  Case 
    When b.Aangepas = 'JA' AND a.Datum >= b.DatumAangepas THEN b.Suiker_onder_Aangepas  
    When b.Aangepas2 = 'JA' AND a.Datum >= b.DatumAangepas2 THEN b.Suiker_onder_Aangepas2  
    ELSE  Suiker_onder  
  End as UPDVAL
FROM penalisasietrash a 
JOIN parsreelstrash As b On a.Kult = b.Kultivar And a.KlasBeskrywing = b.Klasbeskrywing;

... then check the results of your query logic row by row .