Trigger Row Level :OLD and :NEW

Row level triggers can access new and existing values of columns using the ":NEW.column-name" and ":OLD.column-name" 

references, bearing in mind the following restrictions.

Row-level INSERT triggers : Only ":NEW" references are possible as there is no existing row.

Row-level UPDATE triggers : Both ":NEW" and ":OLD" references are possible. ":NEW" represents the new value presented in the 

DML statement that caused the trigger to fire. ":OLD" represents the existing value in the column, prior to the update being 

applied.

Row-level DELETE triggers : Only ":OLD" references are possible as there is no new data presented in the triggering 

statement, just the existing row that is to be deleted.

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old value in column nUsablepledgestock=100

operation =100+100

new value in column nUsablepledgestock after update (stored)=200

then

:old.nUsablepledgestock=100

:new.nUsablepledgestock=200

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