how to keep the mdp_matrix not update if the t_src_sales_tmpl is null

We did not to update the sales_data and mdp_matrix records if the t_src_sales_tmpl columns is null

for example,in the table,we have columnsA and columnsB,and it comes from the columnsA1 and columnsB1 in the  t_src_sales_tmpl,

before update

table	columnsA	columnsB
mdp_matrix	a	b

table	columnsA1	columnsB1
t_src_sales_tmpl	a1

update the t_src_sales_tmpl value into the mdp_matrix,then the mdp_matrix update like this:

table	columnsA	columnsB
mdp_matrix	a1

but we need the filed columnsB in the mdp_matrix should not update,we need the result like this: