Help required for update SQL

Question

Hi Guys,
I am looking for some help to frame the update query.
I have two tables temp_1 and temp_2.
I want to update temp_2 based on data from temp_1. However i do not want to make an update where temp_1 has null value.
One approach i can think of is creating two different update statements but is there any chance to get it done through
only one statement. Also i would like to add here that temP_1 has 250K data and i going to make update on approx 500K
rows from temp_2. So i need to look from performance point of view as well.
Temp_1                       temp_2
Col1   Col2  Col3        col1     col2    col3
1 abc   abc                  1        null    null
2 abc   null                  2         xyz     xyz
3 null  null                   3          xyz     null
After update i am looking forward to get data in below format.
temp_2
col1     col2    col3
1        abc     abc
2        abc     xyz
3        xyz     null  

Answer 1

USE TESTdb
go
create table express (col1 int, col2 varchar(25), col3 varchar(25))
go
create table express2 (col1 int, col2 varchar(25), col3 varchar(25))
go
insert into express values (1,'abc','abc'),(2,'abc',null),(3,null,null)
insert into express2 values (1,null,null),(2,'xyz','xyz'),(3,'xyz',null)
select * from express
select * from express2
update express2
set col2= case when x.col2 is null and e.col2 is not null then e.col2
else x.col2
end ,
col3= case when x.col3 is null and e.col3 is not null then e.col3
else x.col3
end
from express e inner join express2 x on e.col1=x.col1
--drop table express, express2
ebro