如何使用表連接優化此查詢？

Question

有人可以請幫我在優化以下查詢......

UPDATE tab1 SET col1 = 'NA' where 
col2 =(Select col1 from tab2 where col2 is null and 
col3 = (Select col1 from tab3 where col2 = 100)); 

Answer 1

update t1 
SET col1 = 'NA' 
from tab1 t1 
inner join tab2 t2 on t1.col2=t2.col1 and t2.col2 is null 
inner join tab3 t3 on t2.col3=t3.col1 and t3.col2 = 100