Oracle null values are converted into actual values. The first reaction is to use nvl. However, there is another function that can be used: coalesce(col1,col2,col3,....), coalesce supports multiple parameters and can easily return the first A non-null value.
select
coalesce(col1,col2,col3,col4) from dual
Description: When col1 is empty, the value is col2, when col2 is empty, the value is col3, and when col3 is empty, the value is col4
Equivalent to
select nvl(nvl(nvl(col1,col2),col3),col4) from dual
The following table:
select coalesce(c1,c2,c3,c4) from mh ;
The result is as follows: