将计算列插入现有表

Question

I have a column with a string of text inside of it that varies in length. I want to return the first 4 characters in the string starting from position 1 and output them into a table only if there is a direct match. So if I want to return just PDFG and not return ADHR or any other combination then the below code works just fine for me.

use DB
select 
substring(description, 1, 4) as newcol from table1 where substring(description, 1, 4) like '%ABCD%'

however I would like to persist this calculated column into an existing table, so something like this;

use DB
alter table table1
add newcol as ("and then the rest of the code above")

I am not sure how to reorder my code to fit the new query any help is appreciated.

Here is some sample data:

• PDFG_2013 AHSDHDF
• ADHR_2310 ADGDGEE
• DATW_5142 NFBSAEE

The output from this should be stored in newcol within an existing table called table1. The only value in new column from the sample data should be PDFG

Answer 1

Use a case expression to return the first 4 characters if matching, otherwise null.

Alter table table1
add newcol as case when description like 'ABCD%' then substring(description, 1, 4) end

Or, even simplier

Alter table table1
add newcol as case when description like 'ABCD%' then 'ABCD' end