How to use alias column name in oracle query - sql

Question

I have a two table one is Mn_Fdd_Cong and another is test_t .Here i have to split a column value in table Mn_Fdd_Cong based on character underscore and take first part and join with second table. But in my query alias is not working
select regexp_substr(a.Ids, '[^_]+', 1, 1) as "Id" ,a.*,b.*
from Mn_Fdd_Cong a
left join test_t b on a.Id = b.Site_Id where b.G_VAL='U'

Answer 1

You can't use it that way; either extract "new" ID in a subquery (or a CTE) and then re-use it in "main" query, or - simpler - use it directly in JOIN operation.
I used simple substr + instr combination instead of regular expressions (performs better on large data sets).
select a.*, b.*
from md_fdd_cong a join test_t b on b.site_id = substr(a.ids, 1, instr(a.ids, '_') - 1)
where b.g_val = 'U'
Here's the 1st option I suggested (a CTE):
select x.*, b.*
from (select a.*,
substr(a.ids, 1, instr(a.ids, '_') - 1) id --> this is ID ...
from md_fdd_cong a
) x
join test_t b on b.site_id = x.id --> ... which is then used here (as "x.id")
where b.g_val = 'U';

Answer 2

You can't use an alias you define in your select in the join (logically, the join happens before the select). You could duplicate the calculation
select regexp_substr(a.Ids, '[^_]+', 1, 1) as "Id" ,a.*,b.*
from Mn_Fdd_Cong a
left join test_t b on regexp_substr(a.Ids, '[^_]+', 1, 1) = b.Site_Id
where b.G_VAL='U'
Or you could nest the calculation. Personally, I'd put the mn_fdd_cong manipulation into a common table expression (CTE). Note that if you put an identifier like Id in double-quotes, you're creating a case-sensitive identifier so subsequent references to it would need to use the same case-sensitive identifier and would need to use the double quotes.
with new_a as (
select regexp_substr(a.Ids, '[^_]+', 1, 1) as "Id" ,a.*
from Mn_Fdd_Cong a
)
select a.*, b.*
from new_a a
join test_t b on a."Id" = b.Site_Id
where b.G_VAL='U'