How to use a variable AS a where clause? - sql

Question

I have one where clause which I have to use multiple times. I am quite new to Oracle SQL, so please forgive me for my newbe mistakes :). I have read this website, but could not find the answer :(. Here's the SQL statement:
var condition varchar2(100)
exec :condition := 'column 1 = 1 AND column2 = 2, etc.'
Select a.content, b.content
from
(Select (DBMS_LOB.SUBSTR(ost_bama_vrij_veld.inhoud,3)) as content
from table_name
where category = X AND :condition
group by (DBMS_LOB.SUBSTR(ost_bama_vrij_veld.inhoud,3))
) A
,
(Select (DBMS_LOB.SUBSTR(ost_bama_vrij_veld.inhoud,100)) as content
from table_name
where category = Y AND :condition
group by (DBMS_LOB.SUBSTR(ost_bama_vrij_veld.inhoud,100))) B
GROUP BY
a.content, b.content
The content field is a CLOB field and unfortunately all values needed are in the same column. My query does not work ofcourse.

Answer 1

You can't use a bind variable for that much of a where clause, only for specific values. You could use a substitution variable if you're running this in SQL*Plus or SQL Developer (and maybe some other clients):
define condition = 'column 1 = 1 AND column2 = 2, etc.'
Select a.content, b.content
from
(Select (DBMS_LOB.SUBSTR(ost_bama_vrij_veld.inhoud,3)) as content
from table_name
where category = X AND &condition
...
From other places, including JDBC and OCI, you'd need to have the condition as a variable and build the query string using that, so it's repeated in the code that the parser sees. From PL/SQL you could use dynamic SQL to achieve the same thing. I'm not sure why just repeating the conditions is a problem though, binding arguments if values are going to change. Certainly with two clauses like this it seems a bit pointless.
But maybe you could approach this from a different angle and remove the need to repeat the where clause. Querying the table twice might not be efficient anyway. You could apply your condition once as a subquery, but without knowing your indexes or the selectivity of the conditions this could be worse:
with sub_table as (
select category, content
from my_table
where category in (X, Y)
and column 1 = 1 AND column2 = 2, etc.
)
Select a.content, b.content
from
(Select (DBMS_LOB.SUBSTR(ost_bama_vrij_veld.inhoud,3)) as content
from sub_table
where category = X
group by (DBMS_LOB.SUBSTR(ost_bama_vrij_veld.inhoud,3))
) A
,
(Select (DBMS_LOB.SUBSTR(ost_bama_vrij_veld.inhoud,100)) as content
from sub_table
where category = Y
group by (DBMS_LOB.SUBSTR(ost_bama_vrij_veld.inhoud,100))) B
GROUP BY
a.content, b.content
I'm not sure what the grouping is for - to eliminate duplicates? This only really makes sense if you have a single X and Y record matching the other conditions, doesn't it? Maybe I'm not following it properly.
You could also use a case statement:
select max(content_x), max(content_y)
from (
select
case when category = X
then DBMS_LOB.SUBSTR(ost_bama_vrij_veld.inhoud,3) end as content_x,
case when category = Y
then DBMS_LOB.SUBSTR(ost_bama_vrij_veld.inhoud,100) end as content_y,
from my_table
where category in (X, Y)
and column 1 = 1 AND column2 = 2, etc.
)