How to Declare, Populate and Use a Variable using Oracle PL/SQL - oracle

Question

I swear this has been asked so many times previously, yet I cannot seem to apply other examples to my use case:
First things first, this query will be executed as part of an Informatica SQL Source Qualifier, and in some circumstances, be passed-through from an SQL Server OpenQuery statement, so please be mindful of this, and
that SQL Plus will not be used be used; Oracle SQL Developer is used only for code development.
My history is primarily SQL Server & Teradata, but as the title suggests, I now have a requirement where I need to declare, populate and use a variable in Oracle, all within the same procedure. Not SP, so no In/Out declarations
In SQL Server, this code will work as expected (line numbers added for clarity):
1. Declare #MaxDate Int
2.
3. With f_data (cal_period) As (Select 201904 As cal_period)
4.
5. Select #MaxDate = Max(cal_period) From f_data
6.
7. Select
8. Case
9. When (#MaxDate%100) < 12 Then #MaxDate+1
10. Else (#MaxDate+100) - ((#MaxDate%100)-1)
11. End As dt
Line 1: These are YYYYMM date periods defined as int
Line 3: I am using an inline view (CTE) here for illustration, and to
make it easier for you to copy and paste, but in reality, this is
actually a physical control table, so would not normally be visible in the
script.
Line 5: Populates the parameter (SQL Server prefixes parameters with
the at symbol) with the single-value resultset
Line 7-11: Is simply the logic to progress the period by one, the
percentage-mark in SQL Server is the Modulus function, Oracle is
written as Mod(#MaxDate,100)
For those unfamiliar with SQL Server, it does not need a reference table such as Dual ("Sys.Dual") in order to execute the query, such that for Oracle a "From Dual" statement is necessary on the missing Line 12
My requirement is essentially a carbon-copy of the above T-SQL, so I need to declare a one-time use variable, to populate that variable with the results of an SQL query, and then to use this variable in a transformation - the result of which is captured to an Informatica and SSIS variable for later use.
So far, I have tried declaring a variable, this seemed to work (by which I mean it didn't return an error):
Declare MaxPeriod Int;
Begin
Select 201904 Into MaxPeriod From Dual;
End;
And populating from an SQL statement is also showing as successfully completed:
Declare MaxPeriod Int;
Begin
Select Max(MaxPeriodVal) Into MaxPeriod From CtrlTable;
End;
Although I can't seem to get beyond this to actually test the variable as put.line statements fail, as do simple Case checks:
Declare MaxPeriod Int;
Begin
Select 201904 Into MaxPeriod From Dual;
End;
Select
Case
When 201904 = MaxPeriod Then 'Match'
Else 'No Match'
End As dteChk
From Dual;
I have attempted to prefix the MaxPeriod in the check with a colon, and, to have prefixed,suffixed/both with an ampersand eg :MaxPeriod; &MaxPeriod; MaxPeriod&; &MaxPeriod&
All of which failed.

Answer 1

The basic issue is a variable scope problem. You're declaring MaxPeriod within the context of a PL/SQL anonymous block, so it will disappear (fall out of scope) when the block ends on line 4.
You could put your entire query inside the PL/SQL block, but there's not an easy way to return an entire result set from a PL/SQL block, so I don't think you want that.
I don't know how your Oracle driver handles native queries, but this might work:
var MaxPeriod number; -- bind variable declared as global scope for this script
Begin -- one of several ways to assign values to bind variables
:MaxPeriod := 201904;
End;
/
Select
Case
When 201904 = :MaxPeriod Then 'Match'
Else 'No Match'
End As dteChk
From Dual;
If the var syntax doesn't work for you to declare a SQL bind variable, then you may have to look into some other way of passing a bind variable for the query string. You could probably pass a null value (for a number datatype, anyway) and then overwrite it in the SQL script.
Alternately, in your original example code, I think I'd use a CTE or an inline view instead of a variable anyway.
With f_data As (Select 201904 As cal_period from dual)
Select
Case
When Mod(MaxDate,100) < 12 Then MaxDate+1
Else (MaxDate+100) - (Mod(MaxDate,100)-1)
End As dt
from (Select Max(cal_period) as MaxDate From f_data) mp

Answer 2

You can use substitution variable using define in sql*plus as following.
Define MaxPeriod := 201904
Select
Case
When &MaxPeriod = MaxPeriod Then 'Match'
Else 'No Match'
End As dteChk
From Dual;
Cheers!!