Same query but different tables - sql

Question

I'm faced with a big query that is generated in a string and executed with "OPEN pCursor FOR vQuery" and I'm trying to get the query out of the string variable and as a proper "compilable" query.
I'm having this problem where a different table is query depending on a variable
vQuery := 'SELECT ...';
IF pVar = 1 Then
vQuery := vQuery || ' FROM table1';
ELSE
vQuery := vQuery || ' FROM table2';
END IF
vQuery := vQuery || ' WHERE ...';
The two tables have pretty much the same column name. Is there a way to have this as a single query
OPEN Pcursorout FOR
SELECT ... FROM CASE WHEN pVar = 1 THEN table1 ELSE table1 END WHERE ...;
Or I'm stuck at having two queries?
IF pVar = 1 Then
OPEN Pcursorout FOR SELECT ... FROM table1 WHERE ...;
ELSE
OPEN Pcursorout FOR SELECT ... FROM table2 WHERE ...;
END IF
The select and where part are large and exactly the same for both table.

Answer 1

You could use a UNION and use your variable pVar to only include the results from one query in the result set.
SELECT t1.col1, t1.col2, ..., t1.col10
FROM table1 t1
WHERE pVar = 1 and ...
UNION
SELECT t2.col1, t2.col2, ..., t2.col10
FROM table1 t2
WHERE pVar <> 1 and ...
This isn't exactly what you asked about -- not being required to have duplicate lists of columns for the two select statements -- but I think it might capture your intent. It will require that the columns selected by both queries have the same datatype so there will be a (somewhat weak) constraint that the columns of both query results are the same. For example, you won't be able to add a new column to one query but not the other.

Answer 2

Perhaps using UNION / UNION ALL to unite both queries? The requirement for using UNION/UNION ALL is that all SELECTs being united must return columns with the same names.
So if you have
SELECT t.f1,
t.f2,
t.f3
FROM t
WHERE ...
and your other query is
SELECT q.f1,
q.f2,
q.f3
FROM q
WHERE ...
you can have both running as a single SQL statement with UNION:
SELECT t.f1,
t.f2,
t.f3
FROM t
WHERE ...
UNION
SELECT q.f1,
q.f2,
q.f3
FROM q
WHERE ...
Keep in mind that if you need to return columns that exist in one table but not in the other, you can still use UNION, just return NULL and name the column correspondingly to the column name in the table that has it.

Answer 3

Its a bit of a kludge and you might need to look at the performance impact, but you could use an inline view that unions the two base tables, with a flag on each part that you then compare to your variable
SELECT ...
FROM (
SELECT 1 as var, table1.*
FROM table1
UNION ALL
SELECT 2 as var, table2.*
FROM table2
) t
WHERE t.var = pVar
AND ...;
Using an inline view means you don't have to duplicate the main select-list or the where clause etc. If the tables have different columns then you can (and maybe should anyway) only select the columns in the inner queries that will be referenced in the outer select-list.