Dynamic select in Oracle - oracle

Question

I want to do something like this:
sql_str := 'select ';
if (user_input = 1) then
sql_str := sql_str || 'a.col1 from tb1 a';
else
sql_str := sql_str || 'a.col1, b.col2, b.col3 from tb1 a, tb2 b';
execute sql_str
I don't have much, not to say any, experience with Oracle's procedures, functions, etc.
I'm having trouble to find out how to create a procedure (or function) in Oracle to perform the above code.
Any help is appreciated. Thank you.

Answer 1

As you are not returning any value we have to go for procedure. Although use case is not clear.
CREATE PROCEDURE dyn_sql_query (user_input IN NUMBER)
IS
sql_str VARCHAR2 (500) := 'SELECT ';
BEGIN
IF (user_input = 1)
THEN sql_str := sql_str || 'a.col1 from tb1 a';
ELSE sql_str := sql_str || 'a.col1, b.col2, b.col3 from tb1 a, tb2 b';
END IF;
EXECUTE IMMEDIATE sql_query;
END;
Here is very basic script for function (notice RETURN) that accepts table name and returns number of rows.
CREATE FUNCTION count_rows (table_name IN VARCHAR2)
RETURN PLS_INTEGER //
IS
sql_query VARCHAR2 (500) := 'SELECT COUNT(*) FROM ' || table_name;
ret_val PLS_INTEGER;
BEGIN
EXECUTE IMMEDIATE sql_query INTO ret_val;
RETURN ret_val;
END;

Answer 2

Using Your Example, Here is One Approach to Dynamic SQL
As others have indicated, there are many examples of dynamic sql on Stack Exchange.
The documentation is very good on this subject. I like the using clause which makes the dynamic sql more extensible.
In practice, the dynamic sql does something: select ... into, a procedural call, dml, dcl.
SCOTT#dev>declare
2 user_input number(1) :=1;
3 sql_str varchar2(1000);
4 begin
5
6 sql_str := 'select ';
7 if (user_input = 1) then
8 -- sql_str := sql_str || 'a.col1 from tb1 a';
9 sql_str := sql_str || '''1'' from dual';
10 dbms_output.put_line(sql_str);
11 else
12 -- sql_str := sql_str || 'a.col1, b.col2, b.col3 from tb1 a, tb2 b';
13 sql_str := sql_str || '''2'' from dual';
14 dbms_output.put_line(sql_str);
15 end if;
16 execute immediate sql_str;
17 end;
18 /
select '1' from dual
PL/SQL procedure successfully completed.
SCOTT#dev>declare
2 user_input number(1) :=2;
3 sql_str varchar2(1000);
4 begin
5
6 sql_str := 'select ';
7 if (user_input = 1) then
8 -- sql_str := sql_str || 'a.col1 from tb1 a';
9 sql_str := sql_str || '''1'' from dual';
10 dbms_output.put_line(sql_str);
11 else
12 -- sql_str := sql_str || 'a.col1, b.col2, b.col3 from tb1 a, tb2 b';
13 sql_str := sql_str || '''2'' from dual';
14 dbms_output.put_line(sql_str);
15 end if;
16 execute immediate sql_str;
17 end;
18 /
select '2' from dual
PL/SQL procedure successfully completed.
With my example as people have commented, a simple select statement usually does not make sense (no one does this in practice).
One can see that my dynamic sql is parsed by looking at v$sql which shows statistics on the shared sql area:
APPS#dev>SELECT
2 sql_id
3 FROM
4 v$sql
5 WHERE
6 1 = 1
7 AND (
8 sql_text = 'select ''1'' from dual'
9 OR
10 sql_text = 'select ''2'' from dual'
11 ) AND
12 parsing_schema_name = 'SCOTT';
SQL_ID
-------------
27q1fj58cnz0k
c9bw73fh2ay8d
addendum based on comments
As Alex cited, the documentation indicates that a select statement without an into clause does not get executed. It is at least parsed and can be seen in the shared sql area, v$sql.
Here is my example with the into clause.
APPS#dev>DECLARE
2 user_input NUMBER(1) := 1;
3 sql_str VARCHAR2(1000);
4 v_val VARCHAR2(1);
5 BEGIN
6 sql_str := 'select ';
7 IF
8 ( user_input = 1 )
9 THEN
10 -- sql_str := sql_str || 'a.col1 from tb1 a';
11 sql_str := sql_str || '''1'' from dual';
12 ELSE
13 -- sql_str := sql_str || 'a.col1,b.col2,b.col3 from tb1 a,tb2 b';
14 sql_str := sql_str || '''2'' from dual';
15 END IF;
16
17 EXECUTE IMMEDIATE sql_str INTO
18 v_val;
19 dbms_output.put_line(sql_str || ' => ' || v_val);
20 END;
21 /
select '1' from dual => 1
PL/SQL procedure successfully completed.

Answer 3

I am not sure if it helps you but you can create a procedure something like below
CREATE OR REPLACE PROCEDURE test_Ali (user_input IN VARCHAR2(2))
IS
sql_str VARCHAR2(1000):= 'select ';
user_input VARCHAR2(2) := '1';
BEGIN
IF user_input = '1' THEN
BEGIN
sql_str := sql_str||'a.col1 from tb1 a';
EXECUTE IMMEDIATE sql_str;
dbms_output.put_line (sql_str) ;
END;
ELSE
BEGIN
sql_str := sql_str||'a.col1, b.col2, b.col3 from tb1 a, tb2 b';
EXECUTE IMMEDIATE sql_str;
dbms_output.put_line (sql_str) ;
END;
END IF;
END;