Query table A or table B based on some condition - sql

Question

in an Oracle DB (12) I have 2 tables:
table: STEP_DETAILS
+-----------+---------+---------------+
| record_id | step_id | material_type |
+===========+=========+===============+
| 1 | 1 | in |
+-----------+---------+---------------+
| 2 | 1 | in |
+-----------+---------+---------------+
| 3 | 1 | out |
+-----------+---------+---------------+
| 4 | 2 | in |
+-----------+---------+---------------+
| 5 | 2 | out |
+-----------+---------+---------------+
| 6 | 2 | out |
+-----------+---------+---------------+
table: ACTIONS_DETAILS
+-----------+-----------+---------------+
| record_id | action_id | material_type |
+===========+===========+===============+
| 1 | 11 | in |
+-----------+-----------+---------------+
| 2 | 11 | out |
+-----------+-----------+---------------+
| 3 | 12 | in |
+-----------+-----------+---------------+
| 4 | 12 | out |
+-----------+-----------+---------------+
all id-columns are of type INTEGER.
I need to count the input materials for both tables.
in a PL/SQL block I have the following functions, each has 'almost' the same query:
--count from step_details:
FUNCTION get_step_input_count(p_step_id step_details.step_id%TYPE)
RETURN INTEGER
IS
l_count INTEGER := 0;
BEGIN
SELECT COUNT(1)
INTO l_count
FROM step_details
WHERE step_id = p_step_id
AND material_type = 'in';
RETURN l_count;
END get_step_input_count;
--count from action_details:
FUNCTION get_action_input_count(p_action_id action_details.action_id%TYPE)
RETURN INTEGER
IS
l_count INTEGER := 0;
BEGIN
SELECT COUNT(1)
INTO l_count
FROM action_details
WHERE action_id = p_action_id
AND material_type = 'in';
RETURN l_count;
END get_action_input_count;
is it possible to write one single SELECT-statement that can query one of the 2 tables each time based on some condition, so I will eventually write one function that uses one query instead of 2 functions, something like:
FUNCTION get_input_count(p_parent_id integer,
p_from varchar2)
RETURN INTEGER
IS
l_count INTEGER := 0;
BEGIN
SELECT COUNT(1)
INTO l_count
FROM (when p_from = 'S' then 'step_details'
when p_from = 'A' then 'action_details')
WHERE (when p_from = 'S' then 'step_id = p_parent_id'
when p_from = 'A' then 'action_id = p_parent_id')
AND material_type = 'in';
RETURN l_count;
END get_input_count;

Answer 1

You can try with something like the following:
select sum(num_rows)
from
(
select count(*) as num_rows
from tab1 /* first table */
where :param = 1
union all
select count(*) as num_rows
from tab2 /* second table */
where :param = 2
)
Here you use a single select that wraps the UNION ALL of queries from all the possible tables; every table gives its contribution or not, depending on the value of some parameter, so that you only get the rows from the table you want, based on the parameter value.

Answer 2

One option would be using dynamic SQL composed of concatenations for table and column names, and a bind variable for the value which will be common(p_parent_id) for querying from each tables.
SQL> CREATE OR REPLACE FUNCTION get_input_count(p_parent_id INT, p_from VARCHAR2) RETURN INT IS
l_count INT;
crs SYS_REFCURSOR;
v_sql VARCHAR2(32767);
v_from VARCHAR2(32);
v_col VARCHAR2(99);
BEGIN
SELECT DECODE(p_from,'A','actions_details','S','step_details'),
DECODE(p_from,'A','action_id','S','step_id')
INTO v_from, v_col
FROM dual;
v_sql := 'SELECT COUNT(*)
FROM '||v_from||'
WHERE material_type = ''in'' AND '||v_col||' = :prt_id';
OPEN crs FOR v_sql USING p_parent_id;
LOOP
FETCH crs INTO l_count;
EXIT WHEN crs%NOTFOUND;
END LOOP;
CLOSE crs;
RETURN l_count;
END;
/
Demo
where
initializing the value of l_count variable with zero is redundant,
since the query will return zero without exception whenever no
matching records found
the keyword FUNCTION should be prepended by CREATE [OR REPLACE]
ending the stored function with its name is optional(might be
neglected)

Answer 3

You could also use dynamic SQL like below.
SQL injection should be cared in this case.
sql_statement := 'select count(*)';
IF table = 'xxx'
THEN
sql_statement := sql_statement || ' from xxx where material_type = ''in''';
ELSIF table = 'yyy'
sql_statement := sql_statement || ' from yyy where yyyy_type = ''in''';
END IF;
sql_statement := sql_statement blur blur;
EXECUTE IMMEDIATE sql_statement INTO l_count USING p_1;