SQL Question - Interesting One

Question

I need to write a SINGLE SQL statement which has two different
queries, if the first query retrieves records then there is no
need to execute the second one, if the first query doesn't
retrieve the records then I need to make use of second query to
retrieve records.
Is it possible to do in a SINGLE SQL statement? I want to use it
in building a View, i.e, the reason why I am looking for SINGLE
SQL statement
For example,
First Query :- Select Ename from Emp where Deptno=10 and Sal=100;
If this query can retrieve records then I need not execute the
second query
Second Query :- Select Ename from Emp where Deptno=10;
Any help much appreciated?
Thanks,
Venu

Answer 1

John,
My understanding of the requirements is that, if there are rows
where deptno = 10 and sal = 100, then the query should return
only those rows that meet that criteria. However, if there are
no rows where deptno = 10 and sal = 100, then the query should
return all rows where deptno = 10, regardless of the sal.
I included some tests and results below. In the first test, I
used the emp demo table with test data that did not include any
rows where deptno = 10 and sal = 100. I then demonstrated a
query that correctly returned 3 rows where deptno = 10. I then
ran your query which incorrectly resulted in no rows returned.
Then I added a row where deptno = 10 and sal = 100 and re-ran
both queries and they both correctly returned only the 1 row
where deptno = 10 and sal = 100.
In the next test, I created a table similar to your table and
inserted the sample data that you provided, which did not
include any rows where ft_pct = 30. I ran both queries and they
both correctly returned 14 rows where job_no = 'EG01'. Then, I
added a row where ft_pct = 30 and re-ran both queries. My query
correctly returned only the one row where ft_pct = 30. Your
query incorrectly returned 15 rows, which included the original
14 and the one I added, instead of just the one I added.
The manner in which your query performed with your data is what
I expected it to do. However, I don't understand why it seems
to behave differently with the emp demo data. With one table it
produces the wrong results under one condition and with the
other table it produces the wrong results under the other
condition.
Barbara
SQL> -- test using emp demo table:
SQL> -- table structure:
SQL> DESC emp
Name              Null?    Type
EMPNO             NOT NULL NUMBER(4)
ENAME                      VARCHAR2(10)
JOB                        VARCHAR2(9)
MGR                        NUMBER(4)
HIREDATE                   DATE
SAL                        NUMBER(7,2)
COMM                       NUMBER(7,2)
DEPTNO                     NUMBER(2)
SQL> DELETE FROM emp
  2  WHERE  deptno = 10
  3  AND    sal = 100
  4  /
1 row deleted.
SQL> COMMIT
  2  /
Commit complete.
SQL> -- test data without sal = 100:
SQL> SELECT ename, deptno, sal
  2  FROM   emp
  3  /
ENAME          DEPTNO        SAL       
SMITH              20        800       
ALLEN              30       1600       
WARD               30       1250       
JONES              20       2975       
MARTIN             30       1250       
BLAKE              30       2850       
CLARK              10       2450       
SCOTT              20       3000       
KING               10       5000       
TURNER             30       1500       
ADAMS              20       1100       
JAMES              30        950       
FORD               20       3000       
MILLER             10       1300       
14 rows selected.
SQL> -- query with correct results:
SQL> SELECT ename, deptno, sal
  2  FROM   emp
  3  WHERE  deptno = 10
  4  AND    sal = 100
  5  UNION
  6  SELECT ename, deptno, sal
  7  FROM   emp
  8  WHERE  deptno = 10
  9  AND    NOT EXISTS
10           (SELECT ename, deptno, sal
11            FROM   emp
12            WHERE  deptno = 10
13            AND    sal = 100)
14  /
ENAME          DEPTNO        SAL       
CLARK              10       2450       
KING               10       5000       
MILLER             10       1300       
SQL> -- QUERY WITH WRONG RESULT:
SQL> -- ???????????????????????
SQL> SELECT ename, deptno, sal
  2  FROM   emp
  3  WHERE  (deptno=10 and sal=100)
  4  OR     deptno=10
  5  /
no rows selected
SQL> -- ???????????????????????
SQL> -- ABOVE RESULT IS WRONG
SQL> INSERT INTO emp (empno, ename, deptno, sal)
  2  VALUES (9999, 'TEST', 10, 100)
  3  /
1 row created.
SQL> COMMIT
  2  /
Commit complete.
SQL> -- test data with sal = 100
SQL> SELECT ename, deptno, sal
  2  FROM   emp
  3  /
ENAME          DEPTNO        SAL       
SMITH              20        800       
ALLEN              30       1600       
WARD               30       1250       
JONES              20       2975       
MARTIN             30       1250       
BLAKE              30       2850       
CLARK              10       2450       
SCOTT              20       3000       
KING               10       5000       
TURNER             30       1500       
ADAMS              20       1100       
JAMES              30        950       
FORD               20       3000       
MILLER             10       1300       
TEST               10        100       
15 rows selected.
SQL> -- query with correct results:
SQL> SELECT ename, deptno, sal
  2  FROM   emp
  3  WHERE  deptno = 10
  4  AND    sal = 100
  5  UNION
  6  SELECT ename, deptno, sal
  7  FROM   emp
  8  WHERE  deptno = 10
  9  AND    NOT EXISTS
10           (SELECT ename, deptno, sal
11            FROM   emp
12            WHERE  deptno = 10
13            AND    sal = 100)
14  /
ENAME          DEPTNO        SAL       
TEST               10        100       
SQL> -- query with correct results:
SQL> SELECT ename, deptno, sal
  2  FROM   emp
  3  WHERE  (deptno=10 and sal=100)
  4  OR     deptno=10
  5  /
ENAME          DEPTNO        SAL       
TEST               10        100       
SQL> -- test using org_t table:
SQL> CREATE TABLE org_t
  2    (org_cd VARCHAR2 (5) NOT NULL,
  3       job_no VARCHAR2 (4) NOT NULL,
  4       ft_pct NUMBER (3) NOT NULL)
  5  /
Table created.
SQL> INSERT INTO org_t
  2    VALUES ('03538', 'EG01', 40)
  3  /
1 row created.
SQL> INSERT INTO org_t
  2    VALUES ('06763', 'EG01', 40)
  3  /
1 row created.
SQL> INSERT INTO org_t
  2    VALUES ('08026', 'EG01', 40)
  3  /
1 row created.
SQL> INSERT INTO org_t
  2    VALUES ('08105', 'EG01', 40)
  3  /
1 row created.
SQL> INSERT INTO org_t
  2    VALUES ('08200', 'EG01', 40)
  3  /
1 row created.
SQL> INSERT INTO org_t
  2    VALUES ('08232', 'EG01', 40)
  3  /
1 row created.
SQL> INSERT INTO org_t
  2    VALUES ('08233', 'EG01', 40)
  3  /
1 row created.
SQL> INSERT INTO org_t
  2    VALUES ('08286', 'EG01', 40)
  3  /
1 row created.
SQL> INSERT INTO org_t
  2    VALUES ('08909', 'EG01', 40)
  3  /
1 row created.
SQL> INSERT INTO org_t
  2    VALUES ('08984', 'EG01', 40)
  3  /
1 row created.
SQL> INSERT INTO org_t
  2    VALUES ('09073', 'EG01', 40)
  3  /
1 row created.
SQL> INSERT INTO org_t
  2    VALUES ('09723', 'EG01', 1)
  3  /
1 row created.
SQL> INSERT INTO org_t
  2    VALUES ('09724', 'EG01', 100)
  3  /
1 row created.
SQL> INSERT INTO org_t
  2    VALUES ('09752', 'EG01', 1)
  3  /
1 row created.
SQL> COMMIT
  2  /
Commit complete.
SQL> -- test data without ft_pct = 30:
SQL> SELECT org_cd, job_no, ft_pct
  2  FROM   org_t
  3  /
ORG_C JOB_     FT_PCT                  
03538 EG01         40                  
06763 EG01         40                  
08026 EG01         40                  
08105 EG01         40                  
08200 EG01         40                  
08232 EG01         40                  
08233 EG01         40                  
08286 EG01         40                  
08909 EG01         40                  
08984 EG01         40                  
09073 EG01         40                  
09723 EG01          1                  
09724 EG01        100                  
09752 EG01          1                  
14 rows selected.
SQL> -- query with correct results:
SQL> SELECT org_cd, job_no, ft_pct
  2  FROM   org_t
  3  WHERE  job_no = 'EG01'
  4  AND    ft_pct = 30
  5  UNION
  6  SELECT org_cd, job_no, ft_pct
  7  FROM   org_t
  8  WHERE  job_no = 'EG01'
  9  AND    NOT EXISTS
10           (SELECT org_cd, job_no, ft_pct
11            FROM   org_t
12            WHERE  job_no = 'EG01'
13            AND    ft_pct = 30)
14  /
ORG_C JOB_     FT_PCT                  
03538 EG01         40                  
06763 EG01         40                  
08026 EG01         40                  
08105 EG01         40                  
08200 EG01         40                  
08232 EG01         40                  
08233 EG01         40                  
08286 EG01         40                  
08909 EG01         40                  
08984 EG01         40                  
09073 EG01         40                  
09723 EG01          1                  
09724 EG01        100                  
09752 EG01          1                  
14 rows selected.
SQL> -- query with correct results:
SQL> SELECT org_cd,job_no,ft_pct
  2  FROM   org_t
  3  WHERE  (job_no = 'EG01' and ft_pct = 30)
  4  OR     job_no = 'EG01'
  5  /
ORG_C JOB_     FT_PCT                  
03538 EG01         40                  
06763 EG01         40                  
08026 EG01         40                  
08105 EG01         40                  
08200 EG01         40                  
08232 EG01         40                  
08233 EG01         40                  
08286 EG01         40                  
08909 EG01         40                  
08984 EG01         40                  
09073 EG01         40                  
09723 EG01          1                  
09724 EG01        100                  
09752 EG01          1                  
14 rows selected.
SQL> INSERT INTO org_t
  2  VALUES ('99999', 'EG01', 30)
  3  /
1 row created.
SQL> COMMIT
  2  /
Commit complete.
SQL> -- test with ft_pct = 30:
SQL> -- query with correct results:
SQL> SELECT org_cd, job_no, ft_pct
  2  FROM   org_t
  3  WHERE  job_no = 'EG01'
  4  AND    ft_pct = 30
  5  UNION
  6  SELECT org_cd, job_no, ft_pct
  7  FROM   org_t
  8  WHERE  job_no = 'EG01'
  9  AND    NOT EXISTS
10           (SELECT org_cd, job_no, ft_pct
11            FROM   org_t
12            WHERE  job_no = 'EG01'
13            AND    ft_pct = 30)
14  /
ORG_C JOB_     FT_PCT                  
99999 EG01         30                  
SQL> -- QUERY WITH WRONG RESULT:
SQL> -- ???????????????????????
SQL> SELECT org_cd,job_no,ft_pct
  2  FROM   org_t
  3  WHERE  (job_no = 'EG01' and ft_pct = 30)
  4  OR     job_no = 'EG01'
  5  /
ORG_C JOB_     FT_PCT                  
03538 EG01         40                  
06763 EG01         40                  
08026 EG01         40                  
08105 EG01         40                  
08200 EG01         40                  
08232 EG01         40                  
08233 EG01         40                  
08286 EG01         40                  
08909 EG01         40                  
08984 EG01         40                  
09073 EG01         40                  
09723 EG01          1                  
09724 EG01        100                  
09752 EG01          1                  
99999 EG01         30                  
15 rows selected.
SQL> -- ???????????????????????
SQL> -- ABOVE RESULT IS WRONG