... Design and View Issue

Question

Hi All,
I have got stuck over a strange requirement.
There are 2 tables which are identical in structure. Both tables displays the same information.
Table structure is
TeacherID TeacherName param1 param2 param3 param4.
Now if the teacher has a special trainingcourse approved, their record is moved to another table. where the value of TeacherID, Teacher name remains same , but Any of the parameter values can differ.
The issue comes when we need to generate the report. The report should pick the value of parameters from Table1 if the teache has NO trainiing, and it should pick the record from Table2 if the teacher has the Training.
How to create a View which will display the information. as Decode function is not working in a Create View Command.
I have put a Flagg field , which turns to 1 whenever the teacher gets the training , and the record is moved/copied to table 2.
But how come we be able to display the report like below.
teacheId Teachername param1 param2 param3 param4...
xxx xxx xx xxx x xxx
where the value should be picked from Table 2 if there is a training , otherwise should be picked from Table1.
Kindly reply early, and suggest anyother possible design ..
Looking foward to your earliest feedback with this difficult design
Regards

Answer 1

There are a couple of options. Unless your tables are pretty small, I would porbably go with:
SELECT teacherid, teachername, param1, param2, param3, param4
FROM table1
WHERE flag <> 1 -- or possibly IS NULL depending on what you put in
UNION ALL       -- for a teacher with no training
SELECT teacherid, teachername, param1, param2, param3, param4
FROM table2Another option would be an outer join and DECODE or CASE. Something along the lines of:
SELECT t1.teacherid, t1.teachername,
       DECODE(flag, 1, t2.param1, t1.param1) param1,
       DECODE(flag, 1, t2.param2, t1.param2) param2,
       DECODE(flag, 1, t2.param3, t1.param3) param3,
       DECODE(flag, 1, t2.param4, t1.param4) param4
FROM table1 t1
   LEFT JOIN table2 t2 ON (t1.teacherid = t2.teacheridA third possibility, particularly if the number of trained teachers is relatively small would be scalar sub-queries like:
SELECT t1.teacherid, t1.teachername,
       DECODE(flag, 1, (SELECT param1 FROM table2 t2
                        WHERE t1.teacherid = t2.teacherid), t1.param1) param1,
       DECODE(flag, 1, (SELECT param2 FROM table2 t2
                        WHERE t1.teacherid = t2.teacherid), t1.param2) param2,
       DECODE(flag, 1, (SELECT param3 FROM table2 t2
                        WHERE t1.teacherid = t2.teacherid), t1.param3) param3,
       DECODE(flag, 1, (SELECT param4 FROM table2 t2
                        WHERE t1.teacherid = t2.teacherid), t1.param4) param4,
FROM table1 t1You would need to test with your data to determine which was better.
John