Question

I have three tables table1, table2 and table 3. I am getting data to a file with sequence of the output as shown below. I want to append the data coming in that file to a table4 that ill create and persist using Java.

Scenario: I want all the data in this three tables into one cloumn where table2 element matches table3 element in C4 then instead of that matched element i want to replace that element with the corresponding data in C5. For example in below mentioned table we see 11 in table2 matches table3 at column C4. Then the output should have 11_1, 11_2 instead of 11.

Help needed

Table1:                  Table2:                          Table3:

C1                       C3                               C4        C5
-----                    ------                           ------    -------
1                        11                               11        11_1
2                        12                               11        11_2
3                        13                               14        14_1
4                        14                               14        14_2
5                        15                               14        14_3
                         16                               null      null 

Code I worked:

SELECT c1 
FROM   (SELECT c1 
        FROM   table1 
        UNION ALL 
        SELECT c3 
        FROM   table2 
        UNION ALL 
        SELECT c5 
        FROM   table3) temp 

This query fetching me everything 🙁

SELECT C1 FROM(SELECT DISTINCT C1 AS C1 FROM table1 UNION ALL SELECT DISTINCT C2 FROM table2)AS 
temp LEFT JOIN table3 ON .temp.C1=table3.C4 CASE WHEN table3.C4=NULL THEN temp.C1 ELSE table3.C5

Output Required:

Table4
C1
-----
1
2
3
4
5
11_1
11_2
12
13
14_1
14_2
14_3
15
16

Help is appreciated

Answer 1

This should work:

SELECT col 
FROM   (SELECT c1 AS col 
        FROM   table1 
        UNION ALL 
        SELECT CASE 
                 WHEN t3.c4 IS NULL THEN t2.c3 
                 ELSE t3.c5 
               end AS col 
        FROM   table2 t2 
               LEFT JOIN table3 t3 
                      ON t3.c4 = t2.c3) temp 

Result

|  COL |
--------
|    1 |
|    2 |
|    3 |
|    4 |
|    5 |
| 11_1 |
| 11_2 |
|   12 |
|   13 |
| 14_1 |
| 14_2 |
| 14_3 |
|   15 |
|   16 |

See the demo