Sql query to create following tables - sql

Question

use the below table 1 to generate table 2
Table 1
Col1 A B C
------------------
N1 1 0 0
N2 0 1 0
N3 1 0 0
Table 2
output
new_col
-------
N1 A
N2 B
N3 A
Also how to use Table 2 to generate table 1 above

Answer 1

Following SQL query can be help to get requested output:
SELECT Col1,IF(A=1,'A',IF(B=1,'B','C')) AS result FROM `table_name`;

Answer 2

You can use conditinal with CONCAT() Function :
SELECT CONCAT(col1,' ',
CASE WHEN A = 1 THEN 'A' ELSE '' END,
CASE WHEN B = 1 THEN 'B' ELSE '' END,
CASE WHEN C = 1 THEN 'C' ELSE '' END)
FROM table1
provided you're on a DBMS with brand name such as MySQL, PostGRES, SQL Server. As an example, Oracle DB won't allow using more than two arguments for CONCAT() Function.
In order to create table2, use for most of the DBMS :
CREATE TABLE table2 AS
SELECT CONCAT(col1,' ',
CASE WHEN A = 1 THEN 'A' ELSE '' END,
CASE WHEN B = 1 THEN 'B' ELSE '' END,
CASE WHEN C = 1 THEN 'C' ELSE '' END) AS new_col
FROM table1
except for SQL Server in which prefer using :
SELECT CONCAT(col1,' ',
CASE WHEN A = 1 THEN 'A' ELSE '' END,
CASE WHEN B = 1 THEN 'B' ELSE '' END,
CASE WHEN C = 1 THEN 'C' ELSE '' END) AS new_col
INTO table2
FROM table1
In order to implement a reverse engineering(go back to original table), you need to consider the dialectics for each seperate database to handle string values. Assume you're using MySQL DB, then consider using :
CREATE TABLE table3 AS
SELECT SUBSTRING(new_col,1,instr(new_col,' ')-1) AS col1,
CASE WHEN instr(new_col,'A') > 0 THEN 1 ELSE 0 END AS A,
CASE WHEN instr(new_col,'B') > 0 THEN 1 ELSE 0 END AS B,
CASE WHEN instr(new_col,'C') > 0 THEN 1 ELSE 0 END AS C
FROM table2
where I used a different table name (table3), since table1 already exists.
Demo
Btw, if Oracle DB is the case, then use :
SELECT CONCAT(CONCAT(col1,' '),
CASE WHEN A = 1 THEN 'A' ELSE '' END||
CASE WHEN B = 1 THEN 'B' ELSE '' END||
CASE WHEN C = 1 THEN 'C' ELSE '' END) AS new_col
FROM table1

Answer 3

You can use one case statement. This works in Oracle.
SELECT col1
|| CASE
WHEN A = 1
THEN 'A'
WHEN B = 1
THEN 'B'
WHEN C = 1
THEN 'C'
END
NEW_COL
FROM table1

Answer 4

It is unclear what you want when there are multiple "1"s in a row. This is a simple solution:
select id, A from t where A = 1
union all
select id, B from t where B = 1
union all
select id, C from t where C = 1;
In databases that support lateral joins, I would recommend:
select t.id, v.which
from t cross join lateral
(values ('A', t.A), ('B', B), ('C', C)
) v(which, val)
where val = 1;