Detect and fix many to many in mysql tables - mysql

Question

I have a Table "Teaches". This has a PK and a Name.
mysql> Select * from Teacher;
+-----+---------+
| TID | Tname |
+-----+---------+
| 1 | RC |
| 2 | Dinesha |
| 3 | JLT |
+-----+---------+
I have another Table "TeachesSubject". It has _surrid which is a PK, the TID references Teacher and SName is the Name of the subject taught.
mysql> select * from TeachesSubject;
+---------+------+-------+
| _surrid | TID | SNAME |
+---------+------+-------+
| 100 | 1 | DBMS |
| 101 | 1 | DM |
| 102 | 2 | DBMS |
| 103 | 2 | OOAD |
| 104 | 3 | OOAD |
| 105 | 3 | SE |
+---------+------+-------+
These two tables are already populated. I now run a query in TeachesSubject that points out the SNAMEs with duplicate values.
mysql> select SNAME from TeachesSubject group by SNAME having count(*)>1;
+-------+
| SNAME |
+-------+
| DBMS |
| OOAD |
+-------+
Once I detect this, I want my sql script to create a new table:
Teaches
_surrId TID
100 1
101 1
100 2
103 2
103 3
105 3
My old Teaches Subject Must look like
mysql> select * from TeachesSubject;
+---------+------+
| _surrid | SNAME |
+---------+-------+
| 100 | DBMS |
| 101 | DM |
| 103 | OOAD |
| 105 | SE |
+---------+------+
How Do I do this?

Answer 1

Here's how you can do it:
CREATE TABLE TeachesSubject2
(`_surrid` int, `SNAME` varchar(4))
;
INSERT INTO TeachesSubject2
SELECT MIN(_surrid), sname
FROM TeachesSubject
GROUP BY sname
;
CREATE TABLE Teaches
(`_surrid` int, `TID` int)
;
INSERT INTO Teaches
SELECT t2._surrid, t1.tid
FROM TeachesSubject AS t1
LEFT OUTER JOIN TeachesSubject2 AS t2
ON t1.sname = t2.sname
;
Working example is here: http://sqlfiddle.com/#!9/13d417/2
In future it would help if you created a fiddle yourself and attempted to write the SQL yourself. Then I wouldn't have to spend my time setting up for the answer, I could just fix whatever problem you are having.