Using Or in a sql statement which is operating in multiple tables - mysql

Question

The Problem:
I have 2 tables i am selecting data from one called Table_Booked which contains one record with a Username and SessionID for each time a user books, another table called Table_Sessions which contains the Session details such as SessionId,Subject,TeacherID,Day,Am_Pm and WhatYear.
Im trying to select all the records from Table_Booked where the username is the same as the current users, and i also want to get the Day & Am_Pm data for each of the sessions the user has booked or the records for the session he is trying to book. The aim is to compare the records and if any occur in the same time slot not allow the user to book the session.
The Code:
Select Table_Session.Day,Table_Session.Am_Pm,Table_Session.SessionId
From Table_Session,Table_Booked
Where Table_Booked.Username = 'G9rpenman' and
((Table_Booked.SessionID = Table_Session.SessionId) Or (Table_Session.SessionId = 9))
When i run this code it fetches the information from Table_Session.SessionId = 9 twice and other tweaks have caused similarly random data selection. I cant find anything about this. Thank you for your help. If you feel i missed any crucial information out let me know and ill edit this.
Heres some screenshots of my tables with the data im using http://imgur.com/a/9fZtW

Answer 1

Simple rule: Never use commas in the FROM clause. Always use explicit JOIN syntax.
The reason for the duplication is the OR. I think this will do what you want:
Select s.Day, s.Am_Pm, s.SessionId
From Table_Booked b join
Table_Session s
on s.SessionID = b.SessionId or s.SessionId = 9
Where b.Username = 'G9rpenman' ;
If not, you may need union:
Select s.Day, s.Am_Pm, s.SessionId
From Table_Booked b join
Table_Session s
on s.SessionID = b.SessionId
Where b.Username = 'G9rpenman'
union
Select s.Day, s.Am_Pm, s.SessionId
from Table_Session s
where s.SessionId = 9;

Answer 2

Why don't you try a JOIN statement as opposed to selecting from both tables the way you are?
What's the output of the following query :
SELECT
TS.Day
,TS.Am_Pm
,TS.SessionId
FROM Table_Session TS
INNER JOIN Table_Booked TB ON TS.SessionId = TB.SessionID
OR TS.SessionId = 9
WHERE TB.Username = 'G9rpenman'
Could you post some sample data from both tables so we can see what you're trying to compare to provide more help?