Assuming you're joining on columns with no duplicates, which is a very common case:
An inner join of A and B gives the result of A intersect B, i.e. the inner part of a Venn diagram intersection.
An outer join of A and B gives the results of A union B, i.e. the outer parts of a Venn diagram union.
Examples
Suppose you have two tables, with a single column each, and data as follows:
A B
- -
1 3
2 4
3 5
4 6
Note that (1,2) are unique to A, (3,4) are common, and (5,6) are unique to B.
Inner join
An inner join using either of the equivalent queries gives the intersection of the two tables, i.e. the two rows they have in common.
select * from a INNER JOIN b on a.a = b.b;
select a.*, b.* from a,b where a.a = b.b;
a | b
--+--
3 | 3
4 | 4
Left outer join
A left outer join will give all rows in A, plus any common rows in B.
select * from a LEFT OUTER JOIN b on a.a = b.b;
select a.*, b.* from a,b where a.a = b.b(+);
a | b
--+-----
1 | null
2 | null
3 | 3
4 | 4
Right outer join
A right outer join will give all rows in B, plus any common rows in A.
select * from a RIGHT OUTER JOIN b on a.a = b.b;
select a.*, b.* from a,b where a.a(+) = b.b;
a | b
-----+----
3 | 3
4 | 4
null | 5
null | 6
Full outer join
A full outer join will give you the union of A and B, i.e. all the rows in A and all the rows in B. If something in A doesn't have a corresponding datum in B, then the B portion is null, and vice versa.
select * from a FULL OUTER JOIN b on a.a = b.b;
a | b
-----+-----
1 | null
2 | null
3 | 3
4 | 4
null | 6
null | 5
For InnoDB, the following seems to work: create the new empty database, then rename each table in turn into the new database:
RENAME TABLE old_db.table TO new_db.table;
You will need to adjust the permissions after that.
For scripting in a shell, you can use either of the following:
mysql -u username -ppassword old_db -sNe 'show tables' | while read table; \
do mysql -u username -ppassword -sNe "rename table old_db.$table to new_db.$table"; done
OR
for table in `mysql -u root -ppassword -s -N -e "use old_db;show tables from old_db;"`; do mysql -u root -ppassword -s -N -e "use old_db;rename table old_db.$table to new_db.$table;"; done;
Notes:
- There is no space between the option
-p and the password. If your database has no password, remove the -u username -ppassword part.
If some table has a trigger, it cannot be moved to another database using above method (will result Trigger in wrong schema error). If that is the case, use a traditional way to clone a database and then drop the old one:
mysqldump old_db | mysql new_db
If you have stored procedures, you can copy them afterwards:
mysqldump -R old_db | mysql new_db
Best Solution
There is no general answer; you'll have to be more specific. All I can provide are general principles.
All else being equal, you'll be better off with a well-normalized database without redundant information, for a number of reasons. But there are situations where redundant information could save your program a lot of time. One example is text formatted with Markdown: you need to store the original markup to allow for editing, but formatting the source every time you need the output may be extremely taxing on the system. Therefore, you might add a redundant column to store the formatted output and assume the additional responsibility of ensuring that that column is kept up-to-date.
All I know about your situation is that the postulated extra column would save a query. The only correct answer to that is that you should probably keep your table clean and minimal unless you know that the performance benefit of saving one query will make up for it. Remember, premature optimization is the root of all evil – you may find that your application runs more than fast enough anyways. If find while profiling that the extra query is a significant bottleneck, then you might consider adding the column.
Again, without more knowledge of your situation, it is impossible to provide a specific or concrete recommendation, but I hope that I've at least helped you to come to a decision.