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MySQL includes some extensions that you probably will not find in
other SQL databases. Be warned that if you use them, your code will not be
portable to other SQL servers. In some cases, you can write code that
includes MySQL extensions, but is still portable, by using comments
of the form /*! ... */. In this case, MySQL will parse and
execute the code within the comment as it would any other MySQL
statement, but other SQL servers will ignore the extensions. For example:
SELECT /*! STRAIGHT_JOIN */ col_name FROM table1,table2 WHERE ...
If you add a version number after the '!', the syntax will only be
executed if the MySQL version is equal or newer than the used
version number:
CREATE /*!32302 TEMPORARY */ TABLE (a int);
The above means that if you have 3.23.02 or newer, then MySQL will use
the TEMPORARY keyword.
MySQL extensions are listed below:
We try to make MySQL follow the ANSI SQL standard and the
ODBC SQL standard, but in some cases MySQL does some things
differently:
The following functionality is missing in the current version of
MySQL. For a prioritized list indicating when new extensions may be
added to MySQL, you should consult
the online
MySQL TODO list. That is the latest version of the TODO list in
this manual. See section F List of things we want to add to MySQL in the future (The TODO).
The following will not yet work in MySQL:
SELECT * FROM table1 WHERE id IN (SELECT id FROM table2);
SELECT * FROM table1 WHERE id NOT IN (SELECT id FROM table2);
However, in many cases you can rewrite the query without a sub select:
SELECT table1.* FROM table1,table2 WHERE table1.id=table2.id;
SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id where table2.id IS NULL
For more complicated sub queries you can create temporary tables to hold the
sub query.
MySQL only supports INSERT ... SELECT ... and
REPLACE ... SELECT ... Independent sub-selects will be probably
be available in 3.24.0. You can now use the function IN() in
other contexts, however.
MySQL doesn't yet support the Oracle SQL extension:
SELECT ... INTO TABLE .... MySQL supports instead the
ANSI SQL syntax INSERT INTO ... SELECT ..., which is basically
the same thing.
Alternatively, you can use SELECT INTO OUTFILE... or CREATE
TABLE ... SELECT to solve your problem.
Transactions are not supported. MySQL shortly will support atomic
operations, which are like transactions without rollback. With atomic
operations, you can execute a group of INSERT/SELECT/whatever
commands and be guaranteed that no other thread will interfere. In this
context, you won't usually need rollback. Currently, you can prevent
interference from other threads by using the LOCK TABLES and
UNLOCK TABLES commands.
See section 7.23 LOCK TABLES/UNLOCK TABLES syntax.
A stored procedure is a set of SQL commands that can be compiled and stored
in the server. Once this has been done, clients don't need to keep reissuing
the entire query but can refer to the stored procedure. This provides better
performance because the query has to be parsed only once and less information
needs to be sent between the server and the client. You can also raise the
conceptual level by having libraries of functions in the server.
A trigger is a stored procedure that is invoked when a particular event
occurs. For example, you can install a stored procedure that is triggered
each time a record is deleted from a transaction table and that automatically
deletes the corresponding customer from a customer table when all his
transactions are deleted.
The planned update language will be able to
handle stored procedures, but without triggers. Triggers usually slow
down everything, even queries for which they are not needed.
To see when MySQL might get stored procedures, see section F List of things we want to add to MySQL in the future (The TODO).
Note that foreign keys in SQL are not used to join tables, but are used
mostly for checking referential integrity. If you want to get results from
multiple tables from a SELECT statement, you do this by joining
tables!
SELECT * from table1,table2 where table1.id = table2.id;
See section 7.12 JOIN syntax. See section 8.3.5 Using foreign keys.
The FOREIGN KEY syntax in MySQL exists only for compatibility
with other SQL vendors' CREATE TABLE commands; it doesn't do
anything. The FOREIGN KEY syntax without ON DELETE ... is
mostly used for documentation purposes. Some ODBC applications may use this
to produce automatic WHERE clauses, but this is usually easy to
override. FOREIGN KEY is sometimes used as a constraint check, but
this check is unnecessary in practice if rows are inserted into the tables in
the right order. MySQL only supports these clauses because some
applications require them to exist (regardless of whether or not they
work!).
In MySQL, you can work around the problem of ON DELETE
... not being implemented by adding the appropriate DELETE statement to
an application when you delete records from a table that has a foreign key.
In practice this is as quick (in some cases quicker) and much more portable
than using foreign keys.
In the near future we will extend the FOREIGN KEY implementation so
that at least the information will be saved in the table specification file
and may be retrieved by mysqldump and ODBC.
There are so many problems with FOREIGN KEYs that we don't
know where to start:
-
Foreign keys make life very complicated, because the foreign key definitions
must be stored in a database and implementing them would destroy the whole
``nice approach'' of using files that can be moved, copied and removed.
-
The speed impact is terrible for
INSERT and UPDATE statements,
and in this case almost all FOREIGN KEY checks are useless because you
usually insert records in the right tables in the right order, anyway.
-
There is also a need to hold locks on many more tables when updating one
table, because the side effects can cascade through the entire database. It's
MUCH faster to delete records from one table first and subsequently delete
them from the other tables.
-
You can no longer restore a table by doing a full delete from
the table and then restoring all records (from a new source or from a backup).
-
If you have foreign keys you can't dump and restore tables unless you do so
in a very specific order.
-
It's very easy to do ``allowed'' circular definitions that make the
tables impossible to recreate each table with a single create statement, even if
the definition works and is usable.
The only nice aspect of FOREIGN KEY is that it gives ODBC and some
other client programs the ability to see how a table is connected and to use
this to show connection diagrams and to help in building applicatons.
MySQL will soon store FOREIGN KEY definitions so that
a client can ask for and receive an answer how the original connection was
made. The current `.frm' file format does not have any place for it.
MySQL doesn't support views, but this is on the TODO.
Some other SQL databases use `--' to start comments. MySQL
has `#' as the start comment character, even if the mysql
command line tool removes all lines that start with `--'.
You can also use the C comment style /* this is a comment */ with
MySQL.
See section 7.28 Comment syntax.
MySQL 3.23.3 and above supports the `--' comment style
only if the comment is followed by a space. This is because this
degenerate comment style has caused many problems with automatically
generated SQL queries that have used something like the following code,
where we automatically insert the value of the payment for
!payment!:
UPDATE tbl_name SET credit=credit-!payment!
What do you think will happen when the value of payment is negative?
Because 1--1 is legal in SQL, we think it is terrible that
`--' means start comment.
In MySQL 3.23 you can however use: 1-- This is a comment
The following discussing only concerns you if you are running an
MySQL version earlier than 3.23:
If you have a SQL program in a text file that contains `--' comments
you should use:
shell> replace " --" " #" < text-file-with-funny-comments.sql \
| mysql database
instead of the usual:
shell> mysql database < text-file-with-funny-comments.sql
You can also edit the command file ``in place'' to change the `--'
comments to `#' comments:
shell> replace " --" " #" -- text-file-with-funny-comments.sql
Change them back with this command:
shell> replace " #" " --" -- text-file-with-funny-comments.sql
Entry level SQL92. ODBC level 0-2.
MySQL doesn't support COMMIT-ROLLBACK. The problem is
that handling COMMIT-ROLLBACK efficiently would require a
completely different table layout than MySQL uses today.
MySQL would also need extra threads that do automatic cleanups on
the tables and the disk usage would be much higher. This would make
MySQL about 2-4 times slower than it is today. MySQL is
much faster than almost all other SQL databases (typically at least 2-3 times
faster). One of the reasons for this is the lack of
COMMIT-ROLLBACK.
For the moment, we are much more for implementing the SQL server
language (something like stored procedures). With this you would very
seldom really need COMMIT-ROLLBACK. This would also give much
better performance.
Loops that need transactions normally can be coded with the help of
LOCK TABLES, and you don't need cursors when you can update records
on the fly.
We have transactions and cursors on the TODO but not quite prioritized. If
we implement these, it will be as an option to CREATE TABLE. That
means that COMMIT-ROLLBACK will work only on those tables,
so that a speed penalty will be imposed on those table only.
We at TcX have a greater need for a real fast database than a 100%
general database. Whenever we find a way to implement these features without
any speed loss, we will probably do it. For the moment, there are many more
important things to do. Check the TODO for how we prioritize things at
the moment. (Customers with higher levels of support can alter this, so
things may be reprioritized.)
The current problem is actually ROLLBACK. Without ROLLBACK, you
can do any kind of COMMIT action with LOCK TABLES. To support
ROLLBACK, MySQL would have to be changed to store all old
records that were updated and revert everything back to the starting point if
ROLLBACK was issued. For simple cases, this isn't that hard to do (the
current isamlog could be used for this purpose), but it would be much
more difficult to implement ROLLBACK for ALTER/DROP/CREATE
TABLE.
To avoid using ROLLBACK, you can use the following strategy:
-
Use
LOCK TABLES ... to lock all the tables you want to access.
-
Test conditions.
-
Update if everything is okay.
-
Use
UNLOCK TABLES to release your locks.
This is usually a much faster method than using transactions with possible
ROLLBACKs, although not always. The only situation this solution
doesn't handle is when someone kills the threads in the middle of an
update. In this case, all locks will be released but some of the updates may
not have been executed.
You can also use functions to update records in a single operation.
You can get a very efficient application by using the following techniques:
-
Modify fields relative to their current value
-
Update only those fields that actually have changed
For example, when we are doing updates to some customer information, we
update only the customer data that have changed and test only that none of
the changed data, or data that depend on the changed data, have changed
compared to the original row. The test for changed data is done with the
WHERE clause in the UPDATE statement. If the record wasn't
updated, we give the client a message: "Some of the data you have changed
have been changed by another user". Then we show the old row versus the new
row in a window, so the user can decide which version of the customer record
he should use.
This gives us something that is similar to ``column locking'' but is actually
even better, because we only update some of the columns, using values that
are relative to their current values. This means that typical UPDATE
statements look something like these:
UPDATE tablename SET pay_back=pay_back+'relative change';
UPDATE customer
SET
customer_date='current_date',
address='new address',
phone='new phone',
money_he_owes_us=money_he_owes_us+'new_money'
WHERE
customer_id=id AND address='old address' AND phone='old phone';
As you can see, this is very efficient and works even if another client has
changed the values in the pay_back or money_he_owes_us columns.
In many cases, users have wanted ROLLBACK and/or LOCK
TABLES for the purpose of managing unique identifiers for some tables. This
can be handled much more efficiently by using an AUTO_INCREMENT column
and either the SQL function LAST_INSERT_ID() or the C API function
mysql_insert_id(). See section 20.4.29 mysql_insert_id().
At TcX, we have never had any need for row-level locking because we have
always been able to code around it. Some cases really need row
locking, but they are very few. If you want row-level locking, you
can use a flag column in the table and do something like this:
UPDATE tbl_name SET row_flag=1 WHERE id=ID;
MySQL returns 1 for the number of affected rows if the row was
found and row_flag wasn't already 1 in the original row.
You can think of it as MySQL changed the above query to:
UPDATE tbl_name SET row_flag=1 WHERE id=ID and row_flag <> 1;
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