Базы данныхИнтернетКомпьютерыОперационные системыПрограммированиеСетиСвязьРазное
Поиск по сайту:
Подпишись на рассылку:

Назад в раздел

Go to the first, previous, next, last section, table of contents.

5 How standards-compatible is MySQL?

5.1 MySQL extensions to ANSI SQL92

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:

  • The field types MEDIUMINT, SET, ENUM and the different BLOB and TEXT types.
  • All string comparisons are case insensitive by default, with sort ordering determined by the current character set (ISO-8859-1 Latin1 by default). If you don't like this, you should declare your columns with the BINARY attribute or use the BINARY cast, which causes comparisons to be done according to the ASCII order used on the MySQL server host.
  • MySQL maps each database to a directory under the MySQL data directory, and tables within a database to filenames in the database directory. This has two implications:
    • Database names and table names are case sensitive in MySQL on operating systems that have case sensitive filenames (like most Unix systems). If you have a problem remembering table names, adopt a consistent convention, such as always creating databases and tables using lowercase names.
    • Database, table, index, column or alias names may begin with a digit (but may not consist solely of digits).
    • You can use standard system commands to backup, rename, move, delete and copy tables. For example, to rename a table, rename the `.MYD', `.MYI' and `.frm' files to which the table corresponds.
  • In SQL statements, you can access tables from different databases with the db_name.tbl_name syntax. Some SQL servers provide the same functionality but call this User space. MySQL dosen't support tablespaces like in: create table ralph.my_table...IN my_tablespace.
  • LIKE is allowed on numeric columns.
  • Use of INTO OUTFILE and STRAIGHT_JOIN in a SELECT statement. See section 7.12 SELECT syntax.
  • The SQL_SMALL_RESULT option in a SELECT statement.
  • EXPLAIN SELECT to get a description on how tables are joined.
  • Use of index names, indexes on a prefix of a field, and use of INDEX or KEY in a CREATE TABLE statement. See section 7.7 CREATE TABLE syntax.
  • Use of COUNT(DISTINCT list) where 'list' is more than one element.
  • Use of CHANGE col_name, DROP col_name or DROP INDEX in an ALTER TABLE statement. See section 7.8 ALTER TABLE syntax.
  • Use of IGNORE in an ALTER TABLE statement.
  • Use of multiple ADD, ALTER, DROP or CHANGE clauses in an ALTER TABLE statement.
  • Use of DROP TABLE with the keywords IF EXISTS.
  • You can drop multiple tables with a single DROP TABLE statement.
  • The LIMIT clause of the DELETE statement.
  • The DELAYED clause of the INSERT and REPLACE statements.
  • The LOW_PRIORITY clause of the INSERT, REPLACE, DELETE and UPDATE statements.
  • Use of LOAD DATA INFILE. In many cases, this syntax is compatible with Oracle's LOAD DATA INFILE. See section 7.16 LOAD DATA INFILE syntax.
  • The OPTIMIZE TABLE statement. See section 7.9 OPTIMIZE TABLE syntax.
  • The SHOW statement. See section 7.21 SHOW syntax (Get information about tables, columns,...).
  • Strings may be enclosed by either `"' or `'', not just by `''.
  • Use of the escape `' character.
  • The SET OPTION statement. See section 7.25 SET OPTION syntax.
  • You don't need to name all selected columns in the GROUP BY part. This gives better performance for some very specific, but quite normal queries. See section 7.4.13 Functions for use with GROUP BY clauses.
  • To make it easier for users that come from other SQL environments, MySQL supports aliases for many functions. For example, all string functions support both ANSI SQL syntax and ODBC syntax.
  • MySQL understands the || and && operators to mean logical OR and AND, as in the C programming language. In MySQL, || and OR are synonyms, as are && and AND. Because of this nice syntax, MySQL doesn't support the ANSI SQL || operator for string concatenation; use CONCAT() instead. Since CONCAT() takes any number of arguments, it's easy to convert use of the || operator to MySQL.
  • The % operator is a synonym for MOD(). That is, N % M is equivalent to MOD(N,M). % is supported for C programmers and for compatibility with PostgreSQL.
  • The =, <>, <= ,<, >=,>, <<, >>, <=>, AND, OR or LIKE operators may be used in column comparisons to the left of the FROM in SELECT statements. For example:
    mysql> SELECT col1=1 AND col2=2 FROM tbl_name;
  • The LAST_INSERT_ID() function. See section 20.4.29 mysql_insert_id().
  • The REGEXP and NOT REGEXP extended regular expression operators.
  • CONCAT() or CHAR() with one argument or more than two arguments. (In MySQL, these functions can take any number of arguments.)
  • Use of TRIM() to trim substrings. ANSI SQL only supports removal of single characters.
  • The GROUP BY functions STD(), BIT_OR() and BIT_AND().
  • Use of REPLACE instead of DELETE + INSERT. See section 7.15 REPLACE syntax.
  • The FLUSH flush_option statement.
  • The possiblity to set variables in a statement with :=:
    SELECT @a:=SUM(total),@b=COUNT(*),@a/@b AS avg FROM test_table;
    SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;

5.2 Runnning MySQL in ANSI mode

If you start mysqld with the --ansi option, the following behaviour of MySQL changes.

  • || is string concatenation instead of OR.
  • One can have any number of spaces between a function name and the '('. This makes also all function names reserved words.
  • " will be a identifier quote character (like the MySQL ` quote character) and not a string quote character.
  • REAL will be a synonym for FLOAT instead of a synonym of DOUBLE.

5.3 MySQL differences compared to ANSI SQL92

We try to make MySQL follow the ANSI SQL standard and the ODBC SQL standard, but in some cases MySQL does some things differently:

5.4 Functionality missing from MySQL

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).

5.4.1 Sub-selects

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 subqueries you can often create temporary tables to hold the subquery. In some cases, however this option will not work. The most frequently encountered of these cases arises with DELETE statements, for which standard SQL does not support joins (except in sub-selects). For this situation there are two options available until subqueries are supported by MySQL.

The first option is to use a procedural programming language (such as Perl or PHP) to submit a SELECT query to obtain the primary keys for the records to be deleted, and then use these values to construct the DELETE statement (DELETE FROM ... WHERE ... IN (key1, key2, ...)).

The second option is to use interactive SQL to contruct a set of DELETE statements automatically, using the MySQL extension CONCAT() (in lieu of the standard || operator). For example:

SELECT CONCAT('DELETE FROM tab1 WHERE pkid = ', tab1.pkid, ';')
  FROM tab1, tab2
 WHERE tab1.col1 = tab2.col2;

You can place this query in a script file and redirect input from it to the mysql command-line interpreter, piping its output back to a second instance of the interpreter:

prompt> mysql --skip-column-names mydb < myscript.sql | mysql mydb

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.

5.4.3 Transactions

The question is often asked, by the curious and the critical, "Why is MySQl not a transactional database?" or "Why does MySQl not support transactions."

MySQL has made a conscious decision to support another paradigm for data integrity, "atomic operations." It is our thinking and experience that atomic operations offer equal or even better integrity with much better performance. We, nonetheless, appreciate and understand the transactional database paradigm and plan, in the next few releases, on introducing transaction safe tables on a per table basis. We will be giving our users the possibility to decide if they need the speed of atomic operations or if they need to use transactional features in their applications.

How does one use the features of MySQl to maintain rigorous integrity and how do these features compare with the transactional paradigm?

First, in the transactional paradigm, if your applications are written in a way that is dependent on the calling of "rollback" instead of "commit" in critical situations, then transactions are more convenient. Moreover, transactions ensure that unfinished updates or corrupting activities are not commited to the database; the server is given the opportunity to do an automatic rollback and your database is saved.

MySQL, in almost all cases, allows you to solve for potential problems by including simple checks before updates and by running simple scripts that check the databases for inconsistencies and automatically repair or warn if such occurs. Note that just by using the MySQL log or even adding one extra log, one can normally fix tables perfectly with no data integrity loss.

Moreover, "fatal" transactional updates can be rewritten to be atomic. In fact,we will go so far as to say that all integrity problems that transactions solve can be done with LOCK TABLES or atomic updates, ensuring that you never will get an automatic abort from the database, which is a common problem with transactional databases. Not even transactions can prevent all loss if the server goes down. In such cases even a transactional system can lose data. The difference between different systems lies in just how small the time-lap is where they could lose data. No system is 100 % secure, only "secure enough". Even Oracle, reputed to be the safest of transactional databases, is reported to sometimes lose data in such situations.

To be safe with MySQL you only need to have backups and have the update logging turned on. With this you can recover from any situation that you could with any transactional database. It is, of course, always good to have backups, independent of which database you use.

The transactional paradigm has its benefits and its drawbacks. Many users and application developers depend on the ease with which they can code around problems where an "abort" appears or is necessary, and they may have to do a little more work with MySQL to either think differently or write more. If you are new to the atomic operations paradigm, or more familiar or more comfortable with transactions, do not jump to the conclusion that MySQL has not addressed these issues. Reliability and integrity are foremost in our minds. Recent estimates are that there are more than 1,000,000 mysqld servers currently running, many of which are in production environments. We hear very, very seldom from our users that they have lost any data, and in almost all of those cases user error is involved. This is in our opinion the best proof of MySQL's stability and reliability.

Lastly, in situations where integrity is of highest importance, MySQL's current features allow for transaction-level or better reliability and integrity. If you lock tables with LOCK TABLES, all updates will stall until any integrity checks are made. If you only do a read lock (as opposed to a write lock), then reads and inserts are still allowed to happen. The new inserted records will not be seen by any of the clients that have a READ lock until they relaease their read locks. With INSERT DELAYED you can queue insert into a local queue, until the locks are released, without having to have the client to wait for the insert to complete.

Atomic in the sense that we mean it is nothing magical, it only means that you can be sure that while each specific update is running no other user can interfere with it and that there will never be an automatic rollback (which can happen on transaction based systems if you are not very careful). MySQL also guarantees that there will not be any dirty reads.

We have thought quite a bit about integrity and performance and we believe that our atomic operations paradigm allows for both high reliability and extremely high performance, on the order of three to five times the speed of the fastest and most optimally tuned of transactional databases. We didn't leave out transactions because they are hard to do; The main reason we went with atomic operations as opposed to transactions is that by doing this we could apply many speed optimizations that would not otherwise have been possible.

Many of our users who have speed foremost in their minds are not at all concerned about transactions. For them transactions are not an issue. For those of our users who are concerned with or have wondered about transactions vis a vis MySQL, there is a "MySQL way" as we have outlined above.

One final note: we are currently working on a safe replication schema that we believe to be better than any commercial replication system we know of. This system will work most reliably under the atomic operations, non-transactional, paradigm. Stay tuned.

5.4.4 Stored procedures and triggers

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).

5.4.5 Foreign Keys

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 7.13 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. Reasons NOT to use foreign keys

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.

5.4.7 `--' as the start of a comment

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 7.29 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

5.5 What standards does MySQL follow?

Entry level SQL92. ODBC level 0-2.

5.6 How to cope without COMMIT/ROLLBACK

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:

  1. Use LOCK TABLES ... to lock all the tables you want to access.
  2. Test conditions.
  3. Update if everything is okay.
  4. 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
    address='new address',
    phone='new phone',
    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;

Go to the first, previous, next, last section, table of contents.

  • Главная
  • Новости
  • Новинки
  • Скрипты
  • Форум
  • Ссылки
  • О сайте

  • Emanual.ru – это сайт, посвящённый всем значимым событиям в IT-индустрии: новейшие разработки, уникальные методы и горячие новости! Тонны информации, полезной как для обычных пользователей, так и для самых продвинутых программистов! Интересные обсуждения на актуальные темы и огромная аудитория, которая может быть интересна широкому кругу рекламодателей. У нас вы узнаете всё о компьютерах, базах данных, операционных системах, сетях, инфраструктурах, связях и программированию на популярных языках!
     Copyright © 2001-2020
    Реклама на сайте