what are the changes in mysql 8 result rowset case? - mysql

when running
SELECT maxlen FROM `information_schema`.`CHARACTER_SETS`;
mysql 5.7 and mysql 8 produce different results:
on mysql 5.7 the results row names are lower cased,
on mysql 8 the results row names are upper cased.
NB : in the CHARACTER_SETS table, the comumn name is MAXLEN (upper cased).
Since I can't find a resource documenting it, my question is :
what are the changes in mysql 8 result rowset case ?


MySQL 8.0 did change the implementation of some views in the INFORMATION_SCHEMA:
https://mysqlserverteam.com/mysql-8-0-improvements-to-information_schema/ says:
Now that the metadata of all database tables is stored in transactional data dictionary tables, it enables us to design an INFORMATION_SCHEMA table as a database VIEW over the data dictionary tables. This eliminates costs such as the creation of temporary tables for each INFORMATION_SCHEMA query during execution on-the-fly, and also scanning file-system directories to find FRM files. It is also now possible to utilize the full power of the MySQL optimizer to prepare better query execution plans using indexes on data dictionary tables.
So it's being done for good reasons, but I understand that it has upset some of your queries when you fetch results in associative arrays based on column name.
You can see the definition of the view declares the column name explicitly in uppercase:
mysql 8.0.14> SHOW CREATE VIEW CHARACTER_SETS\G
*************************** 1. row ***************************
View: CHARACTER_SETS
Create View: CREATE ALGORITHM=UNDEFINED DEFINER=`mysql.infoschema`#`localhost` SQL SECURITY DEFINER VIEW `CHARACTER_SETS` AS
select
`cs`.`name` AS `CHARACTER_SET_NAME`,
`col`.`name` AS `DEFAULT_COLLATE_NAME`,
`cs`.`comment` AS `DESCRIPTION`,
`cs`.`mb_max_length` AS `MAXLEN` -- delimited column explicitly uppercase
from (`mysql`.`character_sets` `cs`
join `mysql`.`collations` `col` on((`cs`.`default_collation_id` = `col`.`id`)))
character_set_client: utf8
collation_connection: utf8_general_ci
You can work around the change in a couple of ways:
You can declare your own column aliases in the case you want when you query a view:
mysql 8.0.14> SELECT MAXLEN AS `maxlen`
FROM `information_schema`.`CHARACTER_SETS` LIMIT 2;
+--------+
| maxlen |
+--------+
| 2 |
| 1 |
+--------+
You could start a habit of querying columns in uppercase prior to 8.0. Here's a test showing results in my 5.7 sandbox:
mysql 5.7.24> SELECT MAXLEN
FROM `information_schema`.`CHARACTER_SETS` LIMIT 2;
+--------+
| MAXLEN |
+--------+
| 2 |
| 1 |
+--------+
Or you could fetch results into a non-associative array, and reference columns by column number, instead of by name.


There is no change in case sensitivity. If you check mysql documentation on identifier case sensitivity, both v5.7 and v8.0 say that field names are case insensitive:
Column, index, stored routine, event, and resource group names are not case-sensitive on any platform, nor are column aliases.
To me this seems more like a display difference.

Related

Is there an equivalent for \gdesc in mysql?

In postgres, whenever I want to print a description of a query, I could add \gdesc as follows:
# select 1 as num \gdesc
Column | Type
--------+---------
num | integer
(1 row)
I was wondering if there's an equivalent in MySQL (or in any other relational database).
I'm aware of DESCRIBE Table, but I'm looking to describe a query, not a table.

Suggested way to resolve column name/type in a view

I have the following problem that I'm trying to find the best solution for. Let's say I have a view such as the following:
CREATE VIEW myView AS (
SELECT
country_code,
other_column,
COUNT(1) as cnt
FROM mytable
JOIN otherDatabase.otherTable ON (id)
GROUP BY 1,2 ORDER BY 1 LIMIT 1
)
What would be the fastest way to resolve the field names and types of the view? For example, on the above I am looking to get something along the lines of:
{
country_code: VARCHAR,
other_column: BOOL,
cnt: INT
}
The first approach is just to run the query (with a limit, if necessary) and then get the types of the result-set from the driver. The downside of this is what if the query takes 50 minutes to resolve?
The second approach I thought of is to 'follow' the columns to get their types and then do some parsing to resolve any expressions/literals/etc. This would involve a lot of code but would be orders of magnitude faster than the above. However, the potential downside of this is we may have access to the view but not have access to a table (possibly in another database on the server) that contains the column type, so it's possible we might not be able to resolve all field names.
What would be the best way to resolve the types of a view? Note I have tagged this as MySQL, but I'm also wondering if there's a more generic way to resolve types or if it's something that is non-standard and more needs to be done on a per-database basis?
Update: I believe the correct answer is just to run a DESCRIBE myView, and that would give me the column names and types without running the query?

In the current version of MySQL at least, INFORMATION_SCHEMA.COLUMNS holds metadata for views as well as base tables:
mysql> create table mytable (id serial primary key, x int);
Query OK, 0 rows affected (0.01 sec)
mysql> create view v as select * from mytable;
mysql> select column_name, data_type from information_schema.columns where table_name='v';
+-------------+-----------+
| COLUMN_NAME | DATA_TYPE |
+-------------+-----------+
| id | bigint |
| x | int |
+-------------+-----------+

A related issue...
SHOW CREATE TABLE myView;
or
SHOW CREATE VIEW myView;
will fully qualify all the columns.
(When writing a JOIN, it is wise to always qualify the column names.)

How to make MySql table data Case Sensitive?

I wanted to enter data in MySql table's primary key field with respect to case sensitivity.
But default it is not considering case sensitivity for table data.
Here is my query.
mysql> select id from product where id = 'a1';
+----+
| id |
+----+
| A1 |
+----+
1 row in set (0.00 sec)
mysql> insert into product values('a1', 'SomeName', 'SomeName', 200, 10, 10);
ERROR 1062 (23000): Duplicate entry 'a1' for key 'product.PRIMARY'
Also i have tried Collation while creating table but not getting result as required.
can any one suggest which collation has to use or any other technique to make table's column domain case sensitive.

ALTER TABLE product
MODIFY COLUMN id VARCHAR(...) COLLATION ..._bin NOT NOT NULL;
Where the ... are the current column size and character set.
The only case-sensitive things I can think of are
base-64
Unix file names
But those do not seem likely as PKs. What is your use case? Most things are better off being case-insensitive.
(A Comment links to a SQL Server suggestion using ALTER DATABASE; that will not work for MySQL since that only changes the default for subsequently created tables.)

What is the default select order in PostgreSQL or MySQL?

I have read in the PostgreSQL docs that without an ORDER statement, SELECT will return records in an unspecified order.
Recently on an interview, I was asked how to SELECT records in the order that they inserted without an PK or created_at or other field that can be used for order. The senior dev who interviewed me was insistent that without an ORDER statement the records will be returned in the order that they were inserted.
Is this true for PostgreSQL? Is it true for MySQL? Or any other RDBMS?

I can answer for MySQL. I don't know for PostgreSQL.
The default order is not the order of insertion, generally.
In the case of InnoDB, the default order depends on the order of the index read for the query. You can get this information from the EXPLAIN plan.
For MyISAM, it returns orders in the order they are read from the table. This might be the order of insertion, but MyISAM will reuse gaps after you delete records, so newer rows may be stored earlier.
None of this is guaranteed; it's just a side effect of the current implementation. MySQL could change the implementation in the next version, making the default order of result sets different, without violating any documented behavior.
So if you need the results in a specific order, you should use ORDER BY on your queries.

Following BK's answer, and by way of example...
DROP TABLE IF EXISTS my_table;
CREATE TABLE my_table(id INT NOT NULL) ENGINE = MYISAM;
INSERT INTO my_table VALUES (1),(9),(5),(8),(7),(3),(2),(6);
DELETE FROM my_table WHERE id = 8;
INSERT INTO my_table VALUES (4),(8);
SELECT * FROM my_table;
+----+
| id |
+----+
| 1 |
| 9 |
| 5 |
| 4 | -- is this what
| 7 |
| 3 |
| 2 |
| 6 |
| 8 | -- we expect?
+----+

In the case of PostgreSQL, that is quite wrong.
If there are no deletes or updates, rows will be stored in the table in the order you insert them. And even though a sequential scan will usually return the rows in that order, that is not guaranteed: the synchronized sequential scan feature of PostgreSQL can have a sequential scan "piggy back" on an already executing one, so that rows are read starting somewhere in the middle of the table.
However, this ordering of the rows breaks down completely if you update or delete even a single row: the old version of the row will become obsolete, and (in the case of an UPDATE) the new version can end up somewhere entirely different in the table. The space for the old row version is eventually reclaimed by autovacuum and can be reused for a newly inserted row.

Without an ORDER BY clause, the database is free to return rows in any order. There is no guarantee that rows will be returned in the order they were inserted.
With MySQL (InnoDB), we observe that rows are typically returned in the order by an index used in the execution plan, or by the cluster key of a table.
It is not difficult to craft an example...
CREATE TABLE foo
( id INT NOT NULL
, val VARCHAR(10) NOT NULL DEFAULT ''
, UNIQUE KEY (id,val)
) ENGINE=InnoDB;
INSERT INTO foo (id, val) VALUES (7,'seven') ;
INSERT INTO foo (id, val) VALUES (4,'four') ;
SELECT id, val FROM foo ;
MySQL is free to return rows in any order, but in this case, we would typically observe that MySQL will access rows through the InnoDB cluster key.
id val
---- -----
4 four
7 seven
Not at all clear what point the interviewer was trying to make. If the interviewer is trying to sell the idea, given a requirement to return rows from a table in the order the rows were inserted, a query without an ORDER BY clause is ever the right solution, I'm not buying it.
We can craft examples where rows are returned in the order they were inserted, but that is a byproduct of the implementation, ... not guaranteed behavior, and we should never rely on that behavior to satisfy a specification.

Hidden Features of MySQL

Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
I've been working with Microsoft SQL Server with many years now but have only just recently started to use MySQL with my web applications, and I'm hungry for knowledge.
To continue with the long line of "hidden feature" questions, I would like to know any hidden or handy features of MySQL which will hopefully improve my knowledge of this open source database.

Since you put up a bounty, I'll share my hard won secrets...
In general, all the SQLs I tuned today required using sub-queries. Having come from Oracle database world, things I took for granted weren’t working the same with MySQL. And my reading on MySQL tuning makes me conclude that MySQL is behind Oracle in terms of optimizing queries.
While the simple queries required for most B2C applications may work well for MySQL, most of the aggregate reporting type of queries needed for Intelligence Reporting seems to require a fair bit of planning and re-organizing the SQL queries to guide MySQL to execute them faster.
Administration:
max_connections is the number of concurrent connections. The default value is 100 connections (151 since 5.0) - very small.
Note:
connections take memory and your OS might not be able to handle a lot of connections.
MySQL binaries for Linux/x86 allow you to have up to 4096 concurrent connections, but self compiled binaries often have less of a limit.
Set table_cache to match the number of your open tables and concurrent connections. Watch the open_tables value and if it is growing quickly you will need to increase its size.
Note:
The 2 previous parameters may require a lot of open files. 20+max_connections+table_cache*2 is a good estimate for what you need. MySQL on Linux has an open_file_limit option, set this limit.
If you have complex queries sort_buffer_size and tmp_table_size are likely to be very important. Values will depend on the query complexity and available resources, but 4Mb and 32Mb, respectively are recommended starting points.
Note: These are "per connection" values, among read_buffer_size, read_rnd_buffer_size and some others, meaning that this value might be needed for each connection. So, consider your load and available resource when setting these parameters. For example sort_buffer_size is allocated only if MySQL needs to do a sort. Note: be careful not to run out of memory.
If you have many connects established (i.e. a web site without persistent connections) you might improve performance by setting thread_cache_size to a non-zero value. 16 is good value to start with. Increase the value until your threads_created do not grow very quickly.
PRIMARY KEY:
There can be only one AUTO_INCREMENT column per table, it must be indexed, and it cannot have a DEFAULT value
KEY is normally a synonym for INDEX. The key attribute PRIMARY KEY can also be specified as just KEY when given in a column definition. This was implemented for compatibility with other database systems.
A PRIMARY KEY is a unique index where all key columns must be defined as NOT NULL
If a PRIMARY KEY or UNIQUE index consists of only one column that has an integer type,
you can also refer to the column as "_rowid" in SELECT statements.
In MySQL, the name of a PRIMARY KEY is PRIMARY
Currently, only InnoDB (v5.1?) tables support foreign keys.
Usually, you create all the indexes you need when you are creating tables.
Any column declared as PRIMARY KEY, KEY, UNIQUE, or INDEX will be indexed.
NULL means "not having a value". To test for NULL, you cannot use the arithmetic comparison operators such as =, <, or <>. Use the IS NULL and IS NOT NULL operators instead:
NO_AUTO_VALUE_ON_ZERO suppresses auto increment for 0 so that only NULL generates the next sequence number. This mode can be useful if 0 has been stored in a table's AUTO_INCREMENT column. (Storing 0 is not a recommended practice, by the way.)
To change the value of the AUTO_INCREMENT counter to be used for new rows:
ALTER TABLE mytable AUTO_INCREMENT = value;
or
SET INSERT_ID = value;
Unless otherwise specified, the value will begin with: 1000000 or specify it thus:
...) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=1
TIMESTAMPS:
Values for TIMESTAMP columns are converted from the current time zone to UTC for storage,
and from UTC to the current time zone for retrieval.
http://dev.mysql.com/doc/refman/5.1/en/timestamp.html
For one TIMESTAMP column in a table, you can assign the current timestamp as the default value and the auto-update value.
one thing to watch out for when using one of these types in a WHERE clause, it is best to do
WHERE datecolumn = FROM_UNIXTIME(1057941242)
and not
WHERE UNIX_TIMESTAMP(datecolumn) = 1057941242.
doing the latter won't take advantage of an index on that column.
http://dev.mysql.com/doc/refman/5.1/en/date-and-time-functions.html
UNIX_TIMESTAMP()
FROM_UNIXTIME()
UTC_DATE()
UTC_TIME()
UTC_TIMESTAMP()
if you convert a datetime to unix timestamp in MySQL:
And then add 24 hours to it:
And then convert it back to a datetime it magically loses an hour!
Here's what's happening. When converting the unix timestamp back to a datetime the timezone is taken into consideration and it just so happens that between the 28th and the 29th of October 2006 we went off daylight savings time and lost an hour.
Beginning with MySQL 4.1.3, the CURRENT_TIMESTAMP(), CURRENT_TIME(), CURRENT_DATE(), and FROM_UNIXTIME() functions return values in the connection's current time zone, which is available as the value of the time_zone system variable. In addition, UNIX_TIMESTAMP() assumes that its argument is a datetime value in the current time zone.
The current time zone setting does not affect values displayed by functions such as UTC_TIMESTAMP() or values in DATE, TIME, or DATETIME columns.
NOTE: ON UPDATE ONLY updates the DateTime if a field is changed If an UPDATE results in no fields being changed then the DateTime is NOT updated!
Addtionally, the First TIMESTAMP is always AUTOUPDATE by default even if not specified
When working with Dates, I almost always convet to Julian Date becuase Data math is then a simple matter of adding or subtracing integers, and Seconds since Midnight for the same reason. It is rare I need time resoultion of finer granularity than seconds.
Both these can be stored as a 4 byte integer, and if space is really tight can be combined into UNIX time (seconds since the epoch 1/1/1970) as an unsigned integer which will be good till around 2106 as:
' secs in 24Hrs = 86400
' Signed Integer max val = 2,147,483,647 - can hold 68 years of Seconds
' Unsigned Integer max val = 4,294,967,295 - can hold 136 years of Seconds
Binary Protocol:
MySQL 4.1 introduced a binary protocol that allows non-string data values to be sent
and returned in native format without conversion to and from string format. (Very usefull)
Aside, mysql_real_query() is faster than mysql_query() because it does not call strlen()
to operate on the statement string.
http://dev.mysql.com/tech-resources/articles/4.1/prepared-statements.html
The binary protocol supports server-side prepared statements and allows transmission of data values in native format. The binary protocol underwent quite a bit of revision during the earlier releases of MySQL 4.1.
You can use the IS_NUM() macro to test whether a field has a numeric type.
Pass the type value to IS_NUM() and it evaluates to TRUE if the field is numeric:
One thing to note is that binary data CAN be sent inside a regular query if you escape it and remember MySQL requires only that backslash and the quote character be escaped.
So that is a really easy way to INSERT shorter binary strings like encrypted/Salted passwords for example.
Master Server:
http://www.experts-exchange.com/Database/MySQL/Q_22967482.html
http://www.databasejournal.com/features/mysql/article.php/10897_3355201_2
GRANT REPLICATION SLAVE ON . to slave_user IDENTIFIED BY 'slave_password'
#Master Binary Logging Config STATEMENT causes replication
to be statement-based - default
log-bin=Mike
binlog-format=STATEMENT
server-id=1
max_binlog_size = 10M
expire_logs_days = 120
#Slave Config
master-host=master-hostname
master-user=slave-user
master-password=slave-password
server-id=2
Binary Log File must read:
http://dev.mysql.com/doc/refman/5.0/en/binary-log.html
http://www.mydigitallife.info/2007/10/06/how-to-read-mysql-binary-log-files-binlog-with-mysqlbinlog/
http://dev.mysql.com/doc/refman/5.1/en/mysqlbinlog.html
http://dev.mysql.com/doc/refman/5.0/en/binary-log.html
http://dev.mysql.com/doc/refman/5.1/en/binary-log-setting.html
You can delete all binary log files with the RESET MASTER statement, or a subset of them with PURGE MASTER
--result-file=binlog.txt TrustedFriend-bin.000030
Normalization:
http://dev.mysql.com/tech-resources/articles/intro-to-normalization.html
UDF functions
http://www.koders.com/cpp/fid10666379322B54AD41AEB0E4100D87C8CDDF1D8C.aspx
http://souptonuts.sourceforge.net/readme_mysql.htm
DataTypes:
http://dev.mysql.com/doc/refman/5.1/en/storage-requirements.html
http://www.informit.com/articles/article.aspx?p=1238838&seqNum=2
http://bitfilm.net/2008/03/24/saving-bytes-efficient-data-storage-mysql-part-1/
One thing to note is that on a mixed table with both CHAR and VARCHAR, mySQL will change the CHAR's to VARCHAR's
RecNum integer_type UNSIGNED NOT NULL AUTO_INCREMENT, PRIMARY KEY (RecNum)
MySQL always represents dates with the year first, in accordance with the standard SQL and ISO 8601 specifications
Misc:
Turing off some MySQl functionality will result in smaller data files
and faster access. For example:
--datadir will specify the data directory and
--skip-innodb will turn off the inno option and save you 10-20M
More here
http://dev.mysql.com/tech-resources/articles/mysql-c-api.html
Download Chapter 7 - Free
InnoDB is transactional but there is a performance overhead that comes with it. I have found MyISAM tables to be sufficient for 90% of my projects.
Non-transaction-safe tables (MyISAM) have several advantages of their own, all of which occur because:
there is no transaction overhead:
Much faster
Lower disk space requirements
Less memory required to perform updates
Each MyISAM table is stored on disk in three files. The files have names that begin with the table name and have an extension to indicate the file type. An .frm file stores the table format. The data file has an .MYD (MYData) extension. The index file has an .MYI (MYIndex) extension.
These Files can be copied to a storage location intact without using the MySQL Administrators Backup feature which is time consuming (so is the Restore)
The trick is make a copy of these files then DROP the table. When you put the files back
MySQl will recognize them and update the table tracking.
If you must Backup/Restore,
Restoring a backup, or importing from an existing dump file can takes a long time depending on the number of indexes and primary keys you have on each table. You can speed this process up dramatically by modifying your original dump file by surrounding it with the following:
SET AUTOCOMMIT = 0;
SET FOREIGN_KEY_CHECKS=0;
.. your dump file ..
SET FOREIGN_KEY_CHECKS = 1;
COMMIT;
SET AUTOCOMMIT = 1;
To vastly increase the speed of the reload, add the SQL command SET AUTOCOMMIT = 0; at the beginning of the dump file, and add the COMMIT; command to the end.
By default, autocommit is on, meaning that each and every insert command in
the dump file will be treated as a separate transaction and written to disk before the next one is started. If you don't add these commands, reloading a large database into InnoDB can take many hours...
The maximum size of a row in a MySQL table is 65,535 bytes
The effective maximum length of a VARCHAR in MySQL 5.0.3 and on = maximum row size (65,535 bytes)
VARCHAR values are not padded when they are stored. Trailing spaces are retained when
values are stored and retrieved, in conformance with standard SQL.
CHAR and VARCHAR values in MySQL are compared without regard to trailing spaces.
Using CHAR will only speed up your access if the whole record is fixed size. That is,
if you use any variable size object, you might as well make all of them variable size.
You gain no speed by using a CHAR in a table that also contains a VARCHAR.
The VARCHAR limit of 255 characters was raised to 65535 characters as of MySQL 5.0.3
Full-text searches are supported for MyISAM tables only.
http://dev.mysql.com/doc/refman/5.0/en/fulltext-search.html
BLOB columns have no character set, and sorting and comparison are based on the
numeric values of the bytes in column values
If strict SQL mode is not enabled and you assign a value to a BLOB or TEXT column that
exceeds the column's maximum length, the value is truncated to fit and a warning is generated.
Useful Commands:
check strict mode:
SELECT ##global.sql_mode;
turn off strict mode:
SET ##global.sql_mode= '';
SET ##global.sql_mode='MYSQL40'
or remove:
sql-mode="STRICT_TRANS_TABLES,...
SHOW COLUMNS FROM mytable
SELECT max(namecount) AS virtualcolumn FROM mytable ORDER BY virtualcolumn
http://dev.mysql.com/doc/refman/5.0/en/group-by-hidden-fields.html
http://dev.mysql.com/doc/refman/5.1/en/information-functions.html#function_last-insert-id
last_insert_id()
gets you the PK of the last row inserted in the current thread max(pkcolname) gets you last PK overall.
Note: if the table is empty max(pkcolname) returns 1 mysql_insert_id() converts the return type of the native MySQL C API function mysql_insert_id() to a type of
long (named int in PHP).
If your AUTO_INCREMENT column has a column type of BIGINT, the value returned by
mysql_insert_id() will be incorrect. Instead, use the internal MySQL SQL function LAST_INSERT_ID() in an SQL query.
http://dev.mysql.com/doc/refman/5.0/en/information-functions.html#function_last-insert-id
Just a note that when you’re trying to insert data into a table and you get the error:
Unknown column ‘the first bit of data what you want to put into the table‘ in ‘field list’
using something like
INSERT INTO table (this, that) VALUES ($this, $that)
it’s because you’ve not got any apostrophes around the values you’re trying to stick into the table. So you should change your code to:
INSERT INTO table (this, that) VALUES ('$this', '$that')
reminder that `` are used to define MySQL fields, databases, or tables, not values ;)
Lost connection to server during query:
http://dev.mysql.com/doc/refman/5.1/en/gone-away.html
http://dev.mysql.com/doc/refman/5.1/en/packet-too-large.html
http://dev.mysql.com/doc/refman/5.0/en/server-parameters.html
http://dev.mysql.com/doc/refman/5.1/en/show-variables.html
http://dev.mysql.com/doc/refman/5.1/en/option-files.html
http://dev.mysql.com/doc/refman/5.1/en/error-log.html
Tuning Queries
http://www.artfulsoftware.com/infotree/queries.php?&bw=1313
Well that should be enough to earn the bonus I would think... The fruits of many hours and many projects with a great free database. I develop application data servers on windows platforms mostly with MySQL. The worst mess I had to straighten out was
The ultimate MySQL legacy database nightmare
This required a series of appplications to process the tables into something usefull using many of the tricks mentioned here.
If you found this astoundingly helpfull, express your thanks by voting it up.
Also check out my other articles and white papers at: www.coastrd.com

One of the not so hidden feature of MySQL is that it's not really good at being SQL compliant, well, not bugs really, but, more gotchas... :-)

A command to find out what tables are currently in the cache:
mysql> SHOW open TABLES FROM test;
+----------+-------+--------+-------------+
| DATABASE | TABLE | In_use | Name_locked |
+----------+-------+--------+-------------+
| test | a | 3 | 0 |
+----------+-------+--------+-------------+
1 row IN SET (0.00 sec)
(From MySQL performance blog)

A command to find out who is doing what:
mysql> show processlist;
show processlist;
+----+-------------+-----------------+------+---------+------+----------------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-------------+-----------------+------+---------+------+----------------------------------+------------------+
| 1 | root | localhost:32893 | NULL | Sleep | 0 | | NULL |
| 5 | system user | | NULL | Connect | 98 | Waiting for master to send event | NULL |
| 6 | system user | | NULL | Connect | 5018 | Reading event from the relay log | NULL |
+-----+------+-----------+---------+---------+-------+-------+------------------+
3 rows in set (0.00 sec)
And you can kill a process with:
mysql>kill 5

I particularly like MySQL's built-in support for inet_ntoa() and inet_aton(). It makes handling of IP addresses in tables very straightforward (at least so long as they're only IPv4 addresses!)

I love on duplicate key (AKA upsert, merge) for all kinds of counters created lazily:
insert into occurances(word,count) values('foo',1),('bar',1)
on duplicate key cnt=cnt+1
You can insert many rows in one query, and immediately handle duplicate index for each of the rows.

Again - not really hidden features, but really handy:
Feature
Easily grab DDL:
SHOW CREATE TABLE CountryLanguage
output:
CountryLanguage | CREATE TABLE countrylanguage (
CountryCode char(3) NOT NULL DEFAULT '',
Language char(30) NOT NULL DEFAULT '',
IsOfficial enum('T','F') NOT NULL DEFAULT 'F',
Percentage float(4,1) NOT NULL DEFAULT '0.0',
PRIMARY KEY (CountryCode,Language)
) ENGINE=MyISAM DEFAULT CHARSET=latin1
Feature: GROUP_CONCAT() aggregate function
Creates a concatenated string of its arguments per detail, and aggregates by concatenating those per group.
Example 1: simple
SELECT CountryCode
, GROUP_CONCAT(Language) AS List
FROM CountryLanguage
GROUP BY CountryCode
Output:
+-------------+------------------------------------+
| CountryCode | List |
+-------------+------------------------------------+
| ABW | Dutch,English,Papiamento,Spanish |
. ... . ... .
| ZWE | English,Ndebele,Nyanja,Shona |
+-------------+------------------------------------+
Example 2: multiple arguments
SELECT CountryCode
, GROUP_CONCAT(
Language
, IF(IsOfficial='T', ' (Official)', '')
) AS List
FROM CountryLanguage
GROUP BY CountryCode
Output:
+-------------+---------------------------------------------+
| CountryCode | List |
+-------------+---------------------------------------------+
| ABW | Dutch (Official),English,Papiamento,Spanish |
. ... . ... .
| ZWE | English (Official),Ndebele,Nyanja,Shona |
+-------------+---------------------------------------------+
Example 3: Using a custom separator
SELECT CountryCode
, GROUP_CONCAT(Language SEPARATOR ' and ') AS List
FROM CountryLanguage
GROUP BY CountryCode
Output:
+-------------+----------------------------------------------+
| CountryCode | List |
+-------------+----------------------------------------------+
| ABW | Dutch and English and Papiamento and Spanish |
. ... . ... .
| ZWE | English and Ndebele and Nyanja and Shona |
+-------------+----------------------------------------------+
Example 4: Controlling the order of the list elements
SELECT CountryCode
, GROUP_CONCAT(
Language
ORDER BY CASE IsOfficial WHEN 'T' THEN 1 ELSE 2 END DESC
, Language
) AS List
FROM CountryLanguage
GROUP BY CountryCode
Output:
+-------------+------------------------------------+
| CountryCode | List |
+-------------+------------------------------------+
| ABW | English,Papiamento,Spanish,Dutch, |
. ... . ... .
| ZWE | Ndebele,Nyanja,Shona,English |
+-------------+------------------------------------+
Feature: COUNT(DISTINCT ) with multiple expressions
You can use multiple expressions in a COUNT(DISTINCT ...) expression to count the number of combinations.
SELECT COUNT(DISTINCT CountryCode, Language) FROM CountryLanguage
Feature / Gotcha: No need to include non-aggregated expressions in the GROUP BY list
Most RDBMS-es enforce a SQL92 compliant GROUP BY which requires all non-aggregated expressions in the SELECT list to appear in the GROUP BY. In these RDBMS-es, this statement:
SELECT Country.Code, Country.Continent, COUNT(CountryLanguage.Language)
FROM CountryLanguage
INNER JOIN Country
ON CountryLanguage.CountryCode = Country.Code
GROUP BY Country.Code
is not valid, because the SELECT list contains the non-aggregated column Country.Continent which does not appear in the GROUP BY list. In these RDBMS-es, you must either modify the GROUP BY list to read
GROUP BY Country.Code, Country.Continent
or you must add some non-sense aggregate to Country.Continent, for example
SELECT Country.Code, MAX(Country.Continent), COUNT(CountryLanguage.Language)
Now, the thing is, logically there is nothing that demands that Country.Continent should be aggreagated. See, Country.Code is the primary key of the Country table. Country.Continent is also a column from the Country table and is thus by definitions functionally dependent upon the primary key Country.Code. So, there must exist exactly one value in Country.Continent for each distinct Country.Code. If you realize that, than you realize that it does not make sense to aggregate it (there is just one value, right) nor to group by it (as it won't make the result more unique as you're already grouping by on the pk)
Anyway - MySQL lets you include non-aggregated columns in the SELECT list without requiring you to also add them to the GROUP BY clause.
The gotcha with this is that MySQL does not protect you in case you happen to use a non-aggregated column. So, a query like this:
SELECT Country.Code, COUNT(CountryLanguage.Language), CountryLanguage.Percentage
FROM CountryLanguage
INNER JOIN Country
ON CountryLanguage.CountryCode = Country.Code
GROUP BY Country.Code
Will be executed without complaint, but the CountryLanguage.Percentage column will contain non-sense (that is to say, of all languages percentages, one of the available values for the percentage will be picked at random or at least outside your control.
See: Debunking Group By Myths

The "pager" command in the client
If you've got, say, 10,000 rows in your result and want to view them (This assumes the "less" and "tee" commands available, which is normally the case under Linux; in Windows YMMV.)
pager less
select lots_of_stuff FROM tbl WHERE clause_which_matches_10k_rows;
And you'll get them in the "less" file viewer so you can page through them nicely, search etc.
Also
pager tee myfile.txt
select a_few_things FROM tbl WHERE i_want_to_save_output_to_a_file;
Will conveniently write to a file.

Some things you may find interesting:
<query>\G -- \G in the CLI instead of the ; will show one column per row
explain <query>; -- this will show the execution plan for the query

Not a hidden feature, but useful nonetheless: http://mtop.sourceforge.net/

Here are some of my tips - I blogged about them in my blog (Link)
You don't need to use '#' sign when declaring variables.
You have to use a delimiter (the default is ';') to demarcate the end of a statement - Link
If you trying to move data between MS-SQL 2005 and mySQL there are a few hoops to jump through - Link
Doing case sensitive matches in mySQL - link

If you're going to be working with large and/or high transaction InnoDb databases learn and understand "SHOW INNODB STATUS" Mysql Performance Blog, it will become your friend.

If using cmdline Mysq, you can interact with the command line (on Linux machines - not sure if there is an equivalent effect on Windows) by using the shriek/exclamation mark. For example:
\! cat file1.sql
will display the code for file1.sql. To save your statement and query to a file, use the tee facility
\T filename
to turn this off use \t
Lastly to run a script you've already saved, use "source filename". Of course, the normal alternative is to direct in the script name when starting mysql from the command line:
mysql -u root -p < case1.sql
Hope that's of use to someone !
Edit: Just remembered another one - when invoking mysql from the command line you can use the -t switch so that output is in table format - a real boon with some queries (although of course terminating queries with \G as mentioned elsewhere here is also helpful in this respect). A lot more on various switches Command Line Tool
Just found out a neat way to change the order of a sort (normally use Case...)
If you want to change the order of a sort (perhaps sort by 1, 4, 3 ,2 instead of 1, 2, 3,4) you can use the field function within the Order by clause.
For example
Order By Field(sort_field,1,4,3,2)
Found this out here Order by day_of_week in MySQL courtesey of user gms8994

I don't think this is MySQL specific, but enlighting for me:
Instead of writing
WHERE (x.id > y.id) OR (x.id = y.id AND x.f2 > y.f2)
You can just write
WHERE (x.id, x.f2) > (y.id, y.f2)

mysqlsla - One of the very commonly used slow query log analysis tool. You can see top 10 worsts queries since u last rolled out slow query logs. It can also tell you the number of times that BAD query was fired and how much total time it took on the server.

Actually documented, but very annoying: automatic conversions for incorrect dates and other incorrect input.
Before MySQL 5.0.2, MySQL is forgiving of illegal or improper data values and coerces them to legal values for data entry. In MySQL 5.0.2 and up, that remains the default behavior, but you can change the server SQL mode to select more traditional treatment of bad values such that the server rejects them and aborts the statement in which they occur.
As for dates: sometimes you'll be "lucky" when MySQL doesn't adjust the input to nearby valid dates, but instead stores it as 0000-00-00 which by definition is invalid. However, even then you might have wanted MySQL to fail rather than silently storing this value for you.

The built-in SQL Profiler.

InnoDB by default stores all tables in one global tablespace that will never shrink.
You can use innodb_file_per_table which will put each table in a separate tablespace that will be deleted when you drop the table or database.
Plan ahead for this since you have to dump and restore the database to reclaim space otherwise.
Using Per-Table Tablespaces

If you insert into datetime column empty string value "", MySQL will retain the value as 00/00/0000 00:00:00. Unlike Oracle, which will save null value.

During my benchmarks with large datasets and DATETIME fields, it's always slower to do this query:
SELECT * FROM mytable
WHERE date(date_colum) BETWEEN '2011-01-01' AND ''2011-03-03';
Than this approach:
SELECT * FROM mytable
WHERE date_column BETWEEN '2011-01-01 00:00:00' AND '2011-03-03 23:59:59'