My SQL Idiosyncrasies That Bite OTN

MySQL Idiosyncrasies
that BITE
MySQL Idiosyncrasies That BITE - 2010.10
Title
Ronald Bradford
http://ronaldbradford.com
OTN LAD Tour 2010
South America
2010.10
Ronald Bradford
http://ronaldbradford.com
@RonaldBradford #OTN #LAOUC

Deﬁnitions
idiosyncrasy
[id-ee-uh-sing-kruh-see, -sin-] –noun,plural-sies.
A characteristic, habit, mannerism, or the
like, that is peculiar to ...
http://dictionary.com

About RDBMS - Oracle != MySQL
Product Age
Development philosophies
Target audience
Developer practices
Installed versions

Expected RDBMS Characteristics
Data Integrity
Transactions
ACID
Data Security
ANSI SQL

These RDBMS characteristics
are NOT enabled by default
in MySQL
WARNING !!!
(*)

MySQL Terminology
Storage Engines
MyISAM - Default
InnoDB - Default from MySQL 5.5
Transactions
Non Transactional Engine(s)
Transactional Engine(s)

1. Data Integrity

Data Integrity - Expected
CREATE TABLE orders (
qty TINYINT UNSIGNED NOT NULL,
created TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET latin1;
INSERT INTO orders(qty) VALUES (0), (100), (255);
SELECT * FROM orders;
+-----+---------------------+
| qty | created |
+-----+---------------------+
| 0 | 2010-10-25 12:22:22 |
| 100 | 2010-10-25 12:22:22 |
| 255 | 2010-10-25 12:22:22 |
+-----+---------------------+
✔

Data Integrity - Unexpected
mysql> INSERT INTO orders (qty) VALUES (-1), (9000);
mysql> SELECT * FROM orders;
+-----+---------------------+
| qty | created |
+-----+---------------------+
| 0 | 2010-10-25 12:22:22 |
| 100 | 2010-10-25 12:22:22 |
| 255 | 2010-10-25 12:22:22 |
| 0 | 2010-10-25 12:23:06 |
| 255 | 2010-10-25 12:23:06 |
+-----+---------------------+
//CREATE TABLE orders (
// qty TINYINT UNSIGNED NOT NULL,
✘

Data Integrity - Unexpected
mysql> SELECT * FROM orders;
+-----+---------------------+
| qty | created |
+-----+---------------------+
| 0 | 2010-10-25 12:22:22 |
| 100 | 2010-10-25 12:22:22 |
| 255 | 2010-10-25 12:22:22 |
| 0 | 2010-10-25 12:23:06 |
| 255 | 2010-10-25 12:23:06 |
+-----+---------------------+
//CREATE TABLE orders (
// qty TINYINT UNSIGNED NOT NULL,
✘
TINYINT is 1 byte.
UNSIGNED supports
values 0-255

Data Integrity - Unexpected (2)
mysql> INSERT INTO customers (name)
-> VALUES ('RONALD BRADFORD'),
-> ('FRANCISCO MUNOZ ALVAREZ');
mysql> SELECT * FROM customers;
+----------------------+
| name |
+----------------------+
| RONALD BRADFORD |
| FRANCISCO MUNOZ ALVA |
+----------------------+
2 rows in set (0.00 sec)
✘

Data Integrity - Warnings (2)
INSERT INTO customers (name) VALUES ('RONALD BRADFORD'),
('FRANCISCO MUNOZ ALVAREZ');
Query OK, 2 rows affected, 1 warning (0.29 sec)
+---------+------+-------------------------------------------+
+---------+------+-------------------------------------------+
| Warning | 1265 | Data truncated for column 'name' at row 2 |
+---------+------+-------------------------------------------+

Data Integrity with Dates - Warnings
mysql> INSERT INTO sample_date (d) VALUES ('2010-02-31');
Query OK, 1 row affected, 1 warning (0.00 sec)
+---------+------+----------------------------------------+
+---------+------+----------------------------------------+
| Warning | 1265 | Data truncated for column 'd' at row 1 |
+---------+------+----------------------------------------+
mysql> SELECT * FROM sample_date;
+------------+
| d |
+------------+
| 0000-00-00 |
+------------+
✘

SHOW WARNINGS
http://dev.mysql.com/doc/refman/5.1/en/show-warnings.html

Using SQL_MODE for Data Integrity
SQL_MODE =
STRICT_ALL_TABLES;
http://dev.mysql.com/doc/refman/5.1/en/server-sql-mode.html
Recommended
Conﬁguration

Data Integrity - Expected
mysql> SET SESSION SQL_MODE=STRICT_ALL_TABLES;
mysql> TRUNCATE TABLE sample_data;
mysql> INSERT INTO sample_data(i) VALUES (0), (100), (255);
mysql> INSERT INTO sample_data (i) VALUES (-1), (9000);
ERROR 1264 (22003): Out of range value for column 'i' at row 1
mysql> SELECT * FROM sample_data;
+-----+------+---------------------+
| i | c | t |
+-----+------+---------------------+
| 0 | NULL | 2010-06-06 14:39:48 |
| 100 | NULL | 2010-06-06 14:39:48 |
| 255 | NULL | 2010-06-06 14:39:48 |
+-----+------+---------------------+
✔

Data Integrity with Dates - Expected
mysql> SET SESSION SQL_MODE=STRICT_ALL_TABLES;
ERROR 1292 (22007): Incorrect date value: '2010-02-31' for
column 'd' at row 1
✔

Data Integrity with Dates - Unexpected
+------------+
| d |
+------------+
| 2010-00-00 |
| 2010-00-05 |
| 0000-00-00 |
+------------+
✘

Using SQL_MODE for Date Integrity
SQL_MODE =
STRICT_ALL_TABLES,
NO_ZERO_DATE,
NO_ZERO_IN_DATE;
Recommended
Conﬁguration

Data Integrity with Dates - Expected
mysql> SET SESSION
SQL_MODE='STRICT_ALL_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE';
mysql> TRUNCATE TABLE sample_date;
ERROR 1292 (22007): Incorrect date value: '2010-00-00' for column
'd' at row 1
'd' at row 1
'd' at row 1
Empty set (0.00 sec)
✔

2.Transactions

Transactions Example - Tables
DROP TABLE IF EXISTS parent;
CREATE TABLE parent (
id INT UNSIGNED NOT NULL AUTO_INCREMENT,
val VARCHAR(10) NOT NULL,
PRIMARY KEY (id),
UNIQUE KEY (val)
DROP TABLE IF EXISTS child;
CREATE TABLE child (
parent_id INT UNSIGNED NOT NULL,
created TIMESTAMP NOT NULL,
PRIMARY KEY (id),
INDEX (parent_id),
FOREIGN KEY (parent_id) REFERENCES parent(id)

Transactions Example - SQL
START TRANSACTION;
INSERT INTO parent(val) VALUES("a");
INSERT INTO child(parent_id,created)
VALUES(LAST_INSERT_ID(),NOW());
# Expecting an error with next SQL
INSERT INTO parent(val) VALUES("a");
ROLLBACK;
SELECT * FROM parent;
SELECT * FROM child;

Transactions Example - Expected
...
mysql> INSERT INTO parent (val) VALUES("a");
ERROR 1062 (23000): Duplicate entry 'a' for key 'val'
mysql> ROLLBACK;
mysql> SELECT * FROM parent;
mysql> SELECT * FROM child;
✔

Transactions Example - Tables
DROP TABLE IF EXISTS parent;
CREATE TABLE parent (
val VARCHAR(10) NOT NULL,
PRIMARY KEY (id),
UNIQUE KEY (val)
) ENGINE=MyISAM DEFAULT CHARSET latin1;
DROP TABLE IF EXISTS child;
CREATE TABLE child (
parent_id INT UNSIGNED NOT NULL,
created TIMESTAMP NOT NULL,
PRIMARY KEY (id),
INDEX (parent_id)
MyISAM is the current
default if not speciﬁed

Transactions Example - Unexpected
mysql> INSERT INTO parent (val) VALUES("a");
ERROR 1062 (23000): Duplicate entry 'a' for key 'val'
mysql> ROLLBACK;
mysql> SELECT * FROM parent;
+----+-----+
| id | val |
+----+-----+
| 1 | a |
+----+-----+
1 row in set (0.00 sec)
mysql> SELECT * FROM child;
+----+-----------+---------------------+
| id | parent_id | created |
+----+-----------+---------------------+
| 1 | 1 | 2010-06-03 13:53:38 |
+----+-----------+---------------------+
✘

Transactions Solution
mysql> SET GLOBAL storage_engine=InnoDB;
# my.cnf
[mysqld]
default-storage-engine=InnoDB
# NOTE: Requires server restart
$ /etc/init.d/mysqld stop
$ /etc/init.d/mysqld start
http://dev.mysql.com/doc/refman/5.1/en/server-options.html#option_mysqld_default-storage-engine
Recommended
Conﬁguration

Why use Non Transactional Tables?
No transaction overhead
Much faster
Less disk writes
Lower disk space requirements
Less memory requirements
http://dev.mysql.com/doc/refman/5.1/en/storage-engine-compare-transactions.html

3.Variable Scope

Variable Scope Example
mysql> CREATE TABLE test1(id INT UNSIGNED NOT NULL);
mysql> SHOW CREATE TABLE test1G
CREATE TABLE `test1` (
`id` int(10) unsigned NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1
MyISAM is the current
default if not speciﬁed

+----------------+--------+
+----------------+--------+
+----------------+--------+
mysql> DROP TABLE test1;
mysql> SHOW CREATE TABLE test1G
CREATE TABLE `test1` (
✘

+----------------+--------+
+----------------+--------+
+----------------+--------+

+----------------+--------+
+----------------+--------+
+----------------+--------+
mysql> SHOW SESSION VARIABLES LIKE 'storage_engine';
+----------------+--------+
+----------------+--------+
+----------------+--------+

+----------------+--------+
+----------------+--------+
+----------------+--------+
mysql> SHOW SESSION VARIABLES LIKE 'storage_engine';
+----------------+--------+
+----------------+--------+
+----------------+--------+
MySQLVariables have
two scopes.
SESSION & GLOBAL

Variable Scope Syntax
SHOW [GLOBAL | SESSION]VARIABLES;
SET [GLOBAL | SESSION] variable = value;
Default is GLOBAL since 5.0.2
http://dev.mysql.com/doc/refman/5.1/en/user-variables.html
http://dev.mysql.com/doc/refman/5.1/en/set-option.html

Variable Scope Precautions
Persistent connections (e.g. Java)
Replication SQL_THREAD
Be careful of existing
connections!

4. Storage Engines

Storage Engines Example - Creation
CREATE TABLE test1 (
id INT UNSIGNED NOT NULL
SHOW CREATE TABLE test1G
*************************** 1. row *************
Table: test1
Create Table: CREATE TABLE `test1` (
✘

Storage Engines Example - Creation
SHOW CREATE TABLE test1G
*************************** 1. row *************
Table: test1
Create Table: CREATE TABLE `test1` (
✘
Even speciﬁed the
storage engine reverted
to the default

Storage Engines Example - Unexpected
SHOW WARNINGS;
+---------+------+-----------------------------------------------+
+---------+------+-----------------------------------------------+
| Warning | 1286 | Unknown table engine 'InnoDB' |
| Warning | 1266 | Using storage engine MyISAM for table 'test1' |
+---------+------+-----------------------------------------------+

Storage Engines Example - Unexpected (2)
mysql> CREATE TABLE test1
) ENGINE=InnDB DEFAULT CHARSET latin1;
+---------+------+-----------------------------------------------+
+---------+------+-----------------------------------------------+
| Warning | 1286 | Unknown table engine 'InnDB' |
| Warning | 1266 | Using storage engine MyISAM for table 'test1' |
+---------+------+-----------------------------------------------+
✘
Be careful of spelling
mistakes as well.

Using SQL_MODE for Storage Engines
SQL_MODE =
STRICT_ALL_TABLES,
NO_ZERO_DATE,
NO_ZERO_IN_DATE,
NO_ENGINE_SUBSTITUTION;
Recommended
Conﬁguration

Storage Engines Example - Solution
SET SESSION SQL_MODE=NO_ENGINE_SUBSTITUTION;
DROP TABLE IF EXISTS test3;
ERROR 1286 (42000): Unknown table engine 'InnoDB'
✔

5.ACID

Atomicity -
All or nothing

Atomicity - Table
DROP TABLE IF EXISTS accounts;
CREATE TABLE accounts(
username VARCHAR(10) NOT NULL,
PRIMARY KEY (id),
UNIQUE KEY (username)

Atomicity - Data
INSERT INTO accounts (username)
VALUES('jane'),('jo'),('joe'),('joel'),('jame'),
('jamel');
mysql> SELECT * FROM accounts;
+----+----------+
| id | username |
+----+----------+
| 1 | jane |
| 2 | jo |
| 3 | joe |
| 4 | joel |
| 5 | jame |
| 6 | jamel |
+----+----------+

Atomicity - Unexpected
mysql> UPDATE accounts SET username=CONCAT(username,'l')
ERROR 1062 (23000): Duplicate entry 'joel' for key
'username'
+----+----------+
| id | username |
+----+----------+
| 1 | janel |
| 2 | jol |
| 3 | joe |
| 4 | joel |
| 5 | jame |
| 6 | jamel |
+----+----------+
✘

Atomicity - Unexpected (2)
mysql> ROLLBACK;
Query OK, 0 rows affected (0.00 sec)
+----+----------+
| id | username |
+----+----------+
| 1 | janel |
| 2 | jol |
| 3 | joe |
| 4 | joel |
...
✘

Atomicity - Unexpected (2)
mysql> ROLLBACK;
+----+----------+
| id | username |
+----+----------+
| 1 | janel |
| 2 | jol |
| 3 | joe |
| 4 | joel |
...
✘
Rollback has no effect
on non transactional
tables

Atomicity - Transactional Table
PRIMARY KEY (id),
INSERT INTO accounts (username) VALUES('jane'),('jo')...
UPDATE accounts SET username=CONCAT(username,'l');
ERROR 1062 (23000): Duplicate entry 'joel' for key 'username'
SELECT * FROM accounts;
+----+----------+
| id | username |
+----+----------+
| 5 | jame |
| 6 | jamel |
| 1 | jane |
| 2 | jo |
| 3 | joe |
| 4 | joel |
✔

PRIMARY KEY (id),
INSERT INTO accounts (username) VALUES('jane'),('jo')...
UPDATE accounts SET username=CONCAT(username,'l');
ERROR 1062 (23000): Duplicate entry 'joel' for key 'username'
SELECT * FROM accounts;
+----+----------+
| id | username |
+----+----------+
| 5 | jame |
| 6 | jamel |
| 1 | jane |
| 2 | jo |
| 3 | joe |
| 4 | joel |
✔
Data remains unchanged

mysql> ROLLBACK;
+----+----------+
| id | username |
+----+----------+
| 5 | jame |
| 6 | jamel |
| 1 | jane |
| 2 | jo |
| 3 | joe |
| 4 | joel |
+----+----------+
✘

mysql> ROLLBACK;
+----+----------+
| id | username |
+----+----------+
| 5 | jame |
| 6 | jamel |
| 1 | jane |
| 2 | jo |
| 3 | joe |
| 4 | joel |
+----+----------+
✘
Data remains, INSERT
was not rolled back?

Atomicity - The culprit
mysql> SHOW GLOBAL VARIABLES LIKE 'autocommit';
+---------------+-------+
+---------------+-------+
| autocommit | ON |
+---------------+-------+

Atomicity - The culprit
mysql> SHOW GLOBAL VARIABLES LIKE 'autocommit';
+---------------+-------+
+---------------+-------+
| autocommit | ON |
+---------------+-------+
The default for
autocommit is ON.
Great care is needed to
change the default.

Atomicity - Recommendations
Always wrap SQL in transactions
START TRANSACTION | BEGIN [WORK]
COMMIT
Recommended
Practice

Isolation -
Transactions do not affect
other transactions

Transaction Isolation - Levels
READ-UNCOMMITTED
READ-COMMITTED
REPEATABLE-READ
SERIALIZABLE
http://dev.mysql.com/doc/refman/5.1/en/set-transaction.html
http://ronaldbradford.com/blog/understanding-mysql-innodb-transaction-isolation-2009-09-24/
MySQL supports 4
isolation levels

Transaction Isolation - Default
The default is
REPEATABLE-READ
SET GLOBAL tx_isolation = 'READ-COMMITTED';
This is a mistake
http://ronaldbradford.com/blog/dont-assume-common-terminology-2010-03-03/

Transaction Isolation - Default
The default is
REPEATABLE-READ
SET GLOBAL tx_isolation = 'READ-COMMITTED';
This is a mistake
http://ronaldbradford.com/blog/dont-assume-common-terminology-2010-03-03/
Common mistake by
Oracle DBA's
supporting MySQL

Oracle READ COMMITTED
!=
MySQL READ-COMMITTED

Transaction Isolation - Binary Logging Errors
SET GLOBAL tx_isolation='READ-COMMITTED';
mysql> insert into test1 values (1,'x');
ERROR 1598 (HY000): Binary logging not possible.
Message: Transaction level 'READ-COMMITTED' in InnoDB is
not safe for binlog mode 'STATEMENT'
✘
As of 5.1 this requires
further conﬁguration
considerations

Durability -
No committed
transactions are lost

Data Safety
MyISAM writes/flushes data every statement
Does not flush indexes (*)
InnoDB can relax durability
flush all transactions per second
innodb_flush_log_at_trx_commit
sync_binlog
innodb_support_xa

Auto Recovery
InnoDB has it
MyISAM does not - (*) Indexes
MYISAM-RECOVER=FORCE,BACKUP
REPAIR TABLE
myisamchk

Recovery Time
InnoDB is consistent
Redo Log ﬁle size
Slow performance in Undo (*)
MyISAM grows with data volume

InnoDB Auto Recovery Example
InnoDB: Log scan progressed past the checkpoint lsn 0 188755039
100624 16:37:44 InnoDB: Database was not shut down normally!
InnoDB: Starting crash recovery.
InnoDB: Reading tablespace information from the .ibd files...
InnoDB: Restoring possible half-written data pages from the doublewrite
InnoDB: buffer...
InnoDB: Doing recovery: scanned up to log sequence number 0 193997824
InnoDB: Doing recovery: scanned up to log sequence number 0 195151897
InnoDB: 1 transaction(s) which must be rolled back or cleaned up
InnoDB: in total 105565 row operations to undo
InnoDB: Trx id counter is 0 18688
100624 16:37:45 InnoDB: Starting an apply batch of log records to the
database...
InnoDB: Progress in percents: 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
19 ... 99
InnoDB: Apply batch completed
InnoDB: Last MySQL binlog file position 0 12051, file name ./binary-log.
000003
InnoDB: Starting in background the rollback of uncommitted transactions
...
✔

MyISAM Repair needed Example
100126 22:44:35 [ERROR] /var/lib/mysql5/bin/mysqld: Table './XXX/
descriptions' is marked as crashed and should be repaired
100126 22:44:35 [Warning] Checking table: './XXX/descriptions'
100126 22:44:35 [ERROR] /var/lib/mysql5/bin/mysqld: Table './XXX/taggings'
is marked as crashed and should be repaired
100126 22:44:35 [Warning] Checking table: './XXX/taggings'
100126 22:44:35 [ERROR] /var/lib/mysql5/bin/mysqld: Table './XXX/vehicle'
is marked as crashed and should be repaired
✘

6. Data Security

User Privileges - Practices
CREATE USER goodguy@localhost IDENTIFIED BY 'sakila';
GRANT CREATE,SELECT,INSERT,UPDATE,DELETE ON odtug.* TO
goodguy@localhost;
CREATE USER superman@'%';
GRANT ALL ON *.* TO superman@'%';
Best Practice
Normal Practice
http://dev.mysql.com/doc/refman/5.1/en/create-user.html
http://dev.mysql.com/doc/refman/5.1/en/grant.html
✔
✘

User Privileges - Normal Bad Practice
GRANT ALL ON *.* TO user@’%’
*.* gives you access to all tables in all schemas
@’%’ give you access from any external location
ALL gives you
ALTER,ALTER ROUTINE, CREATE, CREATE ROUTINE, CREATE TEMPORARY
TABLES, CREATE USER, CREATEVIEW, DELETE, DROP, EVENT, EXECUTE, FILE,
INDEX, INSERT, LOCK TABLES, PROCESS, REFERENCES, RELOAD, REPLICATION
CLIENT, REPLICATION SLAVE, SELECT, SHOW DATABASES, SHOWVIEW,
SHUTDOWN, SUPER,TRIGGER, UPDATE, USAGE

User Privileges - Why SUPER is bad
SUPER
Bypasses read_only
Bypasses init_connect
Can Disable binary logging
Change conﬁguration dynamically
No reserved connection

SUPER and Read Only
$ mysql -ugoodguy -psakila odtug
mysql> insert into test1(id) values(1);
ERROR 1290 (HY000): The MySQL server is running with the
--read-only option so it cannot execute this statement ✔
$ mysql -usuperman odtug
Query OK, 1 row affected (0.01 sec)
✘

SUPER and Read Only
ERROR 1290 (HY000): The MySQL server is running with the
--read-only option so it cannot execute this statement ✔
Query OK, 1 row affected (0.01 sec)
✘
Data inconsistency
now exists

SUPER and Init Connect
#my.cnf
[client]
init_connect=SET NAMES utf8
This specifies to use UTF8 for communication with client
and data
Common conﬁguration
practice to support
UTF8

SUPER and Init Connect (2)

mysql> SHOW SESSION VARIABLES LIKE
'character%';
+--------------------------+----------+
+--------------------------+----------+
| character_set_client | latin1 |
| character_set_connection | latin1 |
| character_set_results | latin1 |
+--------------------------+----------+
+--------------------------+----------+
+--------------------------+----------+
+--------------------------+----------+
✔ ✘

mysql> SHOW SESSION VARIABLES LIKE
'character%';
+--------------------------+----------+
+--------------------------+----------+
| character_set_client | latin1 |
| character_set_connection | latin1 |
| character_set_results | latin1 |
+--------------------------+----------+
+--------------------------+----------+
+--------------------------+----------+
+--------------------------+----------+
✔ ✘
Data integrity can be
compromised

SUPER and Binary Log
mysql> SHOW MASTER STATUS;
+-------------------+----------+--------------+------------------+
| binary-log.000017 | 4488 | | |
+-------------------+----------+--------------+------------------+
mysql> DELETE FROM sales WHERE id=23;
mysql> SET SQL_LOG_BIN=0;
mysql> SET SQL_LOG_BIN=1;
mysql> SHOW MASTER STATUS;
+-------------------+----------+--------------+------------------+
| File | Position | Binlog_Do_DB | Binlog_Ignore_DB |
+-------------------+----------+--------------+------------------+
| binary-log.000017 | 4580 | | |
+-------------------+----------+--------------+------------------+

SUPER and Binary Log (2)
# at 4488
#101025 12:57:24 server id 1 end_log_pos 4580 Query
thread_id=1 exec_time=0 error_code=0
SET @@session.lc_time_names=0/*!*/;
SET @@session.collation_database=DEFAULT/*!*/;
DELETE FROM sales WHERE id=23
/*!*/;
# at 4580
#101025 13:01:26 server id 1 end_log_pos 4672 Query
thread_id=1 exec_time=0 error_code=0
SET TIMESTAMP=1288018886/*!*/;
DELETE FROM sales WHERE id=99
/*!*/;
DELIMITER ;
✘
Data auditability is now
compromised

SUPER and reserved connection
$ mysql -uroot
mysql> show global variables like 'max_connections';
+-----------------+-------+
+-----------------+-------+
| max_connections | 3 |
+-----------------+-------+
mysql> show global status like 'threads_connected';
+-------------------+-------+
+-------------------+-------+
| Threads_connected | 4 |
+-------------------+-------+
✔
When connected
correctly, 1 connection
is reserved for SUPER

7. ANSI SQL

Group By Example
SELECT country, COUNT(*) AS color_count
FROM flags
GROUP BY country;
+-----------+-------------+
| country | color_count |
+-----------+-------------+
| Australia | 3 |
| Canada | 2 |
| Japan | 2 |
| Sweden | 2 |
| USA | 3 |
+-----------+-------------+
SELECT country, COUNT(*)
FROM flags;
+-----------+----------+
| country | COUNT(*) |
+-----------+----------+
| Australia | 12 |
+-----------+----------+
✔
✘

Group By Example (2)
SET SESSION sql_mode=ONLY_FULL_GROUP_BY;
SELECT country, COUNT(*)
FROM flags;
ERROR 1140 (42000): Mixing of GROUP columns (MIN(),MAX(),COUNT(),...)
with no GROUP columns is illegal if there is no GROUP BY clause
http://ExpertPHPandMySQL.com Chapter 1 - Pg 31
✔

Using SQL_MODE for ANSI SQL
SQL_MODE =
STRICT_ALL_TABLES,
NO_ZERO_DATE,
NO_ZERO_IN_DATE,
NO_ENGINE_SUBSTITUTION,
ONLY_FULL_GROUP_BY;
Recommended
Conﬁguration

8. Case Sensitivity

Case Sensitivity String Comparison
SELECT 'OTN' = UPPER('otn');
+----------------------+
| 'OTN' = UPPER('otn') |
+----------------------+
| 1 |
+----------------------+
SELECT 'OTN' = 'OTN' AS same, 'OTN' = 'Otn' as initcap, 'OTN' = 'otn' as
lowercase, 'OTN' = 'otn' COLLATE latin1_general_cs AS different;
+------+---------+-----------+-----------+
| same | initcap | lowercase | different |
+------+---------+-----------+-----------+
| 1 | 1 | 1 | 0 |
+------+---------+-----------+-----------+
SELECT name, address, email
FROM customer
WHERE name = UPPER('bradford')
✘This practice is
unnecessary as does not
utilize index if exists

Case Sensitivity Tables
# Server 1
mysql> INSERT INTO test1 VALUES(1);
mysql> INSERT INTO TEST1 VALUES(2);
# Server 2
mysql> INSERT INTO test1 VALUES(1);
mysql> INSERT INTO TEST1 VALUES(2);
ERROR 1146 (42S02): Table 'test.TEST1' doesn't exist

Case Sensitivity Tables (2) - Operating Speciﬁc Settings
# Server 1 - Mac OS X / Windows Default
mysql> SHOW GLOBAL VARIABLES LIKE 'lower%';
+------------------------+-------+
+------------------------+-------+
| lower_case_file_system | ON |
| lower_case_table_names | 2 |
+------------------------+-------+
# Server 2 - Linux Default
mysql> SHOW GLOBAL VARIABLES LIKE 'lower%';
+------------------------+-------+
+------------------------+-------+
| lower_case_file_system | OFF |
| lower_case_table_names | 0 |
+------------------------+-------+
Be careful.This default
varies depending on
Operating System

Other Gotchas

Other features the BITE
Character Sets
Data/Client/Application/Web
Sub Queries
Rewrite as JOIN when possible
Views
No always efﬁcient
Cascading Conﬁguration Files
SQL_MODE's not to use

Conclusion

DON'T
ASSUME

Conclusion - SQL_MODE
SQL_MODE =
ALLOW_INVALID_DATES,ANSI_QUOTES, ERROR_FOR_DIVISION_ZERO,
HIGH_NOT_PRECEDENCE,IGNORE_SPACE,NO_AUTO_CREATE_USER,
NO_AUTO_VALUE_ON_ZERO,NO_BACKSLASH_ESCAPES,
NO_DIR_IN_CREATE,NO_ENGINE_SUBSTITUTION,
NO_FIELD_OPTIONS,NO_KEY_OPTIONS,NO_TABLE_OPTIONS,
NO_UNSIGNED_SUBTRACTION,NO_ZERO_DATE,
NO_ZERO_IN_DATE,ONLY_FULL_GROUP_BY (5.1.11),
PAD_CHAR_TO_FULL_LENGTH (5.1.20), PIPES_AS_CONCAT,
REAL_AS_FLOAT,STRICT_ALL_TABLES, STRICT_TRANS_TABLES
ANSI, DB2, MAXDB, MSSQL, MYSQL323, MYSQL40, ORACLE,
POSTGRESQL,TRADITIONAL
✔

Conclusion - SQL_MODE
SQL_MODE =
ALLOW_INVALID_DATES,ANSI_QUOTES, ERROR_FOR_DIVISION_ZERO,
HIGH_NOT_PRECEDENCE,IGNORE_SPACE,NO_AUTO_CREATE_USER,
NO_AUTO_VALUE_ON_ZERO,NO_BACKSLASH_ESCAPES,
NO_DIR_IN_CREATE,NO_ENGINE_SUBSTITUTION,
NO_FIELD_OPTIONS,NO_KEY_OPTIONS,NO_TABLE_OPTIONS,
NO_UNSIGNED_SUBTRACTION,NO_ZERO_DATE,
NO_ZERO_IN_DATE,ONLY_FULL_GROUP_BY (5.1.11),
PAD_CHAR_TO_FULL_LENGTH (5.1.20), PIPES_AS_CONCAT,
REAL_AS_FLOAT,STRICT_ALL_TABLES, STRICT_TRANS_TABLES
ANSI, DB2, MAXDB, MSSQL, MYSQL323, MYSQL40, ORACLE,
POSTGRESQL,TRADITIONAL
✔
✘
?

RonaldBradford.com
MySQL4OracleDBA.com

My SQL Idiosyncrasies That Bite OTN

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