[오라클]MySQL Vs. Oracle

2 Oracle and MySQL Compared

This chapter compares the MySQL database and the Oracle database. It includes the following sections:

• Database Security

• Schema Migration

• Data Types

• Data Storage Concepts

2.1 Database Security

This section includes information about security issues with MySQL databases and Oracle databases.

As with Oracle, MySQL users are maintained by the database. MySQL uses a set of grant tables to keep track of users and the privileges that they can have. MySQL uses these grant tables when performing authentication, authorization and access control for users.

2.1.1 Database Authentication

Unlike Oracle (when set up to use database authentication) and most other databases that use only the user name and password to authenticate a user, MySQL uses an additional location parameter when authenticating a user. This location parameter is usually the host name, IP address, or a wildcard (Ò%Ó). With this additional parameter, MySQL may further restrict a user access to the database to a particular host or hosts in a domain. Moreover, this also allows a different password and set of privileges to be enforced for a user depending on the host from which the connection is made. Thus, user scott, who logs on from abc.com may or may not the same as user scott who logs on from xyz.com.

2.1.2 Privileges

The MySQL privilege system is a hierarchical system that works through inheritance. Privileges granted at a higher level are implicitly passed down to all lower levels and may be overridden by the same privileges set at lower levels. MySQL allows privileges to be granted at five different levels, in descending order of the scope of the privileges:

• Global

• Per-host basis

• Database-level

• Table-specific

• Column-specific (single column in a single table

Each level has a corresponding grant table in the database. When performing a privilege check, MySQL checks each of the tables in descending order of the scope of the privileges, and the privileges granted at a lower level take precedence over the same privileges granted at a higher level.

The privileges supported by MySQL are grouped into two types: administrative privileges and per-object privileges. The administrative privileges are global privileges that have server-wide effects and are concerned with the functioning of MySQL. These administrative privileges include the FILE, PROCESS, REPLICATION, SHUTDOWN and SUPER privilege. The per-object privileges affect database objects such tables, columns, indexes, and stored procedures, and can be granted with a different scope. These per-object privileges are named after the SQL queries that trigger their checks.

Unlike in Oracle, there is no concept of role in MySQL. Thus, in order to grant a group of users the same set of privileges, the privileges have to be granted to each user separately. Alternately, though less satisfactory for auditing, users performing tasks as a role may all share a single user account that is designated for the "role" and with the required privileges granted.

2.2 Schema Migration

The schema contains the definitions of the tables, views, indexes, users, constraints, stored procedures, triggers, and other database-specific objects. Most relational databases work with similar objects.

This section contains the following:

• Schema Object Similarities

• Schema Object Names

• Table Design Considerations

• Migrating Multiple Databases

• Schema Migration Considerations for MySQL

2.2.1 Schema Object Similarities

There are many similarities between schema objects in Oracle and MySQL. However, some schema objects differ between these databases. For more information about schema objects, see Oracle SQL Reference.

Table 2-1 shows the differences between Oracle and MySQL.

Table 2-1 Schema Objects in Oracle and MySQL

Oracle	MySQL
AFTER trigger	trigger
BEFORE trigger	trigger
Check constraint	Check constraint
Column default	Column default
Database	Database
Foreign key	Foreign key
Index	Index
Package	N/A
PL/SQL function	Routine
PL/SQL procedure	Routine
Primary key	Primary key
Role	N/A
Schema	Schema
Sequence	AUTO_INCREMENT for a column
Snapshot	N/A
Synonym	N/A
Table	Table
Tablespace	N/A
Temporary table	Temporary table
Trigger for each row	Trigger for each row
Unique key	Unique key
User	User
View	View

2.2.2 Schema Object Names

Oracle is case insensitive to object names, and Oracle schema object names are stored as uppercase.

As in Oracle, column, index, stored procedure, and trigger names as well as column aliases in MySQL are case insensitive on all platforms. However, the case sensitivity of database and tables names for MySQL differs from Oracle. In MySQL, databases correspond to directories within the data directory, and tables correspond to one or more files within the database directory. As such, the case sensitivity of the database and table names is determined by the case sensitivity of the underlying operating systems. This means that database and table names are not case-sensitive in Windows and are case-sensitive in most varieties of Unix. However, MySQL allows users to determine how the database and table names are stored on disk and in their use in MySQL through the lower_case_table_names system variable. Table aliases are case-sensitive in releases before MySQL 4.1.1.

Both Oracle and MySQL let you use reserved words as object names by representing the name with a quoted identifier. However, MySQL allows some reserved words such as DATE and TIMESTAMP to be used as unquoted identifier for object names, although this is not allowed in Oracle. SQL Developer appends an underscore (_) to the name of a MySQL object that is an Oracle reserved word.

MySQL and Oracle have some minor differences in their definition of an identifier. In MySQL, an unquoted identifier may begin with a digit, and double quotation marks are allowed in a quoted identifier; however, neither of these is allowed in an Oracle identifier. In MySQL, the quote character is the backtick (`). If the SQL mode ANSI_QUOTES is set, double quotes can also be used to quote the identifiers. In Oracle, identifiers are quoted using double quotation marks.

You should choose a schema object name that is unique by case and by at least one other characteristic, and ensure that the object name is not a reserved word from either database.

2.2.3 Table Design Considerations

This section discusses table design issues that you need to consider when converting MySQL databases to Oracle. This section includes the following­:

• Character Data Types

• Column Default Value

2.2.3.1 Character Data Types

MySQL and Oracle have some differences in the character types that they support and in the way they store and retrieve the character type values.

MySQL supports the CHAR and VARCHAR type for character type with a length that is less than 65,535 bytes. The CHAR type can have a maximum length of 255 bytes, and as of MySQL 3.23 it may also be declared with a length of 0 byte. Before MySQL 5.0.3, the length specification for the VARCHAR type is the same as the CHAR type. From MySQL 5.0.3 on, the maximum length for the VARCHAR type is 65,535 bytes. Oracle supports four character types: CHAR, NCHAR, NVARCHAR2 and VARCHAR2. The minimum length that can be declared for all Oracle character types is 1 byte. The maximum size allowed for CHAR and NCHAR is 2,000 bytes, and for NVARCHAR2 and VARCHAR2 it is 4,000 bytes.

MySQL CHAR values are right-padded with spaces to the specified length when they are stored, and trailing spaces are removed when the values are retrieved. On the other hand, VARCHAR values are stored using as many characters as are given, but before MySQL 5.0.3 trailing spaces are removed when the values are stored and retrieved. Oracle blank-pads the value for its CHAR and NCHAR type to the column length if the value is shorter than the column length, and trailing spaces are not removed on retrieval. For NVARCHAR2 and VARVHAR2 data type columns, Oracle stores and retrieves the value exactly as is given, including trailing spaces.

If a value is assigned to a character type column that exceeds its specified length, MySQL truncates the value and does not generate an error unless the STRICT SQL mode is set. Oracle generates an error if the value assigned to a character type column exceeds its specified length.

In MySQL, every character type (CHAR, VARCHAR, and TEXT) column has a column character set and collation. If the character set or collation is not explicitly defined in the column definition, the table character set or collation is implied if specified; otherwise, the database character or collation is chosen. In Oracle, the character set for CHAR and VARCHAR2 types is defined by the database character set, and for the character set for NCHAR and NVARCHAR types is defined the national character set.

When declaring a CHAR or VARCHAR type in MySQL, the default length semantics is characters instead of bytes for MySQL 4.1 and later. In Oracle, the default length semantics is bytes for CHAR and VARCHAR2 types and characters for NCHAR and NVARCHAR2 types.

SQL Developer will map MySQL CHAR and VARCHAR types to Oracle CHAR and VARCHAR2 types, respectively. SQL Developer will determine the maximum number of bytes for the Oracle CHAR and VARCHAR2 data type columns from the number of bytes required to hold the maximum length specified for the corresponding MySQL CHAR and VARCHAR data type columns. If the MySQL VARCHAR2 column is such that the data exceeds 4000 bytes, convert the column to an Oracle CLOB data type column.

2.2.3.2 Column Default Value

MySQL differs from Oracle in the way it handles default value for a column that does not allow NULL value.

In MySQL, for a column that does not allow NULL value and for which no data is provided for the column when data is inserted into the table, MySQL determines a default value for the column. This default value is the implicit default value for the column data type. However, if the strict mode is enabled, MySQL generates errors, and for transactional tables it rolls back the insert statement.

In Oracle, when data is inserted into a table, data must be provided for all columns that do not allow NULL value. Oracle does not generate a default value for columns that have the NOT NULL constraint.

2.2.4 Migrating Multiple Databases

SQL Developer supports the migration of multiple MySQL databases if they are on the same MySQL database server.

2.2.5 Schema Migration Considerations for MySQL

Schema migration considerations for MySQL apply in the following areas"

• Databases

• Mapping MySQL Global and Database-Level Privileges to Oracle System Privileges

• Temporary Tables

• Owner of Schema Objects

2.2.5.1 Databases

When migrating MySQL databases to Oracle, SQL Developer maps each MySQL database to a tablespace in Oracle. Database objects, such as tables, indexes and views are stored in the respective tablespaces and are referenced from the Oracle schema for the user that owns them.

2.2.5.2 Mapping MySQL Global and Database-Level Privileges to Oracle System Privileges

SQL Developer does not process all the administrative privileges on MySQL, except the SUPER privilege. Table 2-2 shows the mappings for MySQL per-object privileges granted at the different levels as well as the SUPER privilege granted at the global level.

Table 2-2 MySQL Privileges and Oracle System Privileges

Level	Privilege	System Privilege(s) on Oracle
Global	ALTER	ALTER ANY TABLE, ALTER ANY SEQUENCE, ALTER ANY CUSTER, COMMENT ANY TABLE
Global	ALTER ROUTINE	ALTER ANY PROCEDURE, DROP ANY PROCEDURE
Global	CREATE	CREATE ANY TABLE, CREATE ANY SEQUENCE, CREATE ANY CLUSTER, CREATE DATABASE LINK, COMMENT ANY TABLE
Global	CREATE ROUTINE	CREATE ANY PROCEDURE
Global	CREATE USER	CREATE USER, GRANT ANY PRIVILEGE
Global	CREATE VIEW	CREATE ANY VIEW
Global	DELETE	ALTER ANY TABLE, DROP USER, DELETE ANY TABLE
Global	DROP	DROP ANT TABLE, DROP ANY SEQUENCE, DROP ANY CLUSTER, DROP ANY VIEW
Global	EXECUTE	EXECUTE ANY PROCEDURE
Global	INDEX	CREATE ANY INDEX, ALTER ANY INDEX, DROP ANY INDEX
Global	INSERT	INSERT ANY TABLE
Global	LOCK TABLES	LOCK ANY TABLE
Global	SELECT	SELECT ANY TABLE
Global	SUPER	CREATE ANY TRIGGER, DROP ANY TRIGGER
Global	UPDATE	UPDATE ANY TABLE
Global	USAGE	CREATE SESSION, ALTER SESSION, UNLIMITED TABLESPACE
Database	CREATE	CREATE CLUSTER, CREATE DATABASE LINK, CREATE SEQUENCE, CREATE TABLE
Database	CREATE ROUTINE	CREATE PROCEDURE
Database	CREATE VIEW	CREATE VIEW
Table	CREATE	CREATE TABLE
Table	CREATE VIEW	CREATE VIEW

2.2.5.3 Temporary Tables

SQL Developer does not support the migration of temporary tables.

In MySQL, temporary tables are database objects that are visible only to the current user session and are automatically dropped when the user session ends.

The definition of temporary tables in Oracle differs slightly from MySQL, in that the temporary tables, once created, exist until they are explicitly dropped and they are visible to all sessions with appropriate privileges. However, the data in the temporary tables is visible only to the user session that inserts the data into the table, and the data may persist for the duration of a transaction or a user session.

2.2.5.4 Owner of Schema Objects

SQL Developer creates an Oracle schema for the MySQL root user that owns, for all databases to be migrated, all database objects except stored procedures. For stored procedures, the MySQL users that created them remain the owner. SQL Developer creates an Oracle schema for each MySQL user that is migrated.

2.3 Data Types

This section describes the data types used within Oracle. It shows the MySQL data types and the Oracle equivalent. It includes information about the following:

• Supported Oracle Data Types

• Default Data Type Mappings

• Comparing Data Types

2.3.1 Supported Oracle Data Types

Table 2-3 describes the Oracle data types supported by Oracle SQL Developer.

Table 2-3 Supported Oracle Data Types

Data Type	Description
BLOB	A binary large object. Maximum size is 4 gigabytes.
CHAR (SIZE)	Fixed-length character data of length size bytes. Maximum size is 2000 bytes. Default and minimum size is 1 byte.
CLOB	A character large object containing single-byte characters. Both fixed-width and variable-width character sets are supported, both using the CHAR database character set. Maximum size is 4 gigabytes.
DATE	The DATE data type stores date and time information. Although date and time information can be represented in both CHAR and NUMBER data types, the DATE data type has special associated properties. For each DATE value, Oracle stores the following information: century, year, month, day, hour, minute, and second.
FLOAT	Specifies a floating-point number with decimal precision 38, or binary precision 126.
LONG (SIZE)	Character data of variable length up to 2 gigabytes, or 2^31 -1 bytes.
LONG RAW	Raw binary data of variable length up to 2 gigabytes.
NCHAR (SIZE)	Fixed-length character data of length size characters or bytes, depending on the choice of national character set. Maximum size is determined by the number of bytes required to store each character, with an upper limit of 2000 bytes. Default and minimum size is 1 character or 1 byte, depending on the character set.
NCLOB	A character large object containing multibyte characters. Both fixed-width and variable-width character sets are supported, both using the NCHAR database character set. Maximum size is 4 gigabytes. Stores national character set data.
NUMBER	Number having precision p and scale s. The precision p can range from 1 to 38. The scale s can range from -84 to 127.
NVARCHAR2 (SIZE)	Variable-length character string having maximum length size characters or bytes, depending on the choice of national character set. Maximum size is determined by the number of bytes required to store each character, with an upper limit of 4000 bytes. You must specify size for NVARCHAR2.
RAW (SIZE)	Raw binary data of length size bytes. Maximum size is 2000 bytes. You must specify size for a RAW value.
VARCHAR (SIZE)	The VARCHAR data type is currently synonymous with the VARCHAR2 data type. Oracle recommends that you use VARCHAR2 rather than VARCHAR. In the future, VARCHAR might be defined as a separate data type used for variable-length character strings compared with different comparison semantics. The maximum size is 4000 and the minimum of 1 is the default.
BINARY_DOUBLE	A 64-bit, double-precision floating-point number data type.
BINARY_FLOAT	A 32-bit, single-precision floating-point number data type.

For more information about Oracle data types, see Oracle Database SQL Language Reference.

2.3.2 Default Data Type Mappings

Table 2-4 shows the default settings used by SQL Developer to convert data types from MySQL to Oracle. SQL Developer enables you to change the default setting for certain data types by specifying an alternative type. For information about changing the default data type mappings, see the SQL Developer online help.

Table 2-4 Default Data Type Mappings Used by Oracle SQL Developer

MySQL Data Type	Oracle Data Type
BIGINT	NUMBER(19, 0)
BIT	RAW
BLOB	BLOB, RAW
CHAR	CHAR
DATE	DATE
DATETIME	DATE
DECIMAL	FLOAT (24)
DOUBLE	FLOAT (24)
DOUBLE PRECISION	FLOAT (24)
ENUM	VARCHAR2
FLOAT	FLOAT
INT	NUMBER(10, 0)
INTEGER	NUMBER(10, 0)
LONGBLOB	BLOB, RAW
LONGTEXT	CLOB, RAW
MEDIUMBLOB	BLOB, RAW
MEDIUMINT	NUMBER(7, 0)
MEDIUMTEXT	CLOB, RAW
NUMERIC	NUMBER
REAL	FLOAT (24)
SET	VARCHAR2
SMALLINT	NUMBER(5, 0)
TEXT	VARCHAR2, CLOB
TIME	DATE
TIMESTAMP	DATE
TINYBLOB	RAW
TINYINT	NUMBER(3, 0)
TINYTEXT	VARCHAR2
VARCHAR	VARCHAR2, CLOB
YEAR	NUMBER

Note:

The ENUM and SET data types have no direct mapping in Oracle. SQL Developer maps ENUM columns in MySQL to VARCHAR2 columns in Oracle. It then adds a constraint and a trigger to those columns to ensure that only values that were allowed by the ENUM data type are allowed in the column it was mapped to in Oracle.

2.3.3 Comparing Data Types

This section lists the difference between MySQL and Oracle data types. For some MySQL data types there is more than one alternative Oracle data type. The tables include information about the following:

• Numeric Types

• Date and Time Types

• String Types

2.3.3.1 Numeric Types

When mapping MySQL data types to numeric data types in Oracle, the following conditions apply:

• If there is no precision or scale defined for the destination Oracle data type, precision and scale are taken from the MySQL source data type.

• If there is a precision or scale defined for the destination data type, these values are compared to the equivalent values of the source data type and the maximum value is selected.

The following table compares the numeric types of MySQL to Oracle:

MySQL	Size	Oracle
BIGINT	8 Bytes	NUMBER (19,0)
BIT	approximately (M+7)/8 Bytes	RAW
DECIMAL(M,D)	M+2 bytes if D > 0, M+1 bytes if D = 0 (D+2, if M < D)	FLOAT(24), BINARY_FLOAT
DOUBLE	8 Bytes	FLOAT(24), BINARY_FLOAT, BINARY_DOUBLE
DOUBLE PRECION	8 Bytes	FLOAT(24), BINARY_DOUBLE
FLOAT(25<=X <=53)	8 Bytes	FLOAT(24), BINARY_FLOAT
FLOAT(X<=24)	4 Bytes	FLOAT, BINARY_FLOAT
INT	4 Bytes	NUMBER (10,0)
INTEGER	4 Bytes	NUMBER (10,0)
MEDIUMINT	3 Bytes	NUMBER (7,0)
NUMERIC	M+2 bytes if D > 0, M+1 bytes if D = 0 (D+2, if M < D)	NUMBER
REAL	8 Bytes	FLOAT(24), BINARY_FLOAT
SMALLINT	2 Bytes	NUMBER(5,0)
TINYINT	1 Byte	NUMBER(3,0)

2.3.3.2 Date and Time Types

The following table compares the date and time types of MySQL to Oracle:

MySQL	Size	Oracle
DATE	3 Bytes	DATE
DATETIME	8 Bytes	DATE
TIMESTAMP	4 Bytes	DATE
TIME	3 Bytes	DATE
YEAR	1 Byte	NUMBER

2.3.3.3 String Types

When mapping MySQL data types to character data types in Oracle, the following conditions apply:

• If there is no length defined for the destination data type, the length is taken from the source data type.

• If there is a length defined for the destination data type, the maximum value of the two lengths is taken.

The following table compares the string types of MySQL to Oracle:

Note:

Reference to M indicates the maximum display size. The maximum legal display size is 255. A reference to L applies to a floating point types and indicates the number of digits following the decimal point.

MySQL	Size	Oracle
BLOB	L + 2 Bytes whereas L<2^16	RAW, BLOB
CHAR(m)	M Bytes, 0<=M<=255	CHAR
ENUM (VALUE1, VALUE2, ...)	1 or 2 Bytes depending on the number of enum. values (65535 values max)
LONGBLOB	L + 4 Bytes whereas L < 2 ^ 32	RAW, BLOB
LONGTEXT	L + 4 Bytes whereas L < 2 ^ 32	RAW, CLOB
MEDIUMBLOB	L + 3 Bytes whereas L < 2^ 24	RAW, BLOB
MEDIUMTEXT	L + 3 Bytes whereas L < 2^ 24	RAW, CLOB
SET (VALUE1, VALUE2, ...)	1, 2, 3, 4 or 8 Bytes depending on the number of set members (64 members maximum)
TEXT	L + 2 Bytes whereas L<2^16	VARCHAR2, CLOB
TINYBLOB	L + 1 Bytes whereas L<2 ^8	RAW, BLOB
TINYTEXT	L + 1 Bytes whereas L<2 ^8	VARCHAR2
VARCHAR(m)	L+1 Bytes whereas L<=M and0<=M<=255 before MySQL 5.0.3 (0 <= M <= 65535 in MySQL 5.0.3 and later; effective maximum length is 65,532 bytes)	VARCHAR2, CLOB

2.4 Data Storage Concepts

This section provide a description of the conceptual differences and similarities in data storage for MySQL and Oracle databases.

Data storage is an aspect of MySQL that sets it apart for nearly every database, including Oracle. In MySQL, databases correspond to directories within the data directory of the server. Tables within a database correspond to one or more files within the database directory, depending on the storage engine used for the tables.

A database can contain a mix of tables of different storage engines. A storage engine is responsible for the storage and retrieval of the data for a table.

MySQL offers a variety of storage engines (formerly called table types) to meet the different requirements of the user's environment. Table 2-5 shows the storage engines supported by MySQL.

Table 2-5 Storage Engines Supported by MySQL

Storage Engine	Description
MyISAM	The default non-transactional storage engine that provides full-text indexing and is highly portable
MERGE	A non-transactional storage engine that allows a collection of MyISAM tables with identical column and index information to be used as one
MEMORY (HEAP)	A non-transactional storage engine that stores data in memory
BDB (Berkeley DB)	The first transactional-safe storage engine
InnoDB	A transactional-safe storage engine designed for maximum performance when processing large volume of data and that provides row-level locking
FEDERATED	A storage engine that accesses data in tables of remote databases rather than in local tables
ARCHIVE	A storage engine that can store large amount of data without indexes in very small footprint
CSV	A storage engine that stores data in text file using comma-separated-values format
BLACKHOLE	A storage engine that acts as a "black hole" that accepts data but throws it away and does not store it
EXAMPLE	A "stub" engine that does nothing. Its purpose is to serve as an example that illustrates how to begin writing new engines.
ISAM	The original MySQL storage engine that has been deprecated in favor of the MyISAM storage engine as of version 5.0

Each storage engine has its benefits and drawbacks. Some of features that differentiate the storage engines are transaction, locking, concurrency and portability. The following table summarizes the features for four of the commonly used storage engines.

Table 2-6 Feature Comparison for Common Storage Engines

Feature	MyISAM	Heap	BDB	InnoDB
Transactional	No	No	Yes	Yes
Lock granularity	Table	Table	Page	Row
Storage	A data file (.MYD) and an index file (.MYI) for each table	In-memory	A single data and index file (.db) for each table	A set of data files for all the tables
Portable	Yes	N/A	No	Yes

An Oracle database consists of one or more tablespaces. Tablespaces provide logical storage space that link a database to the physical disks that hold the data. A tablespace is created from one or more data files. Data files are files in the file system or an area of disk space specified by a raw device. A tablespace can be enlarged by adding more data files.

An Oracle database consists of a least a SYSTEM tablespace, where the Oracle tables are stored. It can also consist of user defined tablespaces. A tablespace is the logical storage location for database objects. For example, you can specify where a particular table or index gets created in the tablespace.