Accident may happens, even though it’s costly one. This is especially the case in Oracle database, which stores the database objects physically in data files on hard disk. Fast hand administrators may accidentally delete some of the datafiles, or the datafiles may get corrupted or unreadable on hard disk failure. You may get also some missing datafiles after database recovery process. In any case, missing a or a few datafiles will cause Oracle database to fail to startup, rendering the whole DB inaccessible.

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Oracle RDBMS databases stores data logically in the form of tablespaces and physically in the form of data files. The datafiles are added to the tablespaces as database space requirement grows bigger. However, there are several reasons you may want to remove or delete data files from a tablespace. Such as accidentally add a wrongly sized or unwanted datafile to a tablespace, or the data space usage has became smaller and some data files want to be removed, or attempt to recover Oracle database which fails to start due to missing or corrupted datafiles by removing them, Oracle does not provide an easy way or user interface to delete or drop datafiles from a tablespace. Once a datafile is made part of a tablespace, it can no longer be detached or removed from the tablespace, albeit there are several workarounds.

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phpMyAdmin can be used to export or backup MySQL databases easily. However, if the database size is very big, it probably won’t be a good idea. phpMyAdmin allows users to save database dump as file or display on screen, which involves exporting SQL statements from the server, and transmitting the data across slower network connection or Internet to user’s computer. This process slow the exporting process, increase database locking time and thus MySQL unavailability, slow the server and may simply crash the Apache HTTPD server if too many incoming web connections hogging the system’s resources.

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When creating a new table or altering an existing table with primary keys, unique constraints and indexes, or when defining a new index with Alter Table manipulation statement in MySQL database, the following error may occur and prohibit the the command from completing:

ERROR 1170 (42000): BLOB/TEXT column ‘field_name’ used in key specification without a key length

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Another useful string function in MySQL database is TRIM() which will return a text string after removing the matching leading or trailing characters, also known as prefixes or suffixes. It’s been described by MySQL reference as function that returns the string str with all remstr prefixes or suffixes removed. If none of the specifiers BOTH, LEADING, or TRAILING is given, BOTH is assumed. remstr is optional and, if not specified, spaces are removed.

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MySQL database has a handy and simple string function REPLACE() that allows table data with the matching string (from_string) to be replaced by new string (to_string). This is useful if there is need to search and replace a text string which affects many records or rows, such as change of company name, postcode, URL or spelling mistake.

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Oracle databases reserve some special characters with specific meaning and purpose within Oracle environment. These reserved characters include _ (underscore) wild card character which used to match exactly one character, % (percentage) which used to match zero or more occurrences of any characters and ‘ (apostrophe or quotation mark) which used to mark the value supplied. These special characters will not be interpreted literally when building SQL query in Oracle, and may caused error in results returned especially when performing string search with LIKE keyword. To use these characters so that Oracle can interpret them literally as a part of string value instead of preset mean, escape character has to be assigned.

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In Oracle, PostgreSQL, DB2 and SQLite database system, there is a nice command feature called Create Table As which allows easy duplicating of a table with data from another or a few other tables. The SQL command can also be used to ‘copy and paste’ a table exactly or backup a table data before performing data manipulation query on the original table, just in case the script has error and the backup date can be used to restore to original state. Create Table As will creates a new table built from the content of dataset or result set retrieved by a Select SQL query from a table that already exists within the database.

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