Test Convert the ColumnsYou should test all of the changes before committing them to your database.The first thing to test is that the SQL generated from the conversion script is correct. But for column definitions that have specified lengths, defaults or NOT NULL:MyColumn VARCHAR(255) DEFAULT '' NOT NULLWe need to MODIFY keeping the same attributes, or the column definition will be fundamentally changed (see notes in ALTER TABLE)
To add a column called email to the contacts table created in Create a basic MySQL table with a datatype of VARCHAR(80), use the following SQL statement: ALTER TABLE contacts ADD email VARCHAR(60); This first statement will add the email column to the end of the table. it was of great help Randy It would be helpful to show an MYSQL example along with the SQL example Karan Sachdeva86 thank you Anonymous Thank you for this
You can also run these commands through a GUI or web interface, like the PHPMySqlAdmin tool.) Other MySQL ALTER TABLE ADD COLUMN examples Before I go back to work, here are some other MySQL ALTER TABLE ADD COLUMN examples, showing different approaches
database design - What is the difference between MySQL VARCHAR and TEXT data types? - Database Administrators Stack Exchange
For the specific implementations within MySQL 5.1, the docs for MyISM state: Support for a true VARCHAR type; a VARCHAR column starts with a length stored in one or two bytes. Two bytes will only be needed if part of the column is stored externally in overflow pages or the maximum length exceeds 255 bytes and the actual length exceeds 127 bytes
Different InnoDB storage formats (COMPRESSED, REDUNDANT) use different amounts of page header and trailer data, which affects the amount of storage available for rows. Also, for fixed-length data types the maximum row length is around 8.000 bytes (because of size of database page) Note, that BLOB and TEXT columns are represented only as pointers to their physical storage (so their length in row will always be constant)
MySQL :: MySQL 5.0 Reference Manual :: 11.1.3 String Type Overview
This is useful primarily when you have to be compliant with old applications that depend on the existence of a column but that do not actually use its value. For example, utf8 characters can require up to three bytes per character, so a VARCHAR column that uses the utf8 character set can be declared to be a maximum of 21,844 characters
MySQL :: MySQL 5.0 Reference Manual :: 11.4.3 The BLOB and TEXT Types
BLOB and TEXT differ from VARBINARY and VARCHAR in the following ways: There is no trailing-space removal for BLOB and TEXT columns when values are stored or retrieved. 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
sql - Change column type from varchar to text in all tables at once - Database Administrators Stack Exchange
Having said that, why do you want to change all of the 'description' columns to unlimited size? If a description should only be 100 characters (for example) it would be better to stick to a definition VARCHAR(100) that identifies what the limit should be
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