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Thursday, June 24, 2010

Some notes on SQL: 3 - changing a table

This is the third in a series of blog posts on SQL, the first covered creating a database, the second selecting information from a database. This post covers commands to modify the structure and contents of an existing database. No claim of authority is made for these posts, they are mainly intended as my notes on the topic.

UPDATE and DELETE allow the rows in a table to be either updated or deleted according to a select-like WHERE clause. This is UPDATE, acting on multiple columns:

UPDATE your_table
SET    first_column = 'newvalue',
       second_column = 'another_value'
WHERE  some_column = 'a test';

And this is the DELETE command:

DELETE FROM your_table
WHERE  some_column = 'a test';

In combination with the ALTER keyword, the following operations can be performed:
The CHANGE keyword allows the name and data type of an existing column to be changed.

ALTER TABLE project_table 
CHANGE COLUMN a_silly_column_name a_better_column_name VARCHAR(100), 
CHANGE COLUMN another_poorly_named_column a_better_name VARCHAR(30);

It's necessary to be cautious here because data loss can occur depending on the source and destination types, for example going from VARCHAR(100) to VARCHAR(30) could potentially lose 70 characters.
The MODIFY keyword allows the data type or position of an existing column to be changed.

ALTER TABLE my_table 
MODIFY COLUMN target_column VARCHAR(120), 
MODIFY COLUMN another_column AFTER target_column;

The ADD keyword allows new columns to be added to a table:

ALTER TABLE my_table 

The ADD and MODIFY keywords take position identifiers: FIRST, LAST and BEFORE, AFTER - which require a second column identifier as indicated in the MODIFY example.

In addition RENAME TO allows the table to be renamed:

ALTER TABLE poor_name RENAME TO good_name;

And DROP deletes a column:

ALTER TABLE my_table DROP COLUMN unwanted_column; 

Obviously you should use DROP COLUMN cautiously!


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