ALTER: Altering Tables in SQL: A Comprehensive Guide

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Working with databases is a dynamic task. With the evolution and growth of any application, changes to existing tables and structures become inevitable. In SQL, these modifications can be performed using the ‘ALTER’ statement. This command allows us to add, delete/drop or modify columns in an existing table. It also lets you add and drop various constraints on an existing table.

Using ALTER TABLE

Here’s the basic syntax of the ALTER TABLE command:

Adding Columns

Suppose you have a table named ‘Students’ with columns ‘ID’, ‘Name’, and ‘Age’ and you want to add the column ‘Grade’. Here is how you would do it:

Dropping Columns

The ALTER TABLE…DROP COLUMN command is used to drop column in a table. It is important to note that column you are about to delete should not be part of any primary key or other constraints. If so, first drop the key and constraints, then delete the column.

Suppose now we want to remove the ‘Grade’ column from the ‘Students’ table:

Modifying Columns

To change the data type of a column, the ALTER TABLE…MODIFY command is used. However, you should take care when altering column specifications as this may affect existing data.

For example, to modify the ‘Age’ column from integer to varchar in the ‘Students’ table:

Adding Constraints

Constraints help us control the kinds of data that can go into a table. We can use ALTER TABLE…ADD CONSTRAINT command to add constraints.

Here’s how you add a new Primary Key:

Conclusion

The ALTER TABLE statement in SQL is a potent tool, allowing complete control over our existing database structures. We can add new columns, change existing ones and even add or remove constraints. Before altering anything in your database, be sure to have a good understanding of what outcome each action will lead to and always test on a non-production environment first to avoid any catastrophic mistakes.