SQL ALTER Complete Details

SQL ALTER Complete Details

 

The ALTER statement in SQL is used to modify the structure of a database object, such as a table, index, or view. It can be used to add, modify, or remove columns, change data types, add or remove constraints, and rename objects.                 

Here are some of the most common ALTER operations:

1.      Adding a new column:

 

ALTER TABLE table_name ADD new_column_name data_type;

 

For example, to add a new column called phone_number to the customers table, you would use the following query:

 

ALTER TABLE emp ADD phone_number VARCHAR(20);

 

2.      Modifying an existing column:

 

ALTER TABLE table_name ALTER column_name data_type;

For example, to change the data type of the email column in the customers table from VARCHAR(50) to VARCHAR(100), you would use the following query:

ALTER TABLE customers ALTER email VARCHAR(100);

 

3.      Adding a constraint:

ALTER TABLE table_name ADD CONSTRAINT constraint_name constraint_type;

 For example, to add a primary key constraint to the id column in the customers table, you would use the following query:

ALTER TABLE customers ADD CONSTRAINT pk_customers PRIMARY KEY (id);

 

4.      Removing a constraint:

 

ALTER TABLE table_name DROP CONSTRAINT constraint_name;

For example, to remove the primary key constraint from the id column in the customers table, you would use the following query 

ALTER TABLE customers DROP CONSTRAINT pk_customers;

 

5.      Renaming an object:

 

ALTER TABLE table_name RENAME TO new_table_name;

For example, to rename the customers table to customer_details, you would use the following query:

 

6.      Dropping a column:

ALTER TABLE table_name DROP COLUMN column_name;

7.      ALTER VIEW:

   - Modifying a view's definition: ALTER VIEW view_name AS new_view_definition;

 

8.      ALTER PROCEDURE:

   - Modifying a stored procedure's definition: ALTER PROCEDURE procedure_name new_procedure_definition;

 

9.      ALTER INDEX:

   - Modifying an index's definition or properties: ALTER INDEX index_name ON table_name [options];

 

10.  ALTER DATABASE:

   - Modifying database options, such as collation or recovery model: ALTER DATABASE database_name [options];

 Here are some interview questions and answers related to the ALTER statement:

 

·         What is the difference between the ALTER TABLE and CREATE TABLE statements?

Answer: The ALTER TABLE statement is used to modify the structure of an existing table, while the CREATE TABLE statement is used to create a new table.

·         How do you add a new column to a table that already contains data?

 Answer: When you add a new column to a table that already contains data, the default value for the new column will be inserted for all of the existing rows. You can also specify a different value to be inserted using the DEFAULT clause.

 ·         How do you change the data type of an existing column?

Answer: When you change the data type of an existing column, the data in the column may be converted to the new data type. If the new data type is smaller than the old data type, some of the data may be lost.

 ·         How do you add a constraint to a table?

Answer: When you add a constraint to a table, the constraint will be enforced on all future data that is inserted into the table. Existing data will not be checked for compliance with the constraint until it is updated.

·         How do you remove a constraint from a table?

 Answer: When you remove a constraint from a table, the constraint will no longer be enforced on any data in the table.

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