SQL ALTER TABLE Command — Best Online Learning Website

SQL ALTER TABLE Command

The Alter Table command is used to change definitions of existing tables. Usually, it can add columns to table. Sometimes it can delete columns (depending on privileges) or change their sizes.

In short we can say,

The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.

The ALTER TABLE statement is also used to add and drop various constraints on an existing table.

1. ALTER TABLE — ADD Column

To add a column in a table, use the following syntax:

Syntax :

ALTER TABLE table_name 
ADD column_name datatype;

For instance, to add a new column tel_number of type integer in table Student.

ALTER TABLE Student 
ADD (tel_number integer);

2 ALTER TABLE — DROP COLUMN

To delete a column in a table, use the following syntax (notice that some database systems don’t allow deleting a column):

The following SQL deletes the “tel_numberl” column from the “Student” table:

Example

ALTER TABLE Student 
DROP COLUMN tel_number;

3 ALTER TABLE — ALTER/MODIFY COLUMN

To change the data type of a column in a table, use the following syntax:

SQL Server / MS Access:

ALTER TABLE table_name 
ALTER COLUMN column_name datatype;

My SQL / Oracle (prior version 10G):

ALTER TABLE table_name 
MODIFY COLUMN column_name datatype;

Oracle 10G and later:

ALTER TABLE table_name 
MODIFY column_name datatype;

SQL ALTER TABLE Example

Look at table “persons” given below :

Now we want to add a column named “DateOfBirth” in the “Persons” table.

We use the following SQL statement:

ALTER TABLE Persons 
ADD DateOfBirth date;

Notice that the new column, “DateOfBirth”, is of type date and is going to hold a date. The data type specifies what type of data the column can hold. For a complete reference of all the data types available in MS Access, MySQL, and SQL Server, go to our complete Data Types reference.

The “Persons” table will now look like this:

Change Data Type Example

Now we want to change the data type of the column named “DateOfBirth” in the “Persons” table.

We use the following SQL statement:

ALTER TABLE Persons 
ALTER COLUMN DateOfBirth year;

Notice that the “DateOfBirth” column is now of type year and is going to hold a year in a two- or four-digit format.

DROP COLUMN Example

Next, we want to delete the column named “ Address “ in the “Persons” table.

We use the following SQL statement:

ALTER TABLE Persons 
DROP COLUMN Address;

The “Persons” table will now look like this:

Read More :

DROP TABLE Statement in SQL

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