| ylbtech-SQL-W3School- High: SQL ALTER TABLE statement |
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ALTER TABLE statement
ALTER TABLE statement is used to add an existing table, modify, or delete columns .
SQL ALTER TABLE Syntax
To add columns in a table, use the following syntax:
ALTER TABLE table_name ADD column_name datatype
To delete a column in a table, use the following syntax:
ALTER TABLE table_name DROP COLUMN column_name
Note: Some database systems do not allow this to delete columns in a database table mode (DROP COLUMN column_name).
To change the data type of columns in the table, use the following syntax:
ALTER TABLE table_name ALTER COLUMN column_name datatype
The original table (used in the example):
Persons table:
| Id | LastName | FirstName | Address | City |
|---|---|---|---|---|
| 1 | Adams | John | Oxford Street | London |
| 2 | Bush | George | Fifth Avenue | New York |
| 3 | Carter | Thomas | Changan Street | Beijing |
SQL ALTER TABLE example
Now we want to add a new column called "Birthday" in the table "Persons" in.
We use the following SQL statement:
ALTER TABLE Persons ADD Birthday date
Note that the new column "Birthday" is the type of date, the date can be stored. Data type column specifies the type of data can be stored.
The new "Persons" table like this:
| Id | LastName | FirstName | Address | City | Birthday |
|---|---|---|---|---|---|
| 1 | Adams | John | Oxford Street | London | |
| 2 | Bush | George | Fifth Avenue | New York | |
| 3 | Carter | Thomas | Changan Street | Beijing |
Changing the data type instance
Now we want to change the "Persons" table "Birthday" column data type.
We use the following SQL statement:
ALTER TABLE Persons ALTER COLUMN Birthday year
Note that, "Birthday" data type column is year, can store two or four year format.
Examples of DROP COLUMN
Next, we remove the "Person" table "Birthday" column:
ALTER TABLE Person DROP COLUMN Birthday
Persons table will be like this:
| Id | LastName | FirstName | Address | City |
|---|---|---|---|---|
| 1 | Adams | John | Oxford Street | London |
| 2 | Bush | George | Fifth Avenue | New York |
| 3 | Carter | Thomas | Changan Street | Beijing |
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