Page 22 - Modular_V2.0_SQL_Flipbook

Check Constraint

The CHECK constraint restricts the value range that can be placed in a column. For example, the following
SQL command creates a CHECK constraint on the Marks column when the Students table is created.

The CHECK constraint ensures that the marks of students must be 33, or greater. The CHECK
constraint rejects the operation with an error.
CREATE TABLE Students
Hands On
(
StudentID INTEGER, Create a Books table with BookID, Title,
FirstName CHAR(30),
Author, and Price columns. Set BookID as
LastName CHAR(30), Marks
the PRIMARY KEY.
INT, CHECK(Marks>=33)
);

MODIFYING A TABLE

Modifying a table means making a change in the base structure of the table. For example, after
creating a table, you realise that you have skipped adding a column named Address to the table
or you want to change the data type of a column or you want to remove a column from the table.
MySQL provides the ALTER TABLE command to modify an existing table.

Adding a Column

SQL provides ALTER TABLE command with ADD COLUMN command to add a column in an existing
table. Syntax to add a column is as follows:

ALTER TABLE table
ADD [COLUMN] column_name datatype [FIRST|AFTER existing_column];

Where, FIRST|AFTER clause allows to specify the position of the column to be added. If you don’t
explicitly specify the position of the new column, MySQL will add it as the last column. For example,
adding the Address column in the Students table:
ALTER TABLE Students
ADD COLUMN Address VARCHAR(50);

The preceding command will add the Address column as the last column in the Students table. To
check the added column, SQL provides the DESC command. The DESC command describes the
structure of the table. For example,
DESC Students;

The DESC command will show the following output:
+-----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+-------------+------+-----+---------+-------+
| StudentID | int | YES | | NULL | |
| FirstName | char(30) | YES | | NULL | |
| LastName | char(30) | YES | | NULL | |
| Marks | int | YES | | NULL | |
| Address | varchar(50) | YES | | NULL | |
+-----------+-------------+------+-----+---------+-------+

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