Page 56 - TCS ICT Book 8

The City School 2021-2022

When you design a database, you divide your information into tables, each of
which has a primary key and then add foreign keys to related tables that reference
those primary keys. These foreign key-primary key pairings form the basis for table

relationships and multi-table queries.

3.7. Constraints in Database

In a database when we define any field as a Primary

Key (PK) there are two rules or constraints applied to
that field automatically. Checkpoint

Not Null: this constraint enforces that this field cannot Databases are structured set
of data.
be left unused. In Database terminology, NOT NULL is Rows are also called records

not considered as empty or zero, it just defined as the Columns are also called fields
data has not been entered yet.

Unique: this constraint enforces that the data entered in this field cannot be repeated

or all the values entered should be unique to each other.

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