Different types of Constraints

Constraints are rules or conditions applied to database tables to enforce data integrity and maintain consistency. They define restrictions on the data that can be inserted, updated, or deleted in a table. Here are some common types of constraints:

SQL Keys are a single or combination of multiple fields in a table. They allow you to create a relationships between two or more tables and maintain uniqueness in a table.

Brief key Description   

Key Description
Primary Key A unique identifier for each record in a table. It ensures that each row has a unique value in the specified column(s)
Foreign Key Establishes a relationship between two tables. It ensures referential integrity by enforcing that values in a column(s) of one table match the values in the primary key column(s) of another table.
Unique Ensures that each value in a specified column(s) is unique across the table
Not Null Ensures that a column(s) cannot have null (empty) values
Check Defines a condition that must be satisfied for a data entry to be valid. It restricts the values that can be inserted or updated in a column.
Default Sets a default value for a column if no explicit value is specified during an insert operation.


These constraints help maintain data consistency, integrity, and reliability within a database system.

Certainly! In the context of a database management system (DBMS), keys are an essential concept that helps establish relationships between tables and ensure data integrity. There are several types of keys used in a database, each serving a specific purpose. 

Here's an explanation of the most common keys:

Types of keys:

Primary Key (PK):

  • A primary key is a unique identifier for each record in a table.
  • It ensures that each row in the table has a unique value in the primary key column.
  • Primary keys are used to uniquely identify records and establish relationships between tables.
  • Only one primary key can exist per table.
  • Examples: Employee ID, Student ID.

Foreign Key (FK):

  • A foreign key is a reference to the primary key of another table.
  • It establishes a relationship between two tables.
  • A foreign key in one table matches the primary key in another table, linking the two tables together.
  • It ensures referential integrity by enforcing that values in the foreign key column(s) exist in the referenced primary key column(s).
  • Multiple foreign keys can exist in a table, depending on the relationships.
  • Examples: Customer ID in an Orders table referencing the Customer table's primary key.

Unique Key:

  • A unique key ensures that each value in a specified column(s) is unique across the table.
  • It prevents duplicate entries in the column(s).
  • Unlike the primary key, a unique key can allow null values, meaning multiple rows can have null values.
  • A table can have multiple unique keys.
  • Examples: Email address, Social Security Number.

Candidate Key:

  • A candidate key is a column or set of columns that can function as a primary key.
  • It satisfies the uniqueness requirement for a primary key.
  • A table can have multiple candidate keys, but only one is chosen as the primary key.
  • Examples: Employee ID, Username.

Super Key:

  • A super key is a set of one or more columns that can uniquely identify a record in a table.
  • It may include additional columns beyond the minimum required for uniqueness.
  • Super keys can contain extraneous attributes that are not necessary for uniqueness.
  • Examples: Employee ID + Email, Customer ID + Order ID.

Composite Key:

  • A composite key is a key that consists of two or more columns.
  • It is used when a single column is not sufficient to uniquely identify a record.
  • The combination of multiple columns creates a unique identifier for a record.
  • Examples: Student ID + Course ID, Employee ID + Project ID.

These keys play a vital role in relational databases, ensuring data integrity, maintaining relationships between tables, and facilitating efficient data retrieval. They are crucial for data modelling, normalization, and establishing the structure and integrity of a database.

Different types of Data Types:

Data types define the kind of data that can be stored in a particular column or field of a database table. The specific data types available may vary depending on the DBMS being used,

Here's an explanation of common data types:

Integer (INT):

  • The integer data type is used to store whole numbers without decimal places.
  • It can hold both positive and negative values.
  • Depending on the specific implementation, integers can have varying storage sizes, such as INT, SMALLINT, or BIGINT, with different ranges.

Floating-Point (FLOAT)/Double Precision (DOUBLE):

  • Floating-point data types are used to store decimal numbers with floating-point precision.
  • They can represent both large and small numbers.
  • Float and double data types differ in their storage size and precision, with double providing higher precision than float.

Character/String (CHAR, VARCHAR):

  • Character and string data types are used to store alphanumeric characters or text strings.
  • CHAR is a fixed-length data type that stores a specific number of characters.
  • VARCHAR is a variable-length data type that can store a flexible number of characters.
  • The maximum number of characters that can be stored in a column depends on the implementation.

Boolean (BOOL):

  • The Boolean data type stores true/false or yes/no values.
  • It is used to represent logical values in a database.
  • Boolean values can be manipulated using logical operations (AND, OR, NOT).

Date/Time (DATE, TIME, DATETIME, TIMESTAMP):

  • Date and time data types are used to store specific points in time or durations.
  • DATE represents a specific date (year, month, day).
  • TIME represents a specific time (hour, minute, second).
  • DATETIME combines date and time values.
  • TIMESTAMP represents a specific point in time, often used for recording the creation or modification of a record.

Binary Large Object (BLOB):

  • The BLOB data type is used to store binary data, such as images, audio, or video files.
  • It can hold a large amount of binary data.
  • BLOB data is typically stored separately from other table data.

These are just a few examples of common data types found in DBMS systems. Different DBMS implementations may have additional data types or variations of the ones mentioned above. It's important to choose the appropriate data type for each column based on the nature of the data it will store, to ensure efficient storage, accurate representation, and optimal data manipulation capabilities.

Data types in a database management system (DBMS) define the kind of data that can be stored in a particular column or field of a table. Each data type has specific characteristics, storage requirements, and range of values. Understanding data types is important for accurately representing and manipulating data

Type Description
Integer Used to store whole numbers (e.g., 1, 100, -10).
Float/Double Used to store decimal numbers with floating-point precision (e.g., 3.14, 2.718).
Character/String Used to store alphanumeric characters or text strings (e.g., "Hello", "John Smith").
BooleanUsed to store true/false values.
Date/TimeUsed to store dates or time values (e.g., "2023-06-23", "14:30:00").
Blob (Binary Large Object) Used to store binary data, such as images, audio, or video files.