Data Types and Constraints in Relational Databases

Data types in databases define the kind of value that each column can hold.

Common Data Types in Relational Databases

Relational databases support a range of data types, each designed to handle specific kinds of data effectively:

Integer: Stores whole numbers. Variants like INT, SMALLINT, and BIGINT cater to different ranges of values.
Decimal and Numeric: Used for precise decimal numbers, often in financial calculations where rounding errors cannot be tolerated.
Float and Double: Handle floating-point numbers, suitable for scientific calculations but with potential for rounding errors.

Char: A fixed-length character type, useful when the length of the data is consistent.
Varchar: A variable-length character type, more flexible and often more space-efficient than Char.
Text: For long-form text data, like articles or descriptions, where length exceeds the typical limits of Char or Varchar.

Date: Stores dates (year, month, day).
Time: Records time of day.
Datetime/Timestamp: Combines date and time into a single entry, essential for recording precise time stamps of events.

Blob: For binary data like images or multimedia files.
Binary: Similar to Char but for binary data, and Varbinary as its variable-length counterpart.

Boolean: Represents true/false values.
Enum: A string object that can only have one value, chosen from a list of predefined values.

Data types are crucial for several reasons:

Data Integrity: By defining the correct data type, databases can enforce the integrity of the data stored. For example, an attempt to store a non-numeric value in an integer column will be rejected.
Optimization: Proper data types improve database performance. For instance, indexing a Varchar field is generally faster than a Text field.
Memory Usage: Efficient data type choices can significantly reduce the storage space a database requires.

Constraints are rules enforced on data columns to ensure the reliability and accuracy of data within a database. Two primary constraints are:

A primary key is a unique identifier for each record in a table. It must contain unique values and cannot be null.
It uniquely identifies a record and is often used as a reference by other tables. This is crucial for maintaining data integrity and for efficient data retrieval.

A foreign key is a column or a set of columns that establishes a link between data in two tables. It acts as a cross-reference between tables as it references the primary key of another table.
Foreign keys are fundamental in maintaining relationships between tables. They ensure referential integrity by only allowing values that are present in the referenced primary key column of another table.

A composite key is a type of candidate key that consists of two or more columns in a table that are used together to create a unique identifier for each record.
Composite keys are especially useful in situations where a single column does not contain enough information to uniquely identify a record.
In many real-world scenarios, a single column isn’t sufficient to uniquely identify a record. Composite keys are crucial in these cases to ensure each record is uniquely identifiable.
They are often used in join tables that link two other tables in many-to-many relationships, helping to normalize data and reduce redundancy.