Understanding Primary Keys, Foreign Keys, and Unique Constraints in SQL

 In SQL, primary keys, foreign keys, and unique constraints are used to ensure data integrity and consistency. 

1. Primary Key:

A primary key is a column or a set of columns in a table that uniquely identifies each row in the table. It is used to enforce data integrity and ensure that there are no duplicate rows in the table. A primary key must be unique, not null, and it should not change over time.

The syntax for defining a primary key is as follows:

CREATE TABLE table_name (
column1 datatype PRIMARY KEY,
column2 datatype,
...
);

2. Foreign Key:

A foreign key is a column or a set of columns in a table that refers to the primary key of another table. It is used to establish a relationship between two tables and enforce referential integrity. A foreign key must have a matching primary key value in the referenced table or have a null value.

The syntax for defining a foreign key is as follows:

    CREATE TABLE table_name (
column1 datatype,
column2 datatype,
FOREIGN KEY (column1)
REFERENCES referenced_table_name (referenced_column1, referenced_column2, ...);
);

3. Unique Constraint:

A unique constraint is a rule that ensures that the values in a column or a set of columns are unique. It is used to enforce data integrity and prevent duplicate values in a table. A unique constraint can be applied to one or more columns in a table.

The syntax for defining a unique constraint is as follows:

 CREATE TABLE table_name (
column1 datatype,
column2 datatype,
CONSTRAINT constraint_name UNIQUE (column1, column2, ...);
);   

          In summary, primary keys are used to uniquely identify rows in a table, foreign keys are used to establish relationships between tables, and unique constraints are used to ensure the uniqueness of values in one or more columns in a table. Understanding these concepts is essential for creating well-designed and structured databases.