What are Constraints in SQL?

In SQL, constraints are rules that define the allowable values and relationships within a database table. They enforce data integrity by preventing invalid or inconsistent data from being inserted, updated, or deleted in the database. Constraints can be specified when creating or altering a table and help ensure that the data in the database remains accurate, consistent, and reliable.

Here are some common types of constraints in SQL:

1. Primary Key Constraint:A primary key constraint uniquely identifies each record in a table and ensures that there are no duplicate values in the specified column(s). It enforces entity integrity by guaranteeing that each row in the table has a unique identifier.Example: PRIMARY KEY (column_name)
2. Foreign Key Constraint:A foreign key constraint establishes a relationship between two tables by linking the primary key column(s) in one table to the foreign key column(s) in another table. It enforces referential integrity by ensuring that values in the foreign key column(s) match values in the corresponding primary key column(s) in the referenced table.Example: FOREIGN KEY (column_name) REFERENCES other_table (other_column)
3. Unique Constraint:A unique constraint ensures that all values in the specified column(s) are unique, similar to a primary key constraint, but it allows NULL values. It prevents duplicate values from being entered in the column(s) but does not automatically create an index like a primary key constraint.Example: UNIQUE (column_name)
4. Check Constraint:A check constraint restricts the values that can be inserted or updated in a column based on a specified condition. It ensures that only values satisfying the condition are allowed in the column.Example: CHECK (column_name > 0)
5. Not Null Constraint:A not null constraint ensures that a column cannot contain NULL values. It requires that every row in the table has a value for the specified column.Example: column_name NOT NULL

Constraints play a crucial role in maintaining data integrity and consistency in a relational database. By defining and enforcing rules at the database level, constraints help prevent data anomalies, maintain the accuracy of the data, and ensure the reliability of the database system.

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