Schema Management
SQL Constraints
Using SQL Constraints
Constraints like PRIMARY KEY, FOREIGN KEY, and UNIQUE enforce data integrity.
Introduction to SQL Constraints
SQL constraints are rules applied to table columns to enforce data integrity and consistency. They ensure that the data stored in the database adheres to specific requirements, preventing invalid data entry. Common types of constraints include PRIMARY KEY, FOREIGN KEY, UNIQUE, CHECK, and NOT NULL.
PRIMARY KEY Constraint
The PRIMARY KEY constraint uniquely identifies each row in a table. It ensures that no two rows have the same value in the specified column(s). A table can have only one primary key, which can consist of one or more columns.
FOREIGN KEY Constraint
The FOREIGN KEY constraint is used to link two tables together. It ensures that the value in a column (or a group of columns) matches a value in the primary key column of another table, thereby maintaining referential integrity.
UNIQUE Constraint
The UNIQUE constraint ensures that all values in a column are different. Unlike the primary key, a table can have multiple UNIQUE constraints. UNIQUE constraints help maintain data uniqueness in non-primary key columns.
CHECK Constraint
The CHECK constraint is used to limit the value range that can be placed in a column. If you define a CHECK constraint on a single column, it allows only certain values for this column. If you define a CHECK constraint on a table, it can limit the values in certain columns based on other columns in the row.
NOT NULL Constraint
The NOT NULL constraint enforces a column to not accept NULL values. This ensures that a column cannot have an undefined value.
Conclusion
SQL constraints are essential for maintaining the integrity and accuracy of the data in your database. By understanding and utilizing these constraints, you can ensure that your data is reliable and adheres to the business rules defined for your application.
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