What are Constraints in SQL?
SQL is an extremely versatile language for creating databases. It enables users to construct tables containing information that adheres to a range of business rules, and which can easily be retrieved from the database. To ensure insertions, updates, or deletions adhere to these rules without disrupting integrity, SQL Constraints have been placed upon tables/columns in order to maintain integrity; their purpose being the maintenance of table integrity.
To set constraints on columns within a database, the CREATE TABLE statement can help. Simply specify the name of the column along with an expression which must evaluate to true in order to fulfill your constraint – ensure this expression relates specifically to that column and only includes values which will be added when you insert data.
Once a constraint is defined, it will prevent values that don’t meet its requirements from being stored in the table. If an attempt to insert such rows leads to violation of constraints and an error message appears; this saves support personnel time from dealing with invalid values entered by other people into their database.
The UNIQUE constraint is used to ensure data uniqueness within a table, guaranteeing no two rows can share identical values for specific columns/combinations of columns. An index created automatically when this constraint is defined will ensure this uniqueness; making this the simplest type of constraint ever introduced onto any given table.
An additional constraint that should not be ignored is NOT NULL. This requirement states that all columns with this restriction must have some value when new records are added to a table, otherwise an error will occur and generate. If one or more fields with NOT NULL constraints is left blank in any row, an error message will be generated as a result.
There are other constraints you can place on tables and columns, but these are among the most essential. They ensure your database data adheres to business rules more easily while making DBA maintenance simpler.
One final constraint found helpful in relational databases is the FOREIGN KEY constraint. A foreign key refers to a column in one table which references another table; this will ensure that data in the foreign column matches existing records from its parent table. A FOREIGN KEY may be defined on individual or multiple columns within a table.