SQL DDL constraints Restrictions on the columns and tables 1SQL DDL Constraints

Different types of constraints Entity constraints – PRIMARY KEY – UNIQUE Domain constraints – CHECK constraints CHECK salary > 0 – DEFAULT value Referential integrity constraints – FOREIGN KEY Column level – Constrains applies to a column Table level – Constrains applies to a table 2SQL DDL Constraints

Checking constraints Constraints are automatically checked by the DBMS every time you try to do – INSERT, – UPDATE – or DELETE SQL DDL Constraints3

Constraints should have a name Constraints (like other database objects) have names. Names are use when constraints are created, altered or dropped. Example names – PK_Student_ID Primary key in the Student table, is ID Default names – SQL Server will generate default names for your constrains Which can be quite unreadable – You might be better of naming the constraints your self SQL DDL Constraints4

Primary key constraint Every table should have a primary key. A primary key is a set of attributes – Often the set has only 1 element The values of the primary key attributes must be unique. Primary key attributes must be NOT NULL Primary keys should generally be ID – Don’t use real data as a primary key – Data type INT. Generated using IDENTITY(1,1) Syntax – Attrib dataType IDENTITY(1,1) PRIMARY KEY 5SQL DDL Constraints

UNIQUE constrains Sometimes a table has more candidate keys – One candidate is selected PRIMARY KEY – Others are declared UNIQUE Syntax – Attrib DataType UNIQUE [NOT NULL] – Unlike PRIMARY KEY a UNIQUE attribute can accept NULL 6SQL DDL Constraints

DEFAULT constraints Default values Used in INSERT statements when no value is supplied Example – enrollmentDate data DEFAULT getDate() 7SQL DDL Constraints

Check constraints Simple business rules Examples – Salary int CHECK (salary > 0) – Month tinyInt CHECK (month BETWEEN 1 AND 12) – Day varchar(10) CHECK (day IN (’Monday’, ’Tuesday’, et.) 8SQL DDL Constraints

Foreign key constraints Dependency between two tables – Referring table Has the foreign key – Referenced table The foreign key reefers to the primary key of this table – Recursive relationship / self referencing table The referring table and the referenced table is the same Examples – Employee has a supervisor / boss, who is another Employee – Category has a super category Syntax – Attrib dataType FOREIGN KEY REFERENCES tableName (attributName) 9SQL DDL Constraints

Cascading actions Generally you cannot update / delete referred rows, but … Syntax – CONSTRAINT someName FOREIGN KEY REFERENCES sometable(someattribute_s) ON UPDATE someActionA ON DELETE someActionB – someAction can be No action: default Cascade: referring rows are updated / deleted Set null: referring values are set to null Set default: referring value are set to their default value SQL DDL Constraints10

Ignoring existing data when you create a constraint Adding a constraint to an existing table can be a problem – If the table has data that does NOT conform with the constraint – It is possible to add a constraint WITHOUT checking the existing rows ALTER TABLE … WITH NOCHECK ADD CONSTRAINT … 11SQL DDL Constraints

Disabling and enabling constraints checking ALTER TABLE … NOCHECK CONSTRAINT constraintName – Checking is disabled ALTER TABLE … CHECK CONSTRAINT constraintName – Checking is enabled 12SQL DDL Constraints