ConstraintsConstraintsLesson 8

Skills MatrixSkills Matrix

ConstraintsConstraints• Domain Integrity: A domain refers to a

column in a table. Domain integrity includes data types, rules, and defaults, constraints, triggers, and XML schema as applied to a column.

• Entity Integrity: Entity integrity applies to rows and includes rules, NULLs, defaults, constraints, and triggers.

• Referential Integrity: Referential integrity applies between tables or columns of the same table and includes constraints and triggers.

Check ConstraintsCheck Constraints

• Use a check constraint in a Transact-SQL statement linked to a field.

• Check constraints restrict the data acceptable in the field even if the data belongs to the correct data type.

Check ConstraintsCheck Constraints• You can’t use default constraints in a few

places, though:– Defaults can’t be used on columns with the

timestamp data type.– Defaults can’t be used on IDENTITY columns.

• IDENTITY columns contain a number automatically incremented with each new record.

– Defaults can’t be used on columns with the ROWGUIDCOL property set. • ROWGUIDCOL indicates that the column includes

a globally unique identifier (GUID) column for the table.

Default ConstraintsDefault Constraints

• If users leave fields blank by not including them in the INSERT or UPDATE statement that they use to add or modify a record, default constraints fill in those fields.

• This can be a big timesaver in a data-entry department—if you use it correctly.

Unique ConstraintsUnique Constraints

• You should use a unique constraint when you need to ensure that no duplicate values can be added to a field.

RulesRules

• Rules (CREATE RULE) store your defined constraints which can then be applied to multiple tables.

• Create it once and use it many times. • Microsoft promises to remove

support for rules in an upcoming version of SQL Server.

Primary Key ConstraintPrimary Key Constraint

• Primary key constraint uses one or more columns in a table. – When using two or more columns, it’s

called a composite primary key. – The primary key uniquely identifies a

row in a table. Some considerations include:

Primary Key ConstraintPrimary Key Constraint

• You may have but one primary key per table.

• The value of the column, or composite of two or more columns, must be unique. A duplicate value will be denied by the RDBMS.

• Null values are not allowed.• Defining a primary key also automatically

generates an index. SQL Server uses the index to enforce uniqueness.

Foreign Key ConstraintForeign Key Constraint

• A foreign key constraint relates to a column or combination of columns used to establish and enforce a link between the data in one or more tables or one or more columns in one table. Some considerations include:– The foreign key must reference a

primary key or unique constraint.– A user must have REFERENCES

permission on a referenced table.

Foreign Key ConstraintForeign Key Constraint– A foreign key constraint that uses only the

REFERENCES clause without the FOREIGN KEY clause refers to a column in the same table.

– Foreign keys do not create indexes automatically.

– The data type of the foreign key and the data type of the column to which it points must match. The names do not need to match.

– You may declare multiple foreign keys in a single table.

SummarySummary

• You learned that tables are wide open to just about any kind of data when they’re first created.

• Users can’t violate the data type of an attribute; other than that, the table accepts anything, including the illogical.

• Guard against a data-entry person making an appointment for a date in the past, for example.

SummarySummary

• To restrict the data your users can enter in a text box in the client application, you learned to create default, check, and unique constraints. – You learned rules are available but

discouraged.

• Primary key and unique constraints guard against duplicate data entry.

Summary for Certification ExaminationSummary for Certification Examination• Know your constraints. Understand the

constraints discussed in this lesson. – Check constraints restrict the data a user

may to enter in a column, even through the data type does not restrict the data.

– Default constraints fill in data for you automatically when you do not specify a value while inserting a new record.

– Unique constraints prevent users from accidentally inserting repetitive values.

• Know that SQL Server still supports rules.