Check constraints are a very useful, though somewhat underutilized feature of DB2. Check constraints enable enhanced data integrity without
requiring procedural logic (such as in stored procedures and triggers). Let’s
examine the basics of table check constraints.
A constraint is basically a restriction placed upon
the data values that can be stored in a column or columns of a table. Of
course, most RDBMS products provide several different types of constraints,
such as referential constraints (to define primary and foreign keys) and unique
constraints (to prohibit duplicates).
Check
constraints place specific data value restrictions on the contents of a column
through the specification of a Boolean expression. The expression is explicitly
defined in the table DDL and is formulated in much the same way that SQL WHERE
clauses are formulated. Any attempt to modify the column data (i.e. during
INSERT and UPDATE processing) will cause the expression to be evaluated. If the
modification conforms to the Boolean expression, the modification is permitted
to continue. If not, the statement will
fail with a constraint violation.
This
functionality is great for simulating the relational concept of a domain. A
domain is basically the set of valid values that a column or data type can take
on. Check constraints only simulate domains, though, because there are other
features provided by domains that are not provided by check constraints. One
such feature is that columns pooled from separate domains must not be compared
or operated on by expressions that require the same type of data for both
operands. For domains to truly be supported the DBMS should support both check
constraints and user-defined data types with strong type checking. This
prohibits allowing ridiculous operations, such as comparing IQ to shoe size or
adding Australian dollars to Euros.
Forming Check Constraints
Check
constraints are written using recognizable SQL syntax. This makes them easy to
implement for anyone who has even a passing familiarity with SQL. The check
constraint consists of two components: a constraint name and a check condition.
The
constraint name is an SQL identifier and is used to reference or identify the
constraint. The same constraint name cannot be specified more than once for the
same table. If a constraint name is not explicitly coded, DB2 will create a
unique name automatically for the constraint.
The
check condition defines the actual constraint logic. The check condition can be defined using any
of the basic predicates (>, <, =, <>, <=, >=), as well as
BETWEEN, IN, LIKE, and NULL.
Furthermore, AND and OR can be used to string conditions together.
There
are, however, restrictions on how check constraints are formulated. Some of
these restrictions include:
·
Limitations on the entire length of the check condition.
·
Other tables may not be accessed in the check condition.
·
Only a limited subset of SQL operations are permitted (for example
subselects and column functions are prohibited in a check constraint).
·
One of the operands (usually the first) of the check constraint must be
the name of a column contained in the table for which the constraint is
defined.
·
The other operand (usually the second) must be either another column
name in the same table or a constant value.
·
If the second operand is a constant, it must be compatible with the
data type of the first operand. If the
second operand is a column, it must be the same data type as the first column
specified.
Check Constraint Examples
Check
constraints enable the DBA or database designer to specify more robust data
integrity rules directly into the database.
Consider the following example:
CREATE TABLE EMP
(EMPNO INTEGER
CONSTRAINT CHECK_EMPNO
CHECK (EMPNO BETWEEN 100 and
25000),
EMP_ADDRESS VARCHAR(70),
EMP_TYPE CHAR(8)
CHECK (EMP_TYPE IN (‘TEMP’, ‘FULLTIME’, ‘CONTRACT’)),
EMP_DEPT CHAR(3) NOT NULL WITH DEFAULT,
SALARY DECIMAL(7,2) NOT NULL
CONSTRAINT CHECK_SALARY
CHECK (SALARY < 50000.00),
COMMISSION DECIMAL(7,2),
BONUS DECIMAL(7,2)
);
The
CREATE statement for the EMP table contains three different check constraints:
1. The name of the first check
constraint for the EMP table is CHECK_EMPNO.
It is defined on the EMPNO column.
The constraint ensures that the EMPNO column can contain values that
range from 100 to 25000 (instead of the domain of all valid integers).
2. The second check constraint
for this table is on the EMP_TYPE column.
This is an example of an unnamed constraint. Though this is possible, it is not
recommended. It is best to always
provide an explicit constraint name in order to ease identification and
administration. This specific constraint
restricts the values that can be placed into EMP_TYPE as: 'TEMP', 'FULLTIME',
and 'CONTRACT'; no other values would be
accepted.
3.
The last check constraint on this table is named CHECK_SALARY. It effectively ensures that no employee can
be entered with a salary of more than $50,000. (Now who would want to work
there?)
Column vs. Table Level
Constraints
The
first check constraint example we reviewed showed a column-level check
constraint. However, check constraints also may be coded at the table-level. A
column-level check constraint is defined in the DDL immediately after the
column. Appropriately enough, a table-level check constraint is defined after
all of the columns of the table have already been defined.
It
is quite common for business rules to require access to multiple columns within
a single table. When this situation
occurs, it is wise to code the business rule into a check constraint at the
table-level, instead of at the column level.
Of course, any column-level check constraint can also be defined at the
table-level, as well. In terms of
functionality, there is no difference between an integrity constraint defined
at the table-level and the same constraint defined at the column-level.
Let’s
augment our sample table DDL to add two table-level check constraints:
CREATE TABLE EMP
(EMPNO INTEGER
CONSTRAINT CHECK_EMPNO
CHECK (EMPNO BETWEEN 100 and
25000),
EMP_ADDRESS VARCHAR(70),
EMP_TYPE CHAR(8)
CHECK (EMP_TYPE IN (‘TEMP’, ‘FULLTIME’, ‘CONTRACT’)),
EMP_DEPT CHAR(3) NOT NULL WITH DEFAULT,
SALARY DECIMAL(7,2) NOT NULL
CONSTRAINT CHECK_SALARY
CHECK (SALARY < 50000.00),
COMMISSION DECIMAL(7,2),
BONUS DECIMAL(7,2),
CONSTRAINT COMM_VS_SALARY
CHECK (SALARY > COMMISSION),
CONSTRAINT COMM_BONUS
CHECK (COMMISSION>0 OR BONUS
> 0),
);
The
CREATE statement for the EMP table has been modified to contain two table-level
check constraints having the following ramifications:
1. The name of the first
table-level check constraint for the EMP table is COMM_VS_SALARY. This constraint will ensure that no employee
can earn more commission than salary.
2. The second table-level check
constraint is named COMM_BONUS. This
constraint will ensure that every employee either earns a commission or a bonus
(or possibly, both).
Check Constraint Benefits
So
what are the benefits of check constraints? The primary benefit is the ability
to enforce business rules directly in each database without requiring
additional application logic. Once defined, the business rule is physically
implemented and cannot be bypassed. Check constraints also provide the
following benefits:
·
Because there is no additional programming required, DBAs can implement
check constraints without involving the application programming staff. This
effectively minimizes the amount of code that must be written by the
programming staff. With the significant
application backlog within most organizations, this can be the most crucial
reason to utilize check constraints.
·
Check constraints provide better data integrity. As check constraints are always executed
whenever the data in the column upon which they are defined is to be modified,
the business rule is not bypassed during ad hoc processing and dynamic SQL.
When business rules are enforced using application programming logic instead,
the rules can not be checked during ad hoc processes.
·
Check constraints promote consistency. Because they are implemented
once, in the table DDL, each constraint is always enforced. Constraints written
in application logic, on the other hand, must be executed within each program
that modifies any data to which the constraint applies. This can cause code
duplication and inconsistent maintenance resulting in inaccurate business rule
support.
·
Typically check constraints coded in DDL will outperform the
corresponding application code.
The
overall impact of check constraints will be to increase application development
productivity while at the same time promoting higher data integrity.
Check Constraints, NULLs,
and Defaults
An
additional consideration for check constraints is the relational NULL. Any
nullable column also defined with a check constraint can be set to null. When
the column is set to null, the check constraint evaluates to unknown. Because null indicates the lack of a value,
the presence of a null will not violate the check constraint.
Additionally,
DB2 provides the ability to specify defaults for table columns – both
system-defined defaults (pre-defined and automatically set by the DBMS) and
user-defined defaults. When a row is inserted or loaded into the table and no
value is specified for the column, the column will be set to the value that has
been identified in the column default specification. For example, we could define a default for
the EMP_TYPE column of our sample EMP table as follows:
EMP_TYPE CHAR(8) DEFAULT ‘FULLTIME’
CHECK (EMP_TYPE IN (‘TEMP’, ‘FULLTIME’, ‘CONTRACT’)),
If
a row is inserted without specifying an EMP_TYPE, the column will default to
the value, ‘FULLTIME’.
A
problem can arise when using defaults with check constraints. Most DBMS
products do not perform semantic checking on constraints and defaults. The
DBMS, therefore, will allow the DBA to define defaults that contradict check
constraints. Furthermore, it is possible
to define check constraints that contradict one another. Care must be taken to
avoid creating this type of problem.
Examples
of contradictory constraints are depicted below:
CHECK
(EMPNO > 10 AND EMPNO <9 o:p="">
In this case, no value is
both greater than 10 and less than 9, so nothing could ever be inserted.
EMP_TYPE CHAR(8) DEFAULT ‘NEW’
CHECK (EMP_TYPE IN (‘TEMP’, ‘FULLTIME’, ‘CONTRACT’)),
In this case, the default
value is not one of the permitted EMP_TYPE values according to the defined
constraint. No defaults would ever be
inserted.
CHECK (EMPNO > 10)
CHECK (EMPNO >= 11)
In this case, the
constraints are redundant. No logical
harm is done, but both constraints will be checked, thereby impacting the
performance of applications that modify the table in which the constraints
exist.
Other
potential semantic problems could occur if the constraints contradicts a
referential integrity DELETE or UPDATE rule, if two constraints are defined on
the same column with contradictory conditions, or if the constraint requires
that the column be NULL, but the column is defined as NOT NULL.
Other Potential Hazards
Take
care when using the LOAD utility on a table with check constraints defined to
it. By specifying the ENFORCE NO parameter you can permit DB2 to load data that
does not conform to the check constraints (as well as the referential
constraints). Although this eases the load process by enabling DB2 to bypass
constraint checking, it will place the table space into a check pending state.
You can run CHECK DATA to clear this state (or force the check pending off by
using START with the FORCE option or the REPAIR utility). If you do not run
CHECK DATA, constraint violations may occur causing dirty data.
Summary
Check
constraints provide a very powerful vehicle for supporting business rules in
the database. They can be used to simulate relational domains. Because check
constraints are non-bypassable, they provide better data integrity than
corresponding logic programmed into the application. It is a wise course of
action to use check constraints in your database designs to support data
integrity, domains, and business rules in all of your relational database
applications.
