4.4. Constraints

Data types are a way to limit the kind of data that can be stored in a table. For many applications, however, the constraint they provide is too coarse. For example, a column containing a product price should probably only accept positive values. But there is no data type that accepts only positive numbers. Another issue is that you might want to constrain column data with respect to other columns or rows. For example, in a table containing employee information, there should only be one row for each employee.

To that end, SQL allows you to define constraints on columns and tables. Constraints give you as much control over the data in your tables as you wish. If a user attempts to store data in a column that would violate a constraint, an error is raised. This applies even if the value came from the default value definition.

4.4.1. Check Constraints

A check constraint is the most generic constraint type. It allows you to specify that the value in a certain column must satisfy an arbitrary expression. For instance, to require positive salaries, you could use:

CREATE TABLE emp (
        empno     NUMBER(4),
        ename     VARCHAR2(10),
        job       VARCHAR2(9),
        mgr       NUMBER(4),
        hiredate  DATE,
        sal       NUMBER(7,2)  CHECK (sal > 0),
        comm      NUMBER(7,2),
        deptno    NUMBER(2)
      )

As you see, the constraint definition comes after the data type, just like default value definitions. Default values and constraints can be listed in any order. A check constraint consists of the key word CHECK followed by an expression in parentheses. The check constraint expression should involve the column thus constrained, otherwise the constraint would not make too much sense.

You can also give the constraint a separate name. This clarifies error messages and allows you to refer to the constraint when you need to change it. The syntax is:

CREATE TABLE emp (
        empno     NUMBER(4),
        ename     VARCHAR2(10),
        job       VARCHAR2(9),
        mgr       NUMBER(4),
        hiredate  DATE,
        sal       NUMBER(7,2)  CONSTRAINT chk_salary CHECK (sal > 0),
        comm      NUMBER(7,2),
        deptno    NUMBER(2)
      )

So, to specify a named constraint, use the key word CONSTRAINT followed by an identifier followed by the constraint definition. A check constraint can also refer to several columns. Say we wanted to limit an employee's total commission (the product of salary and commission) to $5000.00, we would do something like the following:

CREATE TABLE emp (
        empno     NUMBER(4),
        ename     VARCHAR2(10),
        job       VARCHAR2(9),
        mgr       NUMBER(4),
        hiredate  DATE,
        sal       NUMBER(7,2) CONSTRAINT chk_salary CHECK (sal > 0),
        comm      NUMBER(7,2),
        deptno    NUMBER(2),
        CHECK (sal * comm <=5000));

The first two constraints should look familiar. The third one uses a new syntax. It is not attached to a particular column, instead it appears as a separate item in the comma-separated column list. Column definitions and these constraint definitions can be listed in mixed order.

We say that the first two constraints are column constraints, whereas the third one is a table constraint because it is written separately from the column definitions. Column constraints can also be written as table constraints, while the reverse is not necessarily possible. The above example could also be written as

CREATE TABLE emp (
     empno           NUMBER(4),
     ename           VARCHAR2(10),
     job             VARCHAR2(9),
     mgr             NUMBER(4),
     hiredate        DATE,
     sal             NUMBER(7,2),
     comm            NUMBER(7,2),
     deptno          NUMBER(2),
     CHECK (sal > 0),
     CHECK (sal * comm <=5000))

It's a matter of taste.

It should be noted that a check constraint is satisfied if the check expression evaluates to true or the null value. Since most expressions will evaluate to the null value if one operand is null, they will not prevent null values in the constrained columns. To ensure that a column does not contain null values, the not-null constraint described in the next section should be used.

4.4.2. Not-Null Constraints

A not-null constraint simply specifies that a column must not assume the null value. A syntax example:

CREATE TABLE emp (
     empno           NUMBER(4) NOT NULL,
     ename           VARCHAR2(10),
     job             VARCHAR2(9),
     mgr             NUMBER(4),
     hiredate        DATE,
     sal             NUMBER(7,2),
     comm            NUMBER(7,2),
     deptno          NUMBER(2)
);

A not-null constraint is always written as a column constraint. A not-null constraint is functionally equivalent to creating a check constraint CHECK (column_name IS NOT NULL), but in EnterpriseDB creating an explicit not-null constraint is more efficient. The drawback is that you cannot give explicit names to not-null constraints created that way.

Of course, a column can have more than one constraint. Just write the constraints after one another:

CREATE TABLE emp (
    empno            NUMBER(4) NOT NULL,
    ename            VARCHAR2(10),
    job              VARCHAR2(9),
    mgr              NUMBER(4),
    hiredate         DATE,
    sal              NUMBER(7,2) NOT NULL CHECK (sal > 0) ,
    comm             NUMBER(7,2),
    deptno           NUMBER(2),
    CHECK (sal * comm <=5000)
);

The order doesn't matter. It does not necessarily determine in which order the constraints are checked.

The NOT NULL constraint has an inverse: the NULL constraint. This does not mean that the column must be null, which would surely be useless. Instead, this simply defines the default behavior that the column may be null. The NULL constraint is not defined in the SQL standard and should not be used in portable applications. (It was only added to EnterpriseDB to be compatible with some other database systems.) Some users, however, like it because it makes it easy to toggle the constraint in a script file. For example, you could start with

CREATE TABLE emp (
        empno     NUMBER(4) NOT NULL,
        ename     VARCHAR2(10) NOT NULL,
        job       VARCHAR2(9) NULL,
        mgr       NUMBER(4) NULL,
        hiredate  DATE NULL,
        sal       NUMBER(7,2) NOT NULL CHECK (sal > 0),
        comm      NUMBER(7,2) NULL,
        deptno    NUMBER(2) NOT NULL
);

and then insert the NOT key word where desired.

Tip: In most database designs the majority of columns should be marked not null.

4.4.3. Unique Constraints

Unique constraints ensure that the data contained in a column or a group of columns is unique with respect to all the rows in the table. The syntax is

CREATE TABLE emp (
     empno           NUMBER(4) UNIQUE,
     ename           VARCHAR2(10),
     job             VARCHAR2(9),
     mgr             NUMBER(4),
     hiredate        DATE,
     sal             NUMBER(7,2) CHECK (sal > 0),
     comm            NUMBER(7,2),
     deptno          NUMBER(2) 
);

when written as a column constraint, and

CREATE TABLE emp (
     empno           NUMBER(4),
     ename           VARCHAR2(10),
     job             VARCHAR2(9),
     mgr             NUMBER(4),
     hiredate        DATE,
     sal             NUMBER(7,2) CHECK (sal > 0),
     comm            NUMBER(7,2),
     deptno          NUMBER(2),
    UNIQUE (empno)
);

when written as a table constraint.

If a unique constraint refers to a group of columns, the columns are listed separated by commas:

CREATE TABLE emp (
     empno           NUMBER(4),
     ename           VARCHAR2(10),
     job             VARCHAR2(9),
     mgr             NUMBER(4),
     hiredate        DATE,
     sal             NUMBER(7,2) CHECK (sal > 0),
     comm            NUMBER(7,2),
     deptno          NUMBER(2),
     UNIQUE (empno,deptno)
);

It is also possible to assign names to unique constraints:

CREATE TABLE emp (
     empno           NUMBER(4) CONSTRAINT unique_empno UNIQUE,
     ename           VARCHAR2(10),
     job             VARCHAR2(9),
     mgr             NUMBER(4),
     hiredate        DATE,
     sal             NUMBER(7,2) CHECK (sal > 0),
     comm            NUMBER(7,2),
     deptno          NUMBER(2)
);

In general, a unique constraint is violated when there are (at least) two rows in the table where the values of each of the corresponding columns that are part of the constraint are equal. However, null values are not considered equal in this consideration. That means even in the presence of a unique constraint it is possible to store an unlimited number of rows that contain a null value in at least one of the constrained columns. This behavior conforms to the SQL standard, but we have heard that other SQL databases may not follow this rule. So be careful when developing applications that are intended to be portable.

4.4.4. Primary Keys

Technically, a primary key constraint is simply a combination of a unique constraint and a not-null constraint. So, the following two table definitions accept the same data:

CREATE TABLE emp (
     empno           NUMBER(4)  UNIQUE NOT NULL,
     ename           VARCHAR2(10),
     job             VARCHAR2(9),
     mgr             NUMBER(4),
     hiredate        DATE,
     sal             NUMBER(7,2) CHECK (sal > 0),
     comm            NUMBER(7,2),
     deptno          NUMBER(2)
);

CREATE TABLE emp (
     empno           NUMBER(4)  PRIMARY KEY,
     ename           VARCHAR2(10),
     job             VARCHAR2(9),
     mgr             NUMBER(4),
     hiredate        DATE,
     sal             NUMBER(7,2) CHECK (sal > 0),
     comm            NUMBER(7,2),
     deptno          NUMBER(2)
);

Primary keys can also constrain more than one column; the syntax is similar to unique constraints:

CREATE TABLE emp (
     empno           NUMBER(4),
     ename           VARCHAR2(10),
     job             VARCHAR2(9),
     mgr             NUMBER(4),
     hiredate        DATE,
     sal             NUMBER(7,2) CHECK (sal > 0),
     comm            NUMBER(7,2),
     deptno          NUMBER(2),
     PRIMARY KEY (empno, ename) 
);

A primary key indicates that a column or group of columns can be used as a unique identifier for rows in the table. (This is a direct consequence of the definition of a primary key. Note that a unique constraint does not, by itself, provide a unique identifier because it does not exclude null values.) This is useful both for documentation purposes and for client applications. For example, a GUI application that allows modifying row values probably needs to know the primary key of a table to be able to identify rows uniquely.

A table can have at most one primary key (while it can have many unique and not-null constraints). Relational database theory dictates that every table must have a primary key. This rule is not enforced by EnterpriseDB, but it is usually best to follow it.

4.4.5. Foreign Keys

A foreign key constraint specifies that the values in a column (or a group of columns) must match the values appearing in some row of another table. We say this maintains the referential integrity between two related tables.

Let's refer to the dept table that stores the department related information:

CREATE TABLE dept (
    deptno          NUMBER(2) PRIMARY KEY,
    dname           VARCHAR2(14),
    loc             VARCHAR2(13)
);

We want to ensure that the emp table only contains the id's of departments that actually exist. So we define a foreign key constraint in the emp table that references the dept table:

CREATE TABLE emp (
     empno           NUMBER(4)  PRIMARY KEY,
     ename           VARCHAR2(10),
     job             VARCHAR2(9),
     mgr             NUMBER(4),
     hiredate        DATE,
     sal             NUMBER(7,2),
     comm            NUMBER(7,2),
     deptno          NUMBER(2) REFERENCES dept (deptno)
);

Now it is impossible to create employees with deptno entries that do not appear in the dept table. However, keep in mind that the column being referenced in the other table (in this case the deptno column of the dept table) must be the primary key of that table.

We say that in this situation the emp table is the referencing table and the dept table is the referenced table. Similarly, there are referencing and referenced columns.

You can also shorten the above command to

CREATE TABLE emp (
     empno           NUMBER(4)  PRIMARY KEY,
     ename           VARCHAR2(10),
     job             VARCHAR2(9),
     mgr             NUMBER(4),
     hiredate        DATE,
     sal             NUMBER(7,2) CHECK (sal > 0),
     comm            NUMBER(7,2),
     deptno          NUMBER(2) REFERENCES dept 
);

A foreign key can also constrain and reference a group of columns. As usual, it then needs to be written in table constraint form. Here is a contrived syntax example:

CREATE TABLE t1 (
  a integer PRIMARY KEY,
  b integer,
  c integer,
  FOREIGN KEY (b, c) REFERENCES other_table (c1, c2)
);

Of course, the number and type of the constrained columns needs to match the number and type of the referenced columns.

A table can contain more than one foreign key constraint. This is used to implement many-to-many relationships between tables. Say we have a table for keeping a track record of all our employees. You could use this table structure:

CREATE TABLE jobhist (
     empno           NUMBER(4) NOT NULL REFERENCES emp,
     startdate       DATE NOT NULL,
     enddate         DATE,
     job             VARCHAR2(9),
     sal             NUMBER(7,2),
     comm            NUMBER(7,2),
     deptno          NUMBER(2) REFERENCES dept,
     chgdesc         VARCHAR2(80),
     PRIMARY KEY (empno, startdate),
     CHECK (startdate <= enddate)
);

We know that the foreign keys disallow creation of history records for employees who do not exist in the emp table. But what if an employee is removed after his/her history is created in the history table? SQL allows you to specify that as well. Intuitively, we have a few options:

Disallow deleting a referenced employee
Delete the employee as well
Something else?

To illustrate this, let's implement the following policy on the many-to-many relationship example above: when someone wants to remove an employees that is still referenced by a history record, we disallow it. If someone removes an employee from the emp table, then the employee's history is also removed from the history table.

CREATE TABLE emp (
     empno           NUMBER(4)  NOT NULL CONSTRAINT emp_pk PRIMARY KEY,
     ename           VARCHAR2(10),
     job             VARCHAR2(9),
     mgr             NUMBER(4),
     hiredate        DATE,
     sal             NUMBER(7,2) CHECK (sal > 0),
     comm            NUMBER(7,2),
     deptno          NUMBER(2) CONSTRAINT emp_ref_dept_fk 
	                 REFERENCES dept(deptno)
);

CREATE TABLE dept (
     deptno          NUMBER(2) NOT NULL CONSTRAINT dept_pk PRIMARY KEY,
     dname           VARCHAR2(14) CONSTRAINT dept_dname_uq UNIQUE,
     loc             VARCHAR2(13)
);


CREATE TABLE jobhist (
     empno           NUMBER(4) NOT NULL,
     startdate       DATE NOT NULL,
     enddate         DATE,
     job             VARCHAR2(9),
     sal             NUMBER(7,2),
     comm            NUMBER(7,2),
     deptno          NUMBER(2),
     chgdesc         VARCHAR2(80),
     CONSTRAINT jobhist_pk PRIMARY KEY (empno, startdate),
     CONSTRAINT jobhist_ref_emp_fk FOREIGN KEY (empno)
         REFERENCES emp(empno) ON DELETE CASCADE,
     CONSTRAINT jobhist_ref_dept_fk FOREIGN KEY (deptno)
         REFERENCES dept (deptno) ON DELETE SET NULL,
     CONSTRAINT jobhist_date_chk CHECK (startdate <= enddate)
);

Restricting and cascading deletes are the two most common options. RESTRICT prevents a statement from deleting a referenced row. NO ACTION means that if any referencing rows still exist when the constraint is checked, an error is raised; this is the default if you do not specify anything. (The essential difference between these choices is that NO ACTION allows the check to be deferred until later in the transaction, whereas RESTRICT does not.) There are two other options: SET NULL and SET DEFAULT. These cause the referencing columns to be set to nulls or default values, respectively, when the referenced row is deleted. Note that these do not excuse you from observing any constraints. For example, if an action specifies SET DEFAULT but the default value would not satisfy the foreign key, the deletion of the primary key will fail.

Analogous to ON DELETE there is also ON UPDATE which is invoked when a primary key is changed (updated). The possible actions are the same.

More information about updating and deleting data is in Chapter 5.

Finally, we should mention that a foreign key must reference columns that either are a primary key or form a unique constraint. If the foreign key references a unique constraint, there are some additional possibilities regarding how null values are matched. These are explained in the reference documentation for CREATE TABLE.