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You use check constraints to ensure that data in a column is within some parameters that you specify. For example, say the salary for an employee in a firm can t be equal to or exceed $100,000 under any circumstances. You can enforce this condition by using the following statement, which uses the CHECK constraint on the SALARY column: SQL> CREATE TABLE employee (employee_id NUMBER, last_name VARCHAR2(30), first_name VARCHAR2(30), department_id NUMBER, salary NUMBER CHECK(salary < 100000));
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Unique constraints are very common in relational databases. These constraints ensure the uniqueness of the rows in a relational table. You may have more than one unique constraint on a table. For example, a unique constraint on the employee_id column ensures that no employee is listed twice in the employee table. In the following example, the first statement specifies a unique constraint on the combination of the dept_name and location columns: SQL> CREATE TABLE dept( dept_no NUMBER(3), dept_name VARCHAR2(15), location VARCHAR2(25), CONSTRAINT dept_name_ukey UNIQUE(dept_Name,location);
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You can also create a unique constraint on the department table by executing the ALTER TABLE statement: SQL> ALTER TABLE dept ADD CONSTRAINT dept_idx UNIQUE(dept_no); Table altered. SQL>
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Referential integrity constraints ensure that values for certain important columns make sense. Suppose you have a parent table that refers to values in another table, as in the case of the dept table and the employee tables. You shouldn t be able to assign an employee to a department in the employee table if the department doesn t exist in the department table. You can ensure the existence of a valid department by using a referential integrity constraint. In this case, the department_id column is the dept table s primary key, and the dept_id column in the employee table, which refers to the corresponding column in the department table, is called the foreign key. The table containing the foreign key is usually referred to as the child table, and the table containing the referenced key is called the parent table. As with all the other types of constraints, you can create the referential integrity constraint at table creation time or later on, with the help of the ALTER TABLE statement: SQL> CREATE TABLE employee (employee_id NUMBER(7), last_name VARCHAR2(30), first name VARCHAR2(30), job VARCHAR2(15), dept_id NUMBER(3) NOT NULL CONSTRAINT dept_fkey REFERENCES dept(dept_id)); The database designates the dept_id column of the employee table as a foreign key because it refers to the dept_id column in the dept table. Note that for a column to serve as the referenced column, it must be unique or be a primary key in the reference table.
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As you saw in the previous section, integrity constraints are defined on tables to ensure that data that violates preset rules doesn t enter the tables. However, during times like data loading, you can t keep the integrity constraints in a valid state, as this will lead to certain problems. Oracle lets you disable constraints when necessary and enable them when you want. Let s examine the various ways you can alter the states of table constraints. During large data loads, using either the SQL*Loader or the Import utility, it may take a considerably longer time to load the data if you have to check for integrity violations for each row inserted into the table. A better strategy would be to disable the constraint, load the data, and worry about any possible insertion of bad data later on. After the load is completed, the constraints are brought into effect again by enabling them. When you disable the constraint as explained here, the database drops the index. It s therefore a better strategy to precreate nonunique indexes for constraints, which the database doesn t have to drop because they can handle duplicates.
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