In SQL, three commands can be used to modify the database: INSERT, DELETE, and UPDATE. We discuss each of these in turn.
The INSERT Command
In its simplest form, INSERT is used to add a single tuple to a relation. We must specify the relation name and a list of values for the tuple. The values should be listed in the same order in which the corresponding attributes were specified in the CREATE TABLE command. For example, to add a new tuple to the EMPLOYEE relation specified in the CREATE TABLE EMPLOYEE … command in before.
we can use U1:
U1: INSERT INTO EMPLOYEE
VALUES ( ‘Richard’, ‘K’, ‘Marini’, ‘653298653’, ‘1962-12-30’, ‘98
Oak Forest, Katy, TX’, ‘M’, 37000, ‘653298653’, 4 );
A second form of the INSERT statement allows the user to specify explicit attribute names that correspond to the values provided in the INSERT command. This is useful if a relation has many attributes but only a few of those attributes are assigned values in the new tuple. However, the values must include all attributes with NOT NULL specification and no default value. Attributes with NULL allowed or DEFAULT values are the ones that can be left out. For example, to enter a tuple for a new EMPLOYEE for whom we know only the Fname, Lname, Dno, and Ssn attributes, we can use U1A:
U1A: INSERT INTO EMPLOYEE (Fname, Lname, Dno, Ssn)
VALUES (‘Richard’, ‘Marini’, 4, ‘653298653’);
Attributes not specified in U1A are set to their DEFAULT or to NULL, and the values are listed in the same order as the attributes are listed in the INSERT command itself. It is also possible to insert into a relation multiple tuples separated by commas in a single INSERT command. The attribute values forming each tuple are enclosed in parentheses.
A DBMS that fully implements SQL should support and enforce all the integrity constraints that can be specified in the DDL. For example, if we issue the command in U2 on the database shown in Figure, the DBMS should reject the operation because no DEPARTMENT tuple exists in the database with Dnumber = 2. Similarly, U2A would be rejected because no Ssn value is provided and it is the primary key, which cannot be NULL.
U3: INSERT INTO EMPLOYEE (Fname, Lname, Ssn, Dno)
VALUES (‘Robert’, ‘Hatcher’, ‘980760540’, 2);
(U2 is rejected if referential integrity checking is provided by DBMS.)
U2A: INSERT INTO EMPLOYEE (Fname, Lname, Dno)
VALUES (‘Robert’, ‘Hatcher’, 5);
(U2A is rejected if NOT NULL checking is provided by DBMS.)
A variation of the INSERT command inserts multiple tuples into a relation in conjunction with creating the relation and loading it with the result of a query. For example, to create a temporary table that has the employee last name, project name, and hours per week for each employee working on a project, we can write the statements in U3A and U3B:
U3A: CREATE TABLE WORKS_ON_INFO
( Emp_name VARCHAR(15),
Hours_per_week DECIMAL(3,1) );
U3B: INSERT INTO WORKS_ON_INFO ( Emp_name, Proj_name,
SELECT E.Lname, P.Pname, W.Hours
FROM PROJECT P, WORKS_ON W, EMPLOYEE E
WHERE P.Pnumber=W.Pno AND W.Essn=E.Ssn;
A table WORKS_ON_INFO is created by U3A and is loaded with the joined information retrieved from the database by the query in U3B. We can now query WORKS_ON_INFO as we would any other relation; when we do not need it any more, we can remove it by using the DROP TABLE command. Notice that the WORKS_ON_INFO table may not be up-to-date; that is, if we update any of the PROJECT, WORKS_ON, or EMPLOYEE relations after issuing U3B, the information in WORKS_ON_INFO may become outdated. We have to create a view to keep such a table up-to-date.
The DELETE Command
The DELETE command removes tuples from a relation. It includes a WHERE clause, similar to that used in an SQL query, to select the tuples to be deleted. Tuples are explicitly deleted from only one table at a time. However, the deletion may propagate to tuples in other relations if referential triggered actions are specified in the referential integrity constraints of the DDL. Depending on the number of tuples selected by the condition in the WHERE clause, zero, one, or several tuples can be deleted by a single DELETE command. A missing WHERE clause specifies that all tuples in the relation are to be deleted; however, the table remains in the database as an empty table. We must use the DROP TABLE command to remove the table definition. The DELETE commands in U4A to U4D, if applied independently to the database, will delete zero, one, four, and all tuples, respectively, from the EMPLOYEE relation:
U4A: DELETE FROM EMPLOYEE
U4B: DELETE FROM EMPLOYEE
U4C: DELETE FROM EMPLOYEE
U4D: DELETE FROM EMPLOYEE;
The UPDATE Command
The UPDATE command is used to modify attribute values of one or more selected tuples. As in the DELETE command, a WHERE clause in the UPDATE command selects the tuples to be modified from a single relation. However, updating a primary key value may propagate to the foreign key values of tuples in other relations if such a referential triggered action is specified in the referential integrity constraints of the DDL. An additional SET clause in the UPDATE command specifies the attributes to be modified and their new values. For example, to change the location and controlling department number of project number 10 to ‘Bellaire’ and 5, respectively, we use U5:
U5: UPDATE PROJECT
SET Plocation = ‘Bellaire’, Dnum = 5
Several tuples can be modified with a single UPDATE command. An example is togive all employees in the ‘Research’ department a 10 percent raise in salary, as shown in U6. In this request, the modified Salary value depends on the original Salary value in each tuple, so two references to the Salary attribute are needed. In the SET clause, the reference to the Salary attribute on the right refers to the old Salary value before modification, and the one on the left refers to the new Salary value after modification:
U6: UPDATE EMPLOYEE
SET Salary = Salary * 1.1
WHERE Dno = 5;
It is also possible to specify NULL or DEFAULT as the new attribute value. Notice that each UPDATE command explicitly refers to a single relation only. To modify multiple relations, we must issue several UPDATE commands.
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