A Guide to SQL, Sixth Edition 1 Chapter 5 Updating Data

2 Objectives Create a new table from an existing table Change data using the UPDATE command Add new data using the INSERT command Use the COMMIT and ROLLBACK commands to make permanent data updates or to reverse updates

3 Objectives Understand transactions and the role of COMMIT and ROLLBACK in supporting transactions Delete data using the DELETE command Use nulls in UPDATE commands Change the structure of an existing table Drop a table

4 Creating a New Table from an Existing Table It is possible to create a new table from data in an existing table Problem: Create a new table named LEVEL1_CUSTOMER containing the following columns from the CUSTOMER table: CUSTOMER_NUM, CUSTOMER_NAME, BALANCE, CREDIT_LIMIT, and REP_NUM. The columns in the new LEVEL1_CUSTOMER table should have the same characteristics as the corresponding columns in the CUSTOMER table.

5 Creating a New Table from an Existing Table Solution: CREATE TABLE LEVEL1_CUSTOMER (CUSTOMER_NUM CHAR(3) PRIMARY KEY, CUSTOMER_NAME CHAR(35), BALANCE DECIMAL(8,2), CREDIT_LIMIT DECIMAL(8,2), REP_NUM CHAR(2) );

6 Creating a New Table from an Existing Table A new table can be described with the CREATE TABLE command A SELECT command selects data from the CUSTOMER table By placing this SELECT command in an INSERT command, the query results are added to the new table

7 Creating a New Table from an Existing Table Problem: Insert into the LEVEL1_CUSTOMER table the customer number, customer name, balance, credit limit, and rep number for customers with credit limits of $7,500.

8 Creating a New Table from an Existing Table Solution: INSERT INTO LEVEL1_CUSTOMER SELECT CUSTOMER_NUM, CUSTOMER_NAME, BALANCE, CREDIT_LIMIT, REP_NUM FROM CUSTOMER WHERE CREDIT_LIMIT = 7500;

9 Changing Existing Data in a Table The data stored in a database is subject to constant change The UPDATE command is used to change rows on which a specific condition is true Format for the UPDATE command: UPDATE SET = A WHERE clause can be included to indicate the row(s) on which the change is to take place

10 Adding New Rows to an Existing Table The INSERT command can be used to: Add data to the tables Update table data Problem: Add customer number 895 to the LEVEL1_CUSTOMER table. The name is Peter and Margaret’s, the balance is 0, the credit limit is $8,000, and the rep number is 20.

11 Adding New Rows to an Existing Table Solution: INSERT INTO LEVEL1_CUSTOMER VALUES ('895','Peter and Margaret''s', 0, 8000, '20');

12 Commit and Rollback Updates to data are only temporary Updates can be reversed (cancelled) at any time during the current work session Updates become permanent automatically when the DBMS is exited Updates can be saved immediately by executing the COMMIT command

13 Commit and Rollback Updates can be cancelled by executing the ROLLBACK command Updates since the last COMMIT command will be reversed The ROLLBACK command reverses only changes made to the data, not the table’s structure

14 Transactions A transaction is a logical unit of work A transaction can be viewed as a sequence of steps that accomplishes a single task It is essential that the entire sequence is completed successfully

15 Transactions The COMMIT and ROLLBACK commands are used with transactions as follows: Before beginning the updates for a transaction, execute the COMMIT command Complete the updates for the transaction. If any update cannot be completed, execute the ROLLBACK command and discontinue the updates for the current transaction Execute the COMMIT command after completing the final update

16 Deleting Existing Rows from a Table The DELETE command is used to delete data from the database The format for the DELETE command is: DELETE WHERE =

17 Deleting Existing Rows from a Table Problem: In the LEVEL1_CUSTOMER table, change the name of customer 356 to Smith Sport, and then delete customer 895.

18 Deleting Existing Rows from a Table Solution: UPDATE LEVEL1_CUSTOMER SET CUSTOMER_NAME = 'Smith Sport‘ WHERE CUSTOMER_NUM = '356'; DELETE FROM LEVEL1_CUSTOMER WHERE CUSTOMER_NUM = '895'; SELECT * FROM LEVEL1_CUSTOMER;

19 Executing a Rollback Problem: Execute a rollback and then display the data in the LEVEL1_CUSTOMER table Solution: ROLLBACK; SELECT * FROM LEVEL1_CUSTOMER;

20 Changing a Value in a Column to Null The value in a column in an existing row can be changed to null To make this type of change, the affected column must accept nulls If NOT NULL was specified for the column when it was created, then changing a value to null is prohibited

21 Changing a Value in a Column to Null The command for changing the value to null is the same as it would be for changing any other value The value NULL is used as the replacement value

22 Changing Table Structures A table’s structure can be changed by using the ALTER TABLE command To add a new column, the ADD clause of the ALTER TABLE command is used The format for adding a new column is: ALTER TABLE ADD

23 Changing Table Structures Problem: Premiere Products decides to maintain a customer type for each customer in the database. These types are R for regular customers, D for distributors, and S for special customers. Add this information in a new column in the LEVEL1_CUSTOMER table.

24 Changing Table Structures Solution: ALTER TABLE LEVEL1_CUSTOMER ADD CUSTOMER_TYPE CHAR(1);

25 Changing Table Structures The characteristics of existing columns can be changed by using the MODIFY clause of the ALTER TABLE command Problem: The length of the CUSTOMER_NAME column in the LEVEL1_CUSTOMER table is too short. Increase its length to 50 characters. In addition, change the CREDIT_LIMIT column so that it cannot accept nulls.

26 Changing Table Structures Solution: ALTER TABLE LEVEL1_CUSTOMER MODIFY CUSTOMER_NAME CHAR(50); ALTER TABLE LEVEL1_CUSTOMER MODIFY CREDIT_LIMIT NOT NULL;

27 Making Complex Changes Some changes to a table’s structure are beyond the capabilities of some DBMSs Examples include: Eliminate a column Change the column order Combine data from two tables into one

28 Making Complex Changes To make complex changes: Use the CREATE TABLE command to describe the new table Insert values into the new table using the INSERT command combined with an appropriate SELECT command

29 Dropping a Table A table that is no longer needed can be deleted by using the DROP TABLE command Problem: Delete the LEVEL1_CUSTOMER table because it is no longer needed in the Premiere Products database. Solution: DROP TABLE LEVEL1_CUSTOMER;

30 Summary To create a new table from an existing table: Create the new table by using the CREATE TABLE command Use an INSERT command containing a SELECT command to select the desired data from the existing table UPDATE command: changes existing data in a table INSERT command: adds new rows to a table

31 Summary COMMIT command: saves updates ROLLBACK command: reverses updates DELETE command: deletes existing rows To add a column to a table, use the ALTER TABLE command with an ADD clause To change the characteristics of a column, use the ALTER TABLE command with a MODIFY clause

32 SQL Project Five Completed Good Luck H. Zamanzadeh