How can you delete data from tables using SQL DB2 in your programs?

1 The DELETE statement

The basic syntax for deleting data from a table using SQL DB2 is: DELETE FROM table_name WHERE condition; The table_name is the name of the table from which you want to delete data, and the condition is an expression that specifies which rows to delete. If you omit the condition, all the rows in the table will be deleted. For example, if you want to delete all the employees who have left the company from the EMPLOYEE table, you can write: DELETE FROM EMPLOYEE WHERE STATUS = 'Terminated'; This statement will delete all the rows in the EMPLOYEE table where the STATUS column has the value 'Terminated'.

3 The DROP statement

Another way to delete data from a table using SQL DB2 is to use the DROP statement. This statement drops the table and all its associated objects, such as indexes, views, triggers, and constraints, from the database. This means that the DROP statement is irreversible and you cannot recover the data or the table definition after dropping it. Therefore, you should use the DROP statement only when you are sure that you do not need the table anymore. For example, if you want to drop the TEMP table, you can write: DROP TABLE TEMP; This statement will drop the TEMP table and all its associated objects from the database.

4 The COMMIT and ROLLBACK statements

When you delete data from tables using SQL DB2 in your programs, you should be aware of the transactional nature of the database. A transaction is a logical unit of work that consists of one or more SQL statements that are executed as a single unit. A transaction can either be committed or rolled back, depending on whether the changes made by the SQL statements are permanent or not. The COMMIT statement makes the changes permanent and ends the transaction, while the ROLLBACK statement undoes the changes and restores the previous state of the database. For example, if you want to delete some data from the EMPLOYEE table, but you are not sure if you want to keep the changes, you can write: DELETE FROM EMPLOYEE WHERE DEPTNO = 10; COMMIT; or DELETE FROM EMPLOYEE WHERE DEPTNO = 10; ROLLBACK; The first pair of statements will delete the rows from the EMPLOYEE table where the DEPTNO column has the value 10, and make the changes permanent. The second pair of statements will delete the same rows, but undo the changes and restore the original data.

5 The best practices and pitfalls

When you delete data from tables using SQL DB2 in your programs, you should always use a condition in the DELETE statement, unless you want to delete all the rows from a table. Failing to do so may result in deleting more data than intended, or even the entire table. Additionally, it is essential to backup your data before using the TRUNCATE or DROP statements, as they are irreversible and cannot be recovered. Testing SQL statements in a development or testing environment before running them in production is also important, as they may have unexpected effects on the data or database performance. Finally, it is important to use the COMMIT and ROLLBACK statements appropriately depending on whether you want to keep or discard changes made by your SQL statements. Otherwise, transactions may remain in an uncommitted state, leading to locking or concurrency issues for other users or programs accessing the database.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?