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6.3 The DELETE Command
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The simplest data manipulation command is DELETE, as shown in the syntax diagram in Figure 6-3. This command also operates at the table level, and you use the WHERE clause to restrict the set of rows you want to delete from the table. If you omit the WHERE clause, the DELETE command results in an empty table.
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Figure 6-3. DELETE command syntax diagram Note the difference between, but do not actually run, the following two commands: drop table departments delete from departments The DROP TABLE command not only removes the contents of the table, but also the table itself, including all dependent objects/structures such as indexes and privileges. DROP TABLE is a data definition (DDL) command. The DELETE command does not change the database structure, but only the contents it is a data manipulation (DML) command. Moreover, the effects of a DROP TABLE command cannot be undone with a ROLLBACK command, as opposed to the effects of a DELETE command, which can. (The ROLLBACK command is introduced in Section 6.5.)
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Note In 7, you will see that there is a different way to get a table back after a DROP TABLE statement.
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Listing 6-6 shows how you can delete a salary grade. Listing 6-6. Example of a DELETE Command select * from salgrades; GRADE LOWERLIMIT UPPERLIMIT BONUS ----- ---------- ---------- ---------1 700 1200 0 2 1201 1400 50 3 1401 2000 100 4 2001 3000 200 5 3001 9999 500
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delete from salgrades where grade = 5; 1 row deleted. select * from salgrades; GRADE LOWERLIMIT UPPERLIMIT BONUS ----- ---------- ---------- ---------1 700 1200 0 2 1201 1400 50 3 1401 2000 100 4 2001 3000 200 rollback; Rollback complete. To illustrate the fact that you can also use subqueries in the FROM clause of the DELETE statement, Listing 6-7 shows an alternative formulation for the same DELETE statement. Again, we use the ROLLBACK command to undo our changes. Listing 6-7. Alternative DELETE Command, Using a Subquery select * from salgrades; GRADE LOWERLIMIT UPPERLIMIT BONUS ----- ---------- ---------- ---------1 700 1200 0 2 1201 1400 50 3 1401 2000 100 4 2001 3000 200 5 3001 9999 500 delete from (select * from salgrades where grade = 5); 1 row deleted. select * from salgrades; GRADE LOWERLIMIT UPPERLIMIT BONUS ----- ---------- ---------- ---------1 700 1200 0 2 1201 1400 50 3 1401 2000 100 4 2001 3000 200
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rollback; Rollback complete. In this case, there are no obvious advantages to using a subquery over using a regular DELETE statement. You can use subqueries in the WHERE clause of the DELETE statement. Listing 6-8 shows how you can use a subquery to filter the rows for the DELETE statement. In this case, the deptname TRAINING is returned from the subquery and used to filter out the deptname for the dept_emp_names table. Again, we use the ROLLBACK command to undo our changes. Listing 6-8. Alternative DELETE Command, Using a Subquery in the WHERE clause DELETE FROM dept_emp_names WHERE deptname = (SELECT dname FROM departments WHERE location = 'DALLAS'); 40 rows deleted. rollback; Rollback complete.
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Deleting rows may seem rather straightforward, but you might encounter complications due to constraint violations. In Listing 6-9, the delete fails because rows exist in a child table. Listing 6-9. Unable to delete due to a constraint violation DELETE FROM employees WHERE deptno IN (SELECT deptno FROM departments WHERE location = 'NEW YORK') / DELETE FROM employees * ERROR at line 1: ORA-02292: integrity constraint (BOOK.D_MGR_FK) violated - child record found The same is true for the UPDATE and INSERT commands, by the way. Constraints are discussed in the next chapter. Because this section is about deleting rows, there is another SQL command that deserves mention here: TRUNCATE. The TRUNCATE command allows you to delete all rows of a table in a more efficient way than with the DELETE command. The TRUNCATE command belongs to the category of the data definition (DDL) commands, and so it is covered in the next chapter.
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