So far we have explained how to add data to tables and how to change data. What remains is to discuss how to remove data that is no longer needed. Just as adding data is only possible in whole rows, you can only remove entire rows from a table. In the previous section we discussed that SQL does not provide a way to directly address individual rows. Therefore, removing rows can only be done by specifying conditions that the rows to be removed have to match. If you have a primary key in the table then you can specify the exact row. But you can also remove groups of rows matching a condition, or you can remove all rows in the table at once.
You use the DELETE command to remove rows; the syntax is very similar to the UPDATE command. For instance, to remove all rows from the emp table that belong to department number 30, use
DELETE FROM emp WHERE deptno = 30;
If you simply write
DELETE FROM emp;
then all rows in the table will be deleted! Caveat programmer.