The statement:
DELETE FROM customer;
deletes all data in the customer table but doesn't remove the table. In contrast, dropping the table removes the data and the table.
A DELETE
statement with a WHERE
clause can remove specific rows; WHERE
clauses are frequently used in querying, and they are explained later in Section 3.8.3. Consider a simple example:
DELETE FROM customer WHERE cust_id = 1;
This deletes the customer with cust_id=1
. Consider another example:
DELETE FROM customer WHERE surname = 'Smith';
This removes all rows for customers with the surname
Smith.
Updating Data
Data can be updated using a similar syntax to that of the INSERT
statement. Consider an example:
UPDATE customer SET email = lower(email);
This replaces the string values of all email
attributes with the same string in lowercase. The function lower( )
is one of many functions discussed later in Section 3.9.
The UPDATE
statement is also often used with the WHERE
clause. For example:
UPDATE customer SET title = 'Dr' WHERE cust_id = 7;
This updates the title
attribute of customer #7. Consider a second example:
UPDATE customer SET zipcode = '3001' WHERE city = 'Melbourne';
This updates the zipcode
of all rows with a city
value Melbourne.